a. o. beall, jr., r. laury, k. dickson, w. c. pusey, iiiobservations made during core description,...
TRANSCRIPT
27. SEDIMENTOLOGY
A.O. Beall, Jr., Continental Oil Company, Ponca City, Oklahoma,R. Laury, Department of Geology, Southern Methodist University, Dallas, Texas,
K. Dickinson, U.S. Geological Survey, Corpus Christi, Texas,and
W.C. Pusey, III, Continental Oil Company, Ponca City, Oklahoma
MAJOR SEDIMENT TYPES
Deep-water sediments recovered from Leg 10 of theDeep Sea Drilling Project in the Gulf of Mexico (Figure1) comprise a spectrum between biogenically dominatedpelagic sediments and terrigenous elastics, usually charac-terized as laminites and turbidites (Beall and Fischer,1969). In the southern Gulf of Mexico, both shallow anddeep-water carbonates comprise important lithologies.This chapter will attempt to review the sedimentologicalattributes of sediments recovered on Leg 10 by discussingobservations made during core description, smear slidestudy, coarse fraction study, X-ray analysis, textural anal-ysis, carbon-carbonate analysis, limited petrographicstudy of thin sections (primarily of carbonates and cherts),and a review of physical properties. Sediments recoveredcan be categorized as follows:
Organogenic pelagic sediments.Nannoplankton oozes and chalks.Volcanic ash deposits and bentonites.Minor radiolarian cherts.
Abyssal plain terrigenous turbidites and laminites.Sand- and silt-dominated turbidites.Clay- and mud-dominated laminites.
Shallow-water carbonates.Limestones.Dolomites.
Deep-water limestones.Calcilutites and calcarenites.Pebbly mudstones.
Descriptions of deep-water pelagites, laminites, andturbidites from the Gulf of Mexico were treated at lengthby Beall and Fischer (1969) and will not be repeated here;sediments of these types recovered during Leg 10 coringwere quite similar in all respects. In order to reduce repeti-tion and still cover pertinent data, discussion of sedimenttypes will proceed without a conventional discussion ofeach sediment type followed by textural data, mineralogi-cal data, and so on. Under the heading of "Textural Com-position of Sediments," for example, sedimentarystructures and general composition will be discussed inrelation to examples cited. The only departure from thisformat will be under the topics of deep-water limestones,shallow-water carbonates, and cherts, which by nature oftheir less common occurrence have received special atten-tion, primarily in terms of petrographic study.
TEXTURAL COMPOSITION OF SEDIMENTSConventional sand-silt-clay analyses were performed
on each core section, where possible, by Deep Sea Drilling
Project shore-based laboratories (this report), with sub-sidiary detailed sieve analyses carried out by Beall. Inaddition, observations by shipboard scientific staff whilepreparing visual core descriptions have greatly enlargedthe data base. Broader interpretations are made possibleby incorporating all data.
Percentages of sand, silt, and clay are summarized inFigure 2 for all sites. Samples were preferentially takenfrom the most representative lithologies, which are mostoften the finest grained. It should be pointed out thatanalyses performed on semi-consolidated ooze or dis-turbed chalk are undoubtedly biased in that percentagesof sand and silt are probably higher than in the originaltextural mix. Nevertheless, valuable information can bededuced from the aforementioned plot.
For purposes of discussion, the above defined texturaldata can be displayed (Figure 2) under four main catego-ries: (1) abyssal plain sediments, (2) continental rise sedi-ments, (3) carbonate slope sediments, and (4) sedimentswhich accumulated on salt structures, primarily pelagic interms of dominant sedimentary composition. The lastcategory is somewhat arbitrary and was selected to explainsome important differences which exist between suchsediments and other depositional settings.
On a sand-silt-clay basis, abyssal plain sediments ex-hibit a skewed distribution of sediment composition (Fig-ure 2). The clays and muds are typically sand poor, as wellas exhibiting other characteristics to be discussed later. Bycomparing abyssal plain sediments with carbonate slopemuds and pelagic sediments capping salt dome structures,a clear indication of textural separation is displayed. The"high" sand content of the pelagic sediments is due to thepresence of abundant planktonic foraminifera tests.
Continental rise sediments are typically finer grainedthan abyssal plain sediments, according to Figure 2. Thisprimarily reflects the lack of coarser-grained "turbidites"at rise positions sampled. Also note that finer-grained riseand abyssal plain sediments are quite comparable in termsof proportions of silt and clay, a prime reflection of theircommon terrigenous source.
Pelagic sediments accumulating on the crests of saltstructures are apparently finer grained than their slopecounterparts. Although not supported by detailed study,it appears likely that higher biogenic productivity on thecontinental slope (off Yucatan), as compared to the bath-ymetrically more remote salt structure sites, may haveresulted in a coarser-grained sediment where planktonicforaminifera, of silt-and sand-size tests, make up a domi-nant component.
699
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
NORTHERN GULF xx
SALT DOME PROVINCE
PLIO PLEISTDEPDCENTER
CARBONATEi
PLATFORM[ I ! !
MISS. ABYSSAL
FAN
CARBONATE
OLIGOCENE
L OUTCROP.
-28
20c20c
100 200
Scale in Miles82°
Figure 1. DSDP Leg 10 drill sites in the Gulf of Mexico.
SAND
DEEP WATERSEDIMENTS
TEXTURAL SUMMARYGULF OF MEXICO
CLAY
SALT BT FtUCTUFtES
CARBONATE SLOPE
COΛITINENTAL FUSE
ABYSSAL PLAIN
SILT
Figure 2. Sand-silt-clay percentages.
In addition to discrimination on the basis of carbonateversus clay mineral composition, detailed investigationshave shown that the abyssal plain muds and clays aretypically enriched in terrigenous organic debris. X-rayanalysis typically shows enrichment of illite and kaolinitein such sediments, whereas pelagites are relatively en-riched in montmorillonite. This mineralogical differentia-tion is thought to reflect the textural composition of thelaminites (being coarser-grained and terrigenous silt-rich)as contrasted with that of the pelagites (representingpelagic outfall).
Abyssal plain turbidites, of prime interest to sedimen-tologists, are quite variable in textural composition. Al-though the term turbidite is often restricted to sand units,the writers accept a broader definition which encom-passes finer textural grades. Turbidites of the Gulf of Mex-ico abyssal plain can be described as poorly to very poorly
700
SEDIMENTOLOGY
sorted silts, sandy silts, silty sands, sands, and slightlygravelly to gravelly sands. One sample of very coarse,sandy gravel has been described from Site 91. It should bepointed out that only "undisturbed" samples were usedfor detailed textural investigation.
Figure 3 shows a plot of median grain-size versus percent less than 0.062 mm. (total silt plus clay fraction), asdetermined by wet sieve analysis. These analyses, reportedin Table 1, are the result of onshore laboratory studies byBeall. All of the samples shown in the plot possess theattributes of turbidites, as defined by textural grading,sharp lower contacts, and sedimentary structures (Bealland Fischer, 1969). The plot clearly indicates the spec-trum of variability involved. The enclosed area labeled"sand," which contains those samples with less than 10per cent silt plus clay, does not exhibit clear trends of percent "matrix" versus median. It can be noted, however,that no samples had less than 3 per cent "matrix." Thiscan be contrasted with shallow-water sediments, and espe-cially strandline sands, where fine sands with 3 per centsilt plus clay are somewhat less common (Beall, 1970).
1001Md 0 vs % <40 (.062mm)
EE •
o 50
<
SAND
0 1 2 3 4 5 » 5
Md 0Figure 3. Median grain-size versus percent silt-clay fraction
from selected samples.
Several examples of turbidite sequences have been se-lected for detailed discussion. Beall and Fischer (1969, p.541-542 and 565-570) previously presented results of adetailed investigation of an Upper Pleistocene turbidite
from the Gulf of Mexico, which exhibited more or lessideal textural grading. That investigation showed thatgrading was exhibited on the basis of median grain size aswell as on the basis of per cent fine fraction. Maximumgrain size also generally reflected the decrease of size up-wards, especially in the abundance of woody debris nearthe top of the unit.
Figure 4 shows a somewhat more complex case of tex-tural grading also from Upper Pleistocene sediments (Site91). Note that sandy coarse silt, with a median of 4.15 </>,is the coarsest sediment contained in the sequence. Ap-proximately 2 meters thick, the overall aspect is a finingupwards sequence. In detail, there is a suggestion of atleast two graded units superimposed. The presence of twocoarser units with apparent massive bedding overlain inturn by horizontally laminated units supports the inter-pretation of at least two genetic units. The upper cross-laminated sequence may also be a complex of yet anothergraded interval. The unconsolidated nature of the coremakes further interpretation difficult. The coarse fractionsof all samples from this sequence were dominated by or-ganic carbonaceous debris, becoming relatively moreabundant upwards as might be expected.
Figure 5 shows two examples of coarse-grained se-quences of Miocene age, one from Site 87 and the otherfrom Site 90. Although plots of textural parameters ap-pear erratic, utilization of sedimentary structures as wellas textural composition facilitates interpretation of thedata. Both sequences apparently contain superimposedgraded units. The absence of fine-grained tops is sugges-tive of erosion prior to deposition. Note that the uppergraded unit of the example from Site 90 appears to bereversely graded, with the maximum grain size occurringapproxiamtely 20 cm above the base.
In summary, the textural data cited above supports thefollowing conclusions:
1) Large thicknesses of sediments with characteristictextural compositions and sedimentary structures corre-sponding to the concept of turbidites are common in theGulf of Mexico Abyssal Plain.
2) Enlargement of the concept of turbidites to includea spectrum of textural grades extending into the silt grades(and possibly finer) appears necessary. The association ofthese sediment types and their location of several hundredmiles from known shelf edges in the Gulf of Mexico arguefor transport by high energy, turbid sediment masses.Thinning of stratigraphic intervals from the modern shelfedge towards the center of the abyssal plain supports suchan interpretation.
3) The presence of gravelly, medium grade sandswithin the Miocene sequence in the center of the abyssalplain was initially surprising. Depositional rates of suchsediments are quite comparable to the high depositionalrates of Upper Pleistocene sediments from the same locale.With no indication of significant changes in depth of watersince Miocene time, it is concluded that grain size is moreclosely related to provenance than to distance from prove-nance. Mineralogical evidence, discussed later, supportsdifferences in provenance for the two intervals under dis-cussion. It might be further suggested that eustaticchanges in sea level during Miocene time within the Gulf
701
oto
Sample
91-1-2(105-107)91-1-2(119-121)91-1-2(130-133)91-1-2(145-147)91-1-3(5-7)91-1-3(26-28)91-1-3(42-44)91-1-3(60-62)91-1-3(83-85)91-1-3(100-102)91-1-3(115-117)91-1-3(130-132)91-1-3(143-145)91-1-6(79-82)91-1-6(91-94)91-2-4(114-116)91-2-4(130-133)91-2-4(145-147)91-2-5(8-10.5)91-3-6(90-92)91-4-6(103-105.5)91-4-6(112-114.5)91-4-6(124.5-127)91-7-1(75-77.5)91-7-1(96-98.5)91-7-1(112-114.5)91-7-1(130-132.5)91-7-1(143-145.5)91-7-2(34-36)91-7-2(57.5-60)91-7-2(73-75.5)91-7-2(91.5-94)91-7-2(111.5-114)91-7-2(128.5-131)91-7-2(141-143.5)91-7-3(15-17.5)91-7-3(35.5-38)91-7-3(52.5-55)91-9-4(40.5-54)91-9-4(53-55.5)91-9-5(73-75.5)91-9-5(81-83.5)91-9-5(89.5-92)91-9-6(8-11)91-9-6(17-19.5)91-9-6(33-36)91-9-6(48-50.5)91-9-6(59-61.5)91-18-6(130-132)91-18-6(147-149)
-1.00
_
-—_————--_
_———-_—
--——Tr—-
0.05TrTr—-—Tr-_-—_
__—-——-
0.530.05
0.280
_
---Tr0.07—Tr--
0.070.07—--—Tr-—---TrTrTrTrTrTr0.10Tr0.12TrTrTr0.07Tr_
0.06Tr0.17——
Tr——Tr0.064.781.52
1.250
_
-Tr-Tr0.21Tr0.10TrTr0.260.42Tr———Tr—Tr---
0.050.050.04Tr0.050.050.220.150.680.130.05Tr0.210.07Tr0.090.290.680.600.590.120.11-
0.550.570.42
14.658.18
2.00
_
-0.08Tr0.060.690.060.570.07Tr0.521.11TrTrTr-
0.05Tr0.07
--
0.100.100.080.050.150.100.490.541.230.500.100.070.600.140.060.122.262.201.800.890.290.44-
0.820.860.66
28.7719.76
TABLE 1ASieve Analyses, DSDP Leg
Per Cent Coarser than
2.520
_
-0.16Tr0.120.960.121.500.140.070.781.520.270.07Tr-
0.15Tr0.14---
0.150.450.270.250.480.391.091.231.971.190.880.111.202.260.681.076.495.913.062.180.401.00Tr1.991.851.20
40.6829.67
2.750
_
-0.38Tr0.301.170.184.400.210.141.242.151.980.140.05—
0.35Tr0.21—--
0.301.020.450.541.070.872.392.852.832.512.792.773.228.083.654.26
14.5912.755.519.390.983.210.055.485.713.60
46.9735.57
3.00
_
-0.690.080.481.380.397.860.420.261.952.703.850.210.10—
0.450.050.28---
0.351.560.540.941.661.353.743.833.443.704.934.945.92
13.847.117.15
23.2719.438.51
14.172.765.920.20
10.4210.707.73
51.1739.04
3.250
_
-0.920.160.711.931.12
17.741.200.865.144.998.960.280.25-
0.500.150.42---
0.503.071.171.923.492.896.898.305.847.029.47
10.8213.9326.1514.7117.1437.5633.8717.3835.3612.6514.12
1.0223.2421.4717.8156.4444.18
10
3.750
0.070.221.680.421.864.687.59
35.116.146.12
15.6912.6822.65
0.542.42Tr0.760.831.34---
0.6010.185.845.868.548.10
16.3327.0127.6819.1220.5322.9235.6650.2535.7840.0758.5052.2037.0360.2141.2930.90
6.6244.6944.9437.1764.1851.92
4.50
0.140.664.661.84
11.3320.4334.6351.8629.8631.5333.3328.8349.42
4.0524.70
0.061.485.31
18.240.26Tr0.072.93
50.5445.7646.2833.4039.0756.9256.4856.0354.6144.7749.067.3479.5371.0868.3875.2064.2760.5274.9171.3147.9037.0864.9164.0560.5573.4461.61
M D 0
HiHiHiHi
» 5 ?-5.2-4.8
4.15-4.98-4.88-5.2 ?-5.7 ?
4.52-6 ?-4.9V.Hi
Hi-6.0-5.1
HiV.HiV.Hi-6.5 ?
4.484.584.554.854.704.374.304.354.404.704.504.13.734.083.983.523.694.153.533.924.684.83.903.904.122.903.61
% FinerThan 40
99.9199.3597.3599.2596.592.085.555.588.087.078.582.069.599.093.599.9899.0598.396.4
V.Hi-100
99.9999.180.087.087.086.085.072.063.063.070.070.067.054.037.052.049.034.042.054.033.047.063.086.047.047.055.032.044.5
Textural Classification(Folk)
9
79
9
SiltSiltSdy crse siltSdy crse siltSdy crse siltSdy crse siltSdy siltSdy siltSdy crse siltSiltCrse siltClay?Mud?SiltSiltClay/Mud ?Clay/Mud?Clay/Mud ?Silt?Sdy crse siltSdy crse siltSdy crse siltSdy crse siltSdy crse siltSdy crse siltSdy crse siltSdy crse siltSdy crse siltSdy crse siltSdy crse siltSdy crse siltSilty, v.f. sdSdy crse siltSilty, v.f. sdSilty, v.f. sdSilty, v.f. sdSdy crse siltSilty, v.f. sdSilty, v.f. sdSdy crse siltSdy crse siltSilty, v.f. sdSilty, v.f. sdSdy crse siltSilty, find sdSilty, v.f. sd, sli. gravelly
>
pww>[―1r
r>
l - <
r̂σ
1o
p
CΛ
W
~_
o
91-20-2(33-36)91-20-2(140-142)91-20-6(46-48)91-20-6(133-135)91-22-1(81-83.5)91-25-1(94-96)91-25-1(100-102)91-25-1(140-142)91-25-2(60-62)91-25-2(128-131)91-25-3(67-69)91-25-3(102-104)91-25-3(132-134)91-25-4(10-11)91-25-4(24-25)91-25-4(41-42)91-25-4(60-62)91-25-4(64-66)91-25-4(98-100)91-25-4(108-110)91-25-4(129-130)
85-2-1(124-126.5)85-2-1(136-138.5)85-2-1(146.5-149)85-2-2(4-6.5)85-2-2(13-15.5)85-5-1(125-127)85-5-2(125-127)
87-1-2(75-76)87-1-2(79.5-82)87-1-2(85-87.5)87-1-2(95-97.5)87-1-2(105-107.5)87-1-2(111.5-112.5)87-1-2(114-116.5)87-1-2(122-124.5)87-1-2(133-135)87-1-2(142.5-145)87-l(CC)
90-10-1(46-49)90-10-490-12(CC)90-13-1(51-53)90-13-3(91-93)90-13-3(111-113)90-13-3(129-131)90-13-3(145-147)90-13-4(11-12)90-13-4(35-40)90-13-4(51-52)90-13-4(57-58)90-13-4(82-84)90-13-4(108-109)
96-1-6(135.5-138)96-1-6(139-141)
0.08-
0.840.48—-_-—_0.050.592.79
12.6719.7531.89
0.55—
0.050.810.13_
-—__-
0.36_
0.09—
0.04___—
0.160.10_
—_
0.13_1.250.29——_
0.12_Tr0.65_
—
0.701.872.862.680.120.060.07Tr
0.903.45
20.0131.4636.8145.6159.3765.62
1.380.144.014.271.57
0.07Tr0.070.122.20.057.25
0.090.330.061.121.020.170.260.130.132.521.76_
0.040.872.009.06
30.4310.98
1.081.183.068.713.144.85
10.13
Tr-
4.649.70
13.1810.2616.412.239.817.21
29.9942.8864.0670.4271.1382.1986.8976.76
4.130.86
19.8818.8445.95
0.651.714.821.869.970.92
35.60
1.777.380.967.216.570.516.481.581.227.957.87
0.050.30
12.0812.5935.9852.8834.7817.5021.4124.6633.4825.8916.9757.65
0.170.10
17.0923.1425.8022.3060.6522.7739.4243.3766.5377.4288.3088.4088.7291.7891.6281.0118.735.09
60.2862.2583.09
2.988.71
25.2012.1137.3517.8471.50
5.2232.14
3.0422.6020.39
2.9223.3713.114.50
18.6721.11
0.601.32
34.0630.2556.1367.4555.0044.2546.1546.2059.0549.3936.7278.46
0.670.45
40.9949.7144.0743.8379.9259.1576.3875.0084.8289.9393.9392.4091.5593.4492.9484.6840.01
9.3181.0880.8790.45
5.8115.9340.1823.6654.8968.4488.34
8.5050.42
7.2236.1933.72
3.9537.6429.26
8.9928.4934.34
2.263.72
50.6543.9067.2175.7165.6358.4260.2858.8772.8164.0150.2385.00
1.572.03
58.7068.1358.5861.3085.0476.088.1485.4890.4393.0595.0993.3392.5494.0693.8486.8056.8214.9087.8486.7592.46
8.2120.8546.9729.8862.1287.0693.26
10.5357.7512.5444.5742.03
5.1546.9940.5312.2734.9842.17
4.616.77
59.2549.9772.7580.0071.4666.7467.4565.8780.5871.7258.0387.94
3.205.20
66.7470.6565.5564.2286.7381.4789.5289.3392.4194.2295.3493.6892.9694.4694.0287.6164.0522.9290.3088.8993.09
9.9523.4550.9134.1065.6091.9794.87
11.4261.6116.8449.7846.56
5.4952.0647.1214.5839.3746.29
6.579.07
63.4253.3076.2182.5074.6472.0071.6370.1284.3875.2462.8889.32
5.008.52
76.7277.2573.7870.7289.2988.6292.3792.4093.9295.1195.8794.2793.8795.2694.9788.5975.0042.4193.0091.6194.22
12.2728.2956.2539.8870.4195.2396.32
13.1066.6425.7356.3253.31
8.5860.0156.6718.7546.1052.05
11.9313.8169.8557.4380.7785.7179.1578.2577.1976.0188.3680.9868.7191.39
9.5515.51
83.5384.6281.4578.4791.5093.1494.3594.7195.2495.7796.5594.9694.8995.9596.3592.4584.7871.9994.8493.9795.16
15.8335.1564.1349.3276.9196.7496.89
14.9672.9345.7366.0562.9918.1872.1467.7028.9056.1761.93
27.5226.5278.2863.1686.7089.1884.4685.1784.6982.7992.6688.4077.0894.01
19.2028.84
87.4787.7286.4783.3692.995.295.3395.8296.1896.6197.1395.8095.8896.9296.9594.7990.3687.2595.9995.2296.04
21.2844.5373.7861.3784.2097.6697.36
17.5279.5172.7877.7473.8348.2085.3180.7551.2570.8672.19
51.2347.0787.6669.8992.4492.9290.0691.7591.1188.9796.4594.7685.6297.28
36.6147.97
2.632.522.602.601.812.392.132.101.651.390.950.700.600.38
-0.03-0.41
2.653.371.841.801.31
» 5~5
2.903.812.322.351.59
» 52.473.892.963.14.552.903.054.483.433.18
4.474.652.482.761.751.101.802.282.122.131.732.022.481.08
-5.04.58
14.514.016.219.0
8.05.95.14.94.13.83.14.84.93.94.47.0
12.921.0
4.75.54.4
82.561.532.046.520.0
2.93.0
84.224.545.529.533.573.522.527.564.038.534
64661834.511.09.2
13.012.012.714.5
5.89.0
19.54.8
76.065.0
Silty, fine sdSilty, fine sdSilty, fine sdSilty, fine sd, sli. gravellyMed. sd, sli. gravellyFine sdFine sdFine sdMedium sdMedium sdSli. gravelly, crse sdSli. gravelly, crse sdSli. gravelly, crse sdGravelly crse sdGravelly, v. crse sdV. crse sdy gravelSli. gravelly, silty fine sdSilty, v.f. sdSli. gravelly, med. sdSli. gravelly, med. sdSli. gravelly, med. sd
Sdy silt (?)Sdy siltSilty, fine sdSilty, v.f. sdSilty, fine sdFine sdSli. gravelly, med. sd
Sdy silt (?)Sli. gravelly, silty fine sdSilty, v.f. sdSilty, fine sdSli. gravelly, silty v.f. sdSdy, crse siltSilty, fine sdSilty, v.f. sdSdy crse siltSli. gravelly, silty v.f. sdSli. gravelly, silty v.f. sd
Sdy, crse siltSdy, crse siltSilty, fine sdSli. gravelly, silty fine sdSilty, med. sdSli. gravelly, med. sdSli. gravelly, med. sdSilty, fine sdSilty, fine sdSilty, fine sdSli. gravelly, med. sdFine sdSilty, fine sdSli. gravelly, med. sd
Sdy crse siltSdy, crse silt
σ
m>-zHO
roo<
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
TABLE IBSieve Analysis, DSDP Leg 1
Sample
1-5-2(93-94)3-2-1(73-74)3-2-1(83-84)3-2-1(93-94)3-2-1(102-103)3-2-1(113-114)3-2-1(123-124)3-2-1(133-134)3-2-1(143-144)3-2-2(3-4)3-2-2(12-14)3-2-2(23-24)3-2-2(33-34)
1.250
_0.040.140.270.050.0170.010.030.090.160.38
2.750
0.13_—
0.8615.2
1.813.73
16.8530.842.1537.4539.423.0
Per Cent Coarser than
3.00
0.18—-1.28
23.42.87.11
28.441.750.5546.0546.829.0
3.250
0.32
1.9426.65
6.7217.1643.953.760.056.156.237.6
3.750
0.69_
0.374.61
35.0528.7544.064.168.573.270.670.052.6
4.50
8.6Tr0.48
23.846.168.274.681.882.086.383.882.168.0
Md0
-5.2V.Hi
Hi5.14.724.123.873.393.172.983.063.063.66
% FinerThan
40
98.8~IOO.O
99.691.561.057.044.029.526.021.024.025.541.5
TexturalClassification
SiltClayMudSiltSandySandySilty,Silty,Silty,Silty,Silty,Silty,Silty,
siltsiltv.f. sdv.f. sdv.f. sdv.f. sdv.f. sdv.f. sdv.f. sd
Φ
MUD
91/1/2-3
V
SILT
SDYCRSESILT
SDY SILT
SDYCRSE SILT
MUD
V7T
MASSIVE'?DISTURBED
MASSIVE
3.OI
4.O S.OII i
5O% 1OO%
100
1500
- 5 0
THAN 40
-100
150
Figure 4. Textural grading in Upper Pleistocene sediments from Hole 91.
704
SEDIMENTOLOGY
Φ
87/1/2
MUD
SANDY SILT\_SU.GRAV..SLTY FINE SD<J
SILTVV.F.S~
2.O 3.O 4.O 5.O
I I I I I I I
SILTYj^lNESD—r-SLL6RAV-SLTY VF. S D — ~
S0\, CRSE SILT~~^-SILTY,FINE S D — ~
SILTY V E S D - üSDY.CRSE SILT—
SLI. GRAV. SLTY.~VF SD *—
BASE NOT OBSERVED
SILTY MED. SO
SLI.GRAV.MED. SO
SILTY FINE SD T — —
SLI.GRAV.MEO.SOFINE SO
SILTY FINE SD
SILTY V.F. SD —SLI. GRAV..MEDSD 1 ^ i ^ ^ | /
BASE NOT OBSERVED
Figure 5. Textural grading in Miocene sediments from Holes 87 and 90.
of Mexico basin were responsible for delivering coarse-grained sediments to the then existent shelf edge whereinitiation of transport out onto the abyssal plain tookplace.
4) The presence of possible reverse grading at the baseof graded beds suggests that reverse grading, as well aseroded (missing) tops of beds, is not an indicator of prox-imity to source. Although beyond the scope of this paper,the writers suggest that the nature of the suspension orturbidity current is responsible for such developments. Itcan be speculated that very high suspended sediment con-centrations are responsible for the effect.
Textural data is lacking for most consolidated carbon-ate sediments. The limited information gained from thinsection petrography will be covered in the next section,along with less common sediment types such as pebblymudstone and cherts.
MINERALOGY AND PETROLOGY OFSEDIMENTS
The mineral composition of the sediments was deter-mined utilizing data from (1) shipboard study of smearslides, (2) qualitative and quantitative onshore study ofselected smear slides by Laury, (3) qualitative study ofcoarse fraction residues by Dickinson, (4) petrographicstudy of selected chert samples by Laury and carbonatesby Pusey and Beall, (5) carbon-carbonate analyses carriedout by Deep Sea Drilling Project staff, and (6) X-ray
diffraction analyses. Plates 1-4 are photomicrographs oftypical sediments recovered.
Common Minerals:
Mineralogically, the sediments are dominated by cal-cite, quartz, and clay minerals. Dolomite, and a relativelysmall number of common terrigenous detrital componentsprovide most of the remaining variation in composition.X-ray diffraction data show a moderately strong inversecorrelation between total carbonate content and total claycontent (Figure 10). As previously discussed, pelagic sedi-ments are strongly biased toward the high carbonate (highcalcite) end of the spectrum, whereas abyssal plain sedi-ments such as laminites and turbidites are enriched in clayminerals.
Clay mineral suites are dominated by detrital "mica,"undoubtedly varying between "illite" in the finer-grainedsediments and commonly biotite in the coarser-grainedturbidite sands on the abyssal plain. Subsidiary clay min-erals include montmorillonite, kaolinite, and chlorite indecreasing abundance. Montmorillonite appears to bemore common in the northern Gulf (at Sites 92 and 91),and locally abundant in bentonitic layers at various locali-ties throughout the area sampled. Whereas the mont-morillonite has both a detrital and a volcanic source, theother clay minerals are apparently detrital in origin.
705
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
Feldspar, represented by both potassium feldspar andplagioclase, is a common constituent of most sedimentsdominated by terrigenous detritus. Figure 6 illustrates aternary plot of relative percentages of quartz, potassiumfeldspar, and plagioclase, as determined by X-ray diffrac-tion. For purposes of discussion, samples have beencategorized as (1) Pleistocene laminites and turbidites, (2)Miocene-Pliocene laminites and turbidites, and (3) allpelagic oozes and chalks. The latter group is not repre-sented if none of the three components was detected byX-ray diffraction. Leg I data have also been included (Rex,1969, p. 353-357).
As shown in Figure 6, pelagic sediments tend to scatterthroughout the diagram, in part a reflection of spurioussmall percentages of one or two components. Quartz ismost common in this sediment type with quartz and plagi-oclase mixtures reflecting the general composition of ter-
rigenous sediments, that is, low in potassium feldspar.Potassium feldspar is not common in any of the sedimenttypes. Note that in some forty samples, quartz was theonly mineral of the three reported.
The terrigenous-dominated sediments can be consid-ered to represent two overlapping compositional popula-tions. Pleistocene (and to some extent Pliocene) sedimentstend to be enriched in quartz and potassium feldspar, ascompared to the remaining Pliocene and Miocene lami-nites and turbidites, which are comparatively plagioclaserich and low in potassium feldspar. These data are takenas suggestive of two distinct terrigenous provenances, theMiocene provenance being enriched in plagioclase feld-spar.
As discussed in the Site Summaries for Sites 87/3, 90,and 91, there is considerable evidence of at least two majorterrigenous source areas represented by abyssal plain
QUARTZ
^ 4 0 SAMPLES
- PLEIST. LAM/TURB.
+ - PLIO-MIOCENE LAM/TURB.
π-ALL PELAGIC OOZES
Relative Percentages of Quartz,Potassium Feldspar, andPlagioclase for
LEGS 10 and 1, DSDP,GULF OF MEXICO SITES
X-RAY ANALYSIS, THIS REPORT
k-FELDSPAR
Figure 6. Ternary plot of quartz, plagioclase feldspar, and potash feldspar as determined by x-ray.
PLAGIOCLASE
706
SEDIMENTOLOGY
deposits at those locales. In addition to differentiation onthe basis of feldspar and quartz, complementary minera-logical distinctions can be summarized as follows:
Pleistocene terrigenous sediments1) High quartz content.2) K-feldspar and plagioclase equal.3) High montmorillonite content.4) Common detrital dolomite.5) Relatively mature heavy mineral suite.6) Low carbonate and volcanic rock fragment content.Miocene terrigenous sediments1) Intermediate to low quartz content.2) Enriched in plagioclase.3) Low montmorillonite content.4) Less common detrital dolomite.5) Less mature heavy suite.6) Relatively high carbonate and volcanic rock frag-
ment content.Figure 7 illustrates thickness relationships of the abys-
sal plain on a cross section from Site 90 on the western riseto a position on the abyssal plain at Site 85, near theCampeche Scarp. Easterly thinning of the Miocene sectionis a clear indication of contribution of detritus from thenorthwestern Gulf of Mexico (refer to Worzel and Bryant,this volume, for related discussions). The thick Pleisto-cene section, on the other hand, thickens toward the northand northeast, while thinning towards the northwest. Thisis taken as an indication of a primary Pleistocene sourceof detritus to the north and northeast.
As noted in discussions for Sites 90 and 91, there isconsiderable evidence, in terms of sediment color and seis-mic wedging, for suggesting a complex interaction of sedi-ment contribution from the Mississippi Fan, the northern
slope complex, and the northwestern (Rio Grande) sloperegions (Figure 1). Although some sampling bias is un-doubtedly involved, it is readily demonstrated that Pleis-tocene turbidites are considerably finer grained than theMiocene turbidites where both intervals have been sam-pled. The very coarse Miocene sands represent the coars-est-grained terrigenous detritus yet recovered from thedeep-water Gulf of Mexico area.
Carbonate minerals, primarily calcite, are apparentlyof biogenic origin. As noted previously, pelagic compo-nents of oozes or chalks consist largely of nannofossils andplanktonic foraminifera tests. Less common componentsare siliceous radiolarian tests, unidentifiable biogenic car-bonate fragments, and trace amounts of terrigenous de-bris. Volcanic glass and volcanic rock fragments arelocally important at most sites. Aragonite, as detected byX-ray diffraction and smear slide study, was noted inshallow cores at Sites 85, 86, 93, 94, 95, and 97. Aragoniteoccurrences, other than inferring disappearance withdepth, are difficult to summarize.
As a product of carbon-carbonate analysis, drilling re-sults can be summarized and interpreted in terms of twodominant trends, as shown by Figure 8. Each hole hasbeen summarized in terms of a mean carbon versus car-bonate average percentage, with the exception of Site 96,where pelagic sediments are overlain by terrigenous abys-sal plain sediments. The overall mean percentages of car-bon and carbonate, including Sites 1 through 3 of Leg 1,are 0.365 and 43.5 respectively. Abyssal plain sites arecharacteristically low in carbonate and high in carbonwhereas pelagic-dominated sites are the inverse. This re-flects the dominantly terrigenous source of most of thecarbon, primarily from terrigenous plants, as suggested by
SIGSBEE DEEPABYSSAL PLAIN-GOLF OF MEXICO
BEALL, 4/7O
20 Miles(Statute)
NORTHMEXICAN
SLOPE
YUCATANPLATFORM
1700
2100
2200
2300 .
2400 •
2500
2600
2900
100 Meters
Figure 7. Thickness and fades relationships ofdeepwater sediments from the southwestern Gulf of Mexico.
707
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
100
90
96(l)
70
60
50
CaC03
40
30
20
-95
-97 ORGANIC CARBON
VS.
CARBONATE,
GULF OF MEXICO HOLES, DSDP
.^87(1)
ORGANIC C
Figure 8. Carbon-carbonate percentages.
coarse fraction study which revealed abundant spores,carbonaceous debris, and woody fragments.
Site 97 occupies a somewhat anomalous position on theplot, an apparent result of a distinctively different deposi-tional setting. Here the Tertiary section contains consider-able terrigenous material (hemipelagic), possiblyfurnished in part by turbidity from the nearby abyssalplain. The underlying Cenomanian deep-water limestonesare also apparently enriched in carbonaceous debris, re-flecting possibly quiet, deep-water sedimentation withnearby sources of terrigenous carbon, but low amounts ofterrigenous clastic detritus. These carbonates will be dis-cussed in more detail in a subsequent section.
Less Common Mineral Occurrences
Volcanic glass and associated volcanic rock fragmentsare important constituents, at least locally, in most of thesites drilled. Ash is typically associated with zeolites, espe-
cially in Miocene and older sediments. Montmorillonite-rich bentonite or altered volcanic ash was infrequentlyencountered. The zeolite species was tentatively identifiedas phillipsite in smear slide description. X-ray diffractiondata combines zeolites under the heading of "clinoptilo-lite."
Palygorskite, on the basis of X-ray diffraction data, wasidentified in minor amounts in Sites 85, 86, 87, 88, 89, 90,and 91. Although these occurrences are difficult to interre-late, it appears that the source of palygorskite is terrige-nous and probably related to proximity to Mexico.Palygorskite was not reported in the northern Gulf ofMexico holes (Sites 1 and 92).
Lower Pleistocene sediments at Site 92 contain abun-dant siderite (?) occurring as well-sorted, silt-size spheru-lites. Although specific X-ray data are lacking, thespherulites consist of single crystals which have syntaxi-ally overgrown a rhombic nucleus. Although similarspherulites have been found in trace amounts in almost all
708
SEDIMENTOLOGY
holes drilled on Leg 10, and a detrital origin might bereferred, the large amount of siderite (?) present at Site 92suggests authigenic or diagenetic origin. X-ray data fromSite 1 of Leg 1 also show siderite in small amounts.
Heavy mineral suites, especially in the terrigenous-dominated sediments, are represented by a relatively di-verse assemblage of minerals. Detrital dolomite rhombs,often with abraded morphology, are common throughoutmost sites. Dolomite appears to be especially common atSites 92 and 1, and is thought to represent evidence of anorthern detrital source (see discussion by Beall andFischer, 1969).
Opaque minerals, generally pyrite, are common in mostsediments studied especially in pelagic sediments. In ter-rigenous-dominated sediments, abundant opaque heavyminerals appear to be in large part ilmenite or possiblyhematite.
Accessory heavy minerals include stable assemblages ofzircon, tourmaline, and rutile. Less stable components,especially common in Miocene abyssal plain sediments,include green and brown varieties of hornblende, biotite,and chlorite aggregates (metamorphic rock fragments ?).Sphene and apatite are apparently rare. From preliminaryinspection of heavy mineral assemblages, it appears thata quantitative study would be valuable in further discrimi-nation of provenance.
Petrology of Chert from Leg 10Chert was recovered from a number of sites on Leg 10,
and ranged in age from Cenomanian (Late Cretaceous)through early Oligocene. Sites 94, 95, 96, and 97, all in thesoutheastern Gulf of Mexico, contained chert in associa-tion with radiolarian-rich sediments. In view of these oc-currences from deep-water sedement, Laury prepared thinsections of most cherts sampled. Chert formation has, inall cases but one, taken place by late post-depositionalreplacement of deep-water, calcareous, foraminiferal nan-noplankton ooze. At each site the degree of silicificationof the carbonates appears to increase with increasing age,attaining a maximum in Eocene and Paleocene sediments.Depth of burial and/or depth of the overlying water massappear to be unrelated to the degree of silicification.
Age and lithologic composition of the host rock appearto be the major parameters controlling the formation ofchert. Based on the stratigraphic occurrence of chertrecovered on other Deep Sea Drilling Project legs to date,chert is not common in rocks of less than 10 to 15 millionyears of age. At the other end of the scale, the youngestcompletely recrystallized cherts obtained on Leg 10 (thathave undergone complete silicification and recrystallizedto microcrystalline alpha quartz) are Late Cretaceous(Campanian) in age. Thus, once silica replacement begins,it may take tens of millions of years for complete recrys-tallization. These figures are in agreement with otherworkers in this area of study (A.G. Fischer, personal com-munication, 1971).
In addition to composition, it is apparent that initialeffective porosity and permeability of the sediment is im-portant in controlling the pattern of replacement. Forexample, in extensively bioturbated chalks, silica replace-ment is extremely patchy and irregular (Plate 5, Figures
2 and 3). Silica fronts tend to follow burrow zones ratherthan proceeding uniformly. Where sediment laminae arepreserved, however, permeability is greatest parallel tostratification and silica fronts parallel those planes (Plate5, Figures 4-6).
The source of silica need not be from within the bed inwhich chert is formed. Dissolution of siliceous microor-ganisms (radiolaria, diatoms, silicoflagellates, sponge spi-cules, etc.) at depth and in appreciable quantity isthe apparent source of silica for most chert observed onLeg 10.
Silicification begins with the replacement of fine mi-critic matrix within foraminiferal tests by opaline silica.Chertification apparently proceeds in the following man-ner. Voids within the tests are quickly filled, either withopal-cristobalite or with fibrous, length-fast chalcedony.Opalization spreads to matrix carbonate outside the tests.Foram test walls are then replaced by chalcedony,whereas test-filling opal which has not been replaced bychalcedony becomes more ordered optically and crystal-lizes into a fine mosaic of alpha-cristobalite (Plate 5, Fig-ure 1). Ultimate maturity in silicification involvesconversion of all opal to cristobalite, and cristobalite tosubequant, microcrystalline alpha-quartz. Fibrous chal-cedony is stable and may persist in highly silicified rocks(Plate 5, Figures 4-6). Fossils may be preserved as ghostsor may be completely obliterated. Finely comminutedplant material and carbonaceous debris are apparentlyunaffected by silicification (Plate 5, Figures 4-6).
The increasing abundance of siliceous microorganismsand occurrence of chert within several stratigraphic hori-zons as drilling proceeded towards the southeastern Gulfof Mexico originally suggested that a silica gradient mightbe present in that region. Inasmuch as the source of thesilica can be thus interpreted as biogenic and pelagic, itappears logical that entry of abundant pelagic organismsvia the Yucatan and Florida straits, especially duringearly Tertiary time, would yield such a gradient.
Petrology of Limestones and Dolomites
Consolidated limestones and dolomites recovered onLeg 10 can be considered in two main categories: (1)shallow-water carbonates consisting of interbedded lime-stone and dolomite and (2) deep-water limestones, consist-ing of interbedded calcilutites, calcarenites, and pebblymudstones. Pusey and Beall prepared a number of thinsections of the above named rock types in order to moreclearly elucidate their composition and depositional his-tory. Most of the following description is taken fromPusey, using the carbonate terminology of Dunham.
SHALLOW-WATER CARBONATES
Shallow-water limestones and dolomites were recov-ered from three sites (86,94, and 95) on the outer Yucatanplatform and slope (Fig. 1). Most of the core recoveryconsisted of core catcher fragments, especially at Site 86.Site 94 Core 39, however, consisted of a relatively undis-turbed segment some 110 cm in length. This sequence waschosen for detailed petrographic investigation and is illus-trated in Figure 9.
709
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
CARBONATE PETROLOGYD.S.D.P. LEG 10, HOLE 94, CORE 39
y~.CORE CATCHER-
ROCK NAME CALCITE(DUNHAM'S CLASSIF.) VS DOLOMITE
(DIAGRAM-MATIC)
MOTTLED PELLETOIDALPACKSTONE
HOMOGENIZED MILIOLIDWACKESTONE
MOTTLED ORBITOLINIDPACKSTONE
MOTTLED MILIOLIDMUDSTONE
STROMATOLITICMUDSTONE-WACKESTONE
FORAMINIFERALGRAINSTONE
MILIOLID GRAINSTONE
TYPE GRAINS IN ORDEROF DECREASING ABUNDANCE
CALCITE
CALCITE
70%; PELLETOIDS, LITHOCLASTS,MILIOLIDS AND OTHER FORAMS,OSTRACODS, GASTROPODS.
15%; MILIOLIDS, OSTRACODS,PELECYPODS, ORBITOLINID FORAMS
70%; ORBITOLINID FORAMS, MILIOLIDS,LARGE PELECYPODS, RED ALGAR,OSTRACODS.
MILIOLIDS, SKELETAL DEBRIS
70%; MILIOLIDS, SMALL FORAMS,PELECYPOD
70%; MILIOLIDS, MOLLUSC FRAG.
MATRIX &POROSITY
MICROSPAR
MICRITE
MICRITE
MICRITE
SPAR CEMENT
5% SPAR25% 0
DEPOSITIONALENVIRONMENT
MARINE, SHALLOW SHELF,SLIGHTLY RESTRICTED
MARINE, SHALLOW SHELF: LOW ENERGY
MARINE, VERY SHALLOW SHELF
SUPRATIDAL MUDFLAT
MARINE, SHALLOW SHELF:MODERATE ENERGY
MARINE, VERY SHALLOW SHELF:MODERATE ENERGY
Figure 9. Sedimentologic and petrologic interpretation of shallow-water Cretaceous carbonates from Hole 94.
In general, the rocks consist of odorous, finely lami-nated lime mud overlying a highly leached section of verypale orange, slightly dolomitic grainstones, packstones,wackestones, and mudstones. Carbonate grains are char-acteristically miliolids, orbitolinid forams, pelecypods,gastropods, ostracods, pellets, and lithoclasts. The grain-stones are characteristically selectively leached with mol-dic porosity and vugs lined with spar infill and dolomite.The common presence of dolomitized, algal mat-typestromatolites with preserved desiccation cracks (Plate 6),and paleontologically determined shallow-water miliolidsin the carbonate sands and muds suggests a shallow-marine to supratidal environment of deposition. Puseystates further that a very shallow-marine environment issuggested by the micrite-rimmed skeletal grains, whichare probably evidence of algal-fungal boring. Miliolid-richcarbonate muds and sands are characteristic of restrictedshallow shelf deposits in the Lower Cretaceous (Edwardsand Glen Rose formations) of Texas and northern Mexico(Behrens, 1965, Griffith et al., 1969).
More highly dolomitized rocks from these sites aregenerally identifiable as grainstones or wackestones.Porosities may exceed 25 per cent in some cases. Thepresence of a mottled to possibly brecciated fabric suggests
Traces of dark black material associated with UpperCretaceous sediments, originally identified as "asphal-tite," was found to be low in organic carbon. Such zonesare now interpreted as iron-rich, residual soil horizons,considerable post-depositional leaching or alteration.Recognizable grains are similar to those described above,possibly a reflection of considerable surface solution ofcarbonate.
It is interesting to note that in the shallow-water UpperCretaceous limestones and dolomites no evidence of reef-building organisms was observed. On the basis of sedi-mentological parameters, carbonate petrography, andpaleontological interpretations, the depositional environ-ment appears to have been shallow shelf to supratidal forthe upper few tens of meters of Upper Cretaceous plat-form carbonates penetrated at Sites 86, 94, and 95. Fur-thermore, the cyclical nature of partial sequences studied(Figure 9) suggests that essentially intertidal conditionsprevailed to some extent. Pusey states that these miliolid-rich mudstones and wackestones are identical with re-stricted, shallow shelf deposits known from other portionsof the Gulf of Mexico. The only major exception appearsto be that echinoderm fragments are extremely rare at thedrilled sites which implies even greater restriction and
710
SEDIMENTOLOGY
isolation from normal marine circulation. One is left toconclude that reef and associated facies lie either seawardof these sites or are poorly developed.
The presence of rudistid reef facies appears assured byrudistid debris seaward of the Campeche Scarp (Site 85).Site 3 of Leg 1 also encountered rudistid fragments indeep-water turbidites of Plio-Pleistocene age. It thus ap-pears evident that reef facies either were, or are, presentnear the present margin of the Campeche Scarp. Follow-ing deposition of the shallow-water carbonates describedabove, the suggestion is that the carbonate platform wasthen emergent and quite possibly subjected to subaerialerosion. No transgressive phase of sedimentation accom-panied by reef development could be discerned at the sitesdrilled, although such could have been removed by ero-sion. Subsequent deposition appears to have been com-prised of relatively deep-water pelagic ooze, suggesting adrowning of the outer portions of the carbonate platform,essentially to present depths of water.
DEEP-WATER LIMESTONES
Deep-water limestones were recovered from Site 97 andconsisted of (carbonate) pebbly mudstone apparentlyoverlying interbedded calcilutite and calcisiltite, all ofEarly Cenomanian (Late Cretaceous) age. The pebblymudstones consist of abundant clasts of limestone of vari-ous colors and hardness, generally recrystallized (micrite)chalk clasts, porous dolomite clasts of probable EarlyCretaceous shelf origin, and mud clasts, often deformed,of slightly clayey to clayey calcilutite (sometimes similarto associated fine-grained interbeds of the sequence de-scribed). Paleontological study suggests that much of thedetritus is shallow water in origin, containing retran-sported shallow-water forams. Several different ages ofsediment are suggested on the basis of disaggregation ofindividual clasts. Present data suggest that (pelagic) slopesediments comprised an important, although not neces-sarily dominant, nearby source of clasts. The considerableamount of miliolid debris reported by McNeely (thisvolume) from these sediments would support a shelf-banksource.
The general occurrence of exotic clasts suspended in amatrix of mixed deep-water ooze-clay and "shallow-water" debris of variable size is common to deep-watersediments of many areas. The rather unique textural mixand fabric of pebbly mudstones have been generally as-cribed to a gravity-dominated process known as lowvelocity mass flow, or simply as gravity mud slides ofsubmarine origin. The association of these sediments withcalcilutite containing a deep-water fauna is suggestive ofnearby areas of high bathymetric relief, such as exist atpresent.
The underlying hard, dense, horizontally to vaguelylaminated to sparsely mottled calcisiltites and calcilutiteswhich occur in Cores 11 and 12 are more difficult tointerpret. On the basis of Pusey's petrographic work, itappears that the dominant particle types contained withinthese sediments are fine-grained detrital carbonate andplanktonic foraminifera tests, with rare, unbroken, thin-walled pelecypod shells, ostracod shells, and miliolids.Microscopically, laminae consist of fossil-rich micrite al-
ternating with micrite. The micrite contains abundantparticulate organic material up to 10 microns in size, con-sisting of translucent, brown globules. Lithologically,these limestones resemble similar lithologies from themid-Paleozoic of the Ouachita fold belt of southeasternOklahoma and west Texas, which are interpreted vari-ously as moderate to deep water in origin.
The association of these rock types with carbonaceousorganic-rich, radiolarian (?), microlaminated chert similarto various deep-water cherts of open ocean origin fromLeg 1 (Beall and Fischer, 1969), and with interbedded,vaguely laminated, clayey, nannofossil-rich calcilutite ofprobable deep-water origin suggests that the dense lime-stones are lithified deep-water carbonate analogues of ter-rigenous clastic hemilaminites and laminites. Thepresence of deep-water limestones during a period whencorrelative shallow-water carbonates were undoubtedlyaccumulating on the nearby carbonate platforms isstrongly suggestive that the area of Site 97 occupied arelatively deep-water position during Early Cretaceoustime. This has important implications as to the structuralevolution of the Gulf of Mexico in Cretaceous time andearlier.
PHYSICAL PROPERTIES OF SEDIMENTS
Any discussion of physical properties of sedimentsnecessarily draws on other types of data such asmineralogy and textural composition. As used here, physi-cal properties refer primarily to those measurements ofphysical parameters taken on board ship and include (1)bulk density as determined by GRAPE (Gamma-Ray At-tenuation Porosity Evaluation), (2) natural gamma-rayemission, measured as counts per 75 seconds, (3) intermit-tent penetrometer tests which utilize measurement ofdepth of penetration of a weighted needle into sediment,(4) intermittent measurement of sonic velocity utilizing a"sediment velocimeter," (5) laboratory determination ofwater content, bulk density, and grain density of selectedsamples, and (6) laboratory determination of pore watersalinity, as derived from measurement of selected, artifi-cially consolidated samples. As will be reviewed in asubsequent section, chromatographic analysis of "naturalgas" samples from selected cores was also carried out andmight be termed a "physical property."
Continuous measurement of gamma-ray emission anddensity can be used to differentiate lithology on a detailedbasis as well as demonstrating broad-scale variations incomposition and degree of consolidation. Beall andFischer (1969) have shown, in the case of Leg 1 sediments,examples of detailed differentiation of various lithologies.Enough of this type of data has now been obtained for thedeep-water Gulf of Mexico to show statistically valid re-sults.
Natural Gamma-Ray
Figure 10 compares average core barrel results of natu-ral gamma-ray count versus per cent total carbonate, astaken from x-ray diffraction analysis. It should be pointedout that individual mineralogic samples are not neces-sarily representative for average composition of an entire
711
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
4 500
4GCC
Y-RAYCOUNTSX BBL)
3500
3000
2500
2000
*
-
-
/
DOMINANTLY. * . . . . . LAMINITES/TURBIDITES
- # .
* /
/
/
i i i i i i
/
/
/
DOMINANTLYPELAGITES
• *
• ••MOST CHALKS
I i i
M
:iPERCENT TOTAL CARBONATE (X-RAY)
Figure 10. Comparison of natural gamma-ray count versuspercent total carbonate (from x-ray diffraction).
core. Nevertheless, a pronounced inverse correlation be-tween carbonate content and gamma-ray count can benoted. As shown on the figure, most of the samples of highgamma-ray count can be characterized as (terrigenousclastic-dominated) laminite or turbidite sequences,whereas the high carbonate sediments are characteristi-cally pelagites. Also, note that most chalks result in plotswith very low gamma-ray counts. Alternatively, a plot ofgamma-ray count versus clay content would also show amarked correlation of similar degree. Zones of high gam-ma-ray emission and with high zeolite or volcanic glasscontent can also be cited as responsible for some of thevariability shown.
Where core sampling is somewhat closely spaced, natu-ral gamma-ray data can be used to characterize verticalsequences of rock, much in the same way that conven-tional gamma-ray surveys are used in subsurface geology.Figure 11 shows a cross section along the outer Yucatanplatform (Sites 86, 94, and 95) where paleontological agedata have been rather uniquely substantiated by gamma-ray correlation. In this case, it is readily recognized thatcorrelative zones persist throughout the region. Post-Oligocene and pre-Upper Cretaceous units are generallymore radioactive as compared to the remainder of thelower Tertiary. The missing upper Tertiary section at Site95 is clearly shown by gamma-ray data. Increasing claycontent toward Site 86 is thought to be responsible forincreasing gamma-ray count in correlative beds, especiallyas seen in Pliocene and Paleocene intervals.
Other examples of gamma-ray correlation can also becited. Tentative gamma-ray correlations were made be-tween Site 91 and Site 87/3. In view of the different systemused to date Site 3, a gamma-ray correlation would sug-gest that Cores 5 and 6 (330) from Site 3 are approxi-mately equivalent, stratigraphically, to the base ofPleistocene sediments as described at Site 91 between
Cores 8 and 9 (517). Further paleontological study of Site3 will be necessary to confirm this correlation. As shownin Figure 7, such a correlation is important in resolvingthe stratigraphic position of the Campeche-Yucatan car-bonate turbidite wedge, as seen at Site 87/3. This wedgeof abyssal plain sediment does not appear to be present interms of a carbonate-dominated lithology at Site 91.Penetrometer
A penetrometer measurement basically defines the abil-ity of a sharp, weighted needle to penetrate sediment. Suchmeasurements, calibrated in thousandths of a millimeter,appear to adequately define the state of consolidation ofsediments, although they are difficult to relate to conven-tional engineering measurements in quantitative terms.For purposes of comparison, penetrometer measurementscorrespond well with geological observations as to state ofconsolidation and are reproducible within a small experi-mental error. An empirical scale of consolidation has beenassigned to penetrometer data for the Gulf of Mexico(Beall and Fischer, 1969). Readings between 0 and 10( × 10 3 mm) are designated as stone, 10 to 30 are semi-consolidated, and sediments with penetration greater than30 are unconsolidated.
Figure 12 shows a summary plot of penetrometer re-sults for Leg 10 (including data from Leg 1). For purposesof comparison, data have been grouped into those catego-ries previously used for comparing mineralogic data.Those categories are (1) slope (pelagic) sediments, (2)abyssal plain or rise sediments, (3) sediments accumulatedon salt structures, and (4) the Straits of Florida site. Notethat the abyssal plain and rise muds are the least con-solidated as compared to the less penetrable carbonatesediments. Penetrometer tests in the pelagic carbonatesediments show the appearance of "chalk" at about 400meters (an average depth for the sites studied).
It is interesting to note that the "salt dome sediments,"which reflect an essentially pelagic mode of deposition forthe most part, are more consolidated than the "normal"pelagic sediments. These sediments appear to be "over-consolidated" with respect to "normally consolidated"sediments of similar lithological composition. This subjectwill be continued under the discussion of bulk densitydata.
The Straits of Florida site is somewhat unique, both insedimentological setting-sediment type and in terms ofpenetrometer results. Here the presence of "soft" chalkwas first noted at a depth of 200 meters below the sedi-ment-water interface, demonstrating a strong dependenceof a "chalk" lithology on composition. Sediment abovethis level is also somewhat "over-consolidated," suggest-ing the possibility that either slow rates of deposition and-/or concurrent intermittent erosion are somehowresponsible. Deep-water limestones and associated pebblymudstones at the base of Site 97 are generally quite con-solidated, with the exception of thin interbeds of pelagicooze, which may represent disturbance during coring.
Bulk Density
Bulk density data are categorized in the same way asthe penetrometer data (Figure 13). Abyssal plain sedi-ments, all of post-Miocene age and largely comprised of
712
HOLE 86Y-RAY
HOLE 94Y-RAY
1000 2000 3000
HOLE 95Y-RAY
1000 2000 3000 SEAFL00R— 0
DEPTHBELOW SEAFLOOR
IN METERS
LOW (<2000 COUNTS) GAMMA SEDIMENT FACIES
HIGH (>2000 COUNTS) GAMMA SEDIMENT FACIES
u> Figure 1 1 . Comparison of paleontological and gamma-ray correlation along the outer Yucatan platform.
gisHHOoa-
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
PENETROMETER(PENETRATION XI0~3MM) 1.4
6MS/CC DENSITY (BULK)1.8 2.0 2.2 2.4
20 40 300
α.
α
SLOPE PELAGIC)SEDIMENTS
STRAIT OF FLORIDA SITE
— SALT DOME SEDIMENTS
ABYSSAL PLAIN/RISESEDIMENTS
600
700
800
Figure 12. Summary plot of penetrometer results versussedimentological assemblages used in this paper; Legs 1and 10 combined for deepwater Gulf of Mexicosediments.
terrigenous clastic mud/clay, define a somewhat broadband of bulk densities with increasing depth. Much of thevariation noted is apparently due to lithologic variability,e.g. carbonate-bearing units versus terrigenous clay-bear-ing units versus quartzose silt/sand-bearing units, but nev-ertheless a definite data field is apparent. Site 1 essentiallydefines the upper limit of bulk density in the field whereasSite 89 defines the lower boundary.
Abyssal plain and rise sediments can be compared withsediments recovered from the crest of salt structures,where considerably denser sediments are apparent. Onesuch example is from Site 2 (Leg 1), where a thin sectionof Plio-Pleistocene clay and clayey ooze directly overliescaprock. Lithological composition appears to be of onlysecondary importance in such cases, however, for sedi-ments from Site 92, with a similar bulk density trend, is
CO
100
200
300
400
500
600
700
800
2.6—r~
2.8
SALT DOMES E D I M E N T S — —(POST MIOCENE)
SLOPEOOZE/CHALK(POST K)
OFF-SHORELA. SHELF(POSTOUGOCENE)
HOLE 97STRAIT OF FLA.(POST ALBIAN)
ABYSSALPLAINSEDIMENTS(POST L. _MIOCENE)
Figure 13. Summary plot of bulk density versus sedimen-tological assemblages used in this paper; Legs 1 and 10combined for deepwater Gulf of Mexico sediments.
comprised almost totally of terrigenous clastic sediments.These data, in conjunction with data from at Site 88,suggest again that sediment from the crests of salt struc-tures exhibit "over-consolidation" with respect to othersediment categories and present depth of burial.
The slope ooze and chalk are characterized by some-what low bulk densities even in the consolidated chalkintervals. The high porosities noted in this type of sedi-ment are apparently related to the high percentage ofbiogenic debris present, notably foraminiferal tests andnannofossil debris. At depth, the older chalks and oozesterminate abruptly at approximately the Early-LateCretaceous boundary. Sediments below this point arehard, recrystallized, moderately dense limestones anddolomites. Bulk densities were not determined on thismaterial but are known to be high from observation andrelatively low porosity values as determined from thinsection data.
Site 97, from the Straits of Florida, again representssomewhat of an anomaly with respect to other sites stud-ied. The sequence of relatively low density pelagic oozesand chalks superimposed on rather dense pebbly mud-stones and limestones with interbedded minor chert isapparent. The dense carbonate rock fragments present inthe pebbly mudstones are apparently responsible, at leastin part, for the high bulk density readings reached in thislower interval.
Consolidation and Pore-water Composition
Interpretations as to consolidation of clastic muds andclays from the abyssal plain can be made by combining
714
SEDIMENTOLOGY
bulk density and penetrometer data. Note that the changefrom rapid consolidation with depth (or low bulk densitiesand high penetrometer values) to lesser rates of consolida-tion with depth (or high bulk densities and low penetrom-eter values) takes place at approximately 400 meters(about 1250 feet) (see site summaries, Sites 90 and 91).These changes take place in what has previously beendescribed as "Stage I" consolidation or simple expulsionof pore water. As such, these sediments have not beenburied to great enough depths to experience the onset ofNaCl filtration. Such is subtantiated by shipboard salinitydetermination of rapidly squeezed pore fluids. Manheim(1969) has shown, on the basis of most Deep Sea DrillingProject sediment fluid extractions to date, that NaCl filtra-tion does not appear to be operative at the shallow depthscored.
Examination of sediments from the crests of salt struc-tures (Sites 2, 92, and 88) suggests that salt diffusion fromunderlying salt can be common in deep-water sediments.These sediments, as previously discussed, generally appearto show anomalously high bulk densities at shallow burialdepths along with an "over-consolidated" appearance asdocumented by penetrometer measurements.
Site 92 had been previously suggested to be underlainby salt, as based on seismic profiling (Beall, 1968). Figure14 shows a detailed summary of Site 92, including general-ized lithology as well as bulk density, penetrometer values,and pore water salinity as determined by the shipboardchemist. Note that the sequence can be separated into anupper unit of unconsolidated to semi-consolidated UpperPleistocene clay and mud which overlies a lower sectionof semi-consolidated to consolidated clay mud and mud-stone claystone of Early Pleistocene age. The top of thesalt (or caprock) was calculated to be at a depth of approx-imately 300 meters, based on seismic profiling.
Physical measurements at Site 92 show a relativelyconsistent and rapid increase in consolidation with depth.There is a somewhat more rapid increase in bulk densityat about 200 meters, which suggests that there may besome missing section. Sediment below that level alsoshows annealed vertical fractures, especially toward thebase of the hole. Salinity determinations were carried outon cored sediment using the techniques of Manheim et al.(1969). Results clearly show a consistent increase ofsalinity with depth, reaching a level of about five timesnormal seawater at a depth of approximately 200 metersbelow the sediment-water interface. This rapid and pro-nounced increase in salinity is apparently due to diffusionof salt through the capping sediments. The mudstoneswhich overlie salt are of deep-water origin and very youngin age, thus removing from consideration effects whichshould be ascribed to age, subaerial exposure, or invasionvia permeable sandstones of older age, as is often the casein shelf and onshore settings. The results cited above arequite comparable to those reported by Manheim and Bis-choff (1969) for sediments on the upper continental slopeof offshore Texas.
These results clearly illustrate the ability of consolidat-ing clays and muds to transmit hypersaline solutions. Fur-thermore, it would appear that this process of diffusionsomehow facilitates "over-consolidation" of sediments. It
SITE 92SIGSBEE SCARP
BULK DENSITYGMS/CC
Iβ 1.9 2.0 2.1 2 2CHOLOCENE FORAM OOZE
LATE PLEISTOCENEFINELY LAMINATED
TO MASSIVE CLAY/MUD[HEMI-LAMINITE]
0 20 40 60 80 100PENETROMETER
EARLY PLEISTOCENEMASSIVE TO LAMINATEDTO BURROWED NANNO-
BEARING CLAY/MUa[ HEMI-PELA6ITE—HEMI-LAMINITE]
ANNEALED VERTICALFEATURES IN LOWERCLAYSTONE/MUDSTONE
APPROXIMATE POSITIONOF TOP OF SALT*
'BASED ON GEOPHYSICAL DATA
Figure 14. Lithology and physical and chemical properties,Site 92.
may be that because of saturation of grain boundary bond-ing sites, mechanical rotation of particles is facilitated.Diagenetic cementation as related to possibly higher ther-mal gradients on salt structures might also be cited. Thedata cited above, in conjunction with these speculations,are yet another example of how data on deep-water sedi-ments may help to provide answers to puzzling geologicalphenomena in shallow-water sequences where a consider-ably greater range of variables is involved.
Natural Gas
Odors of natural gas were detected in a number of coresearly in Leg 10, suggesting that continuous monitoring offree gas from within the core liner would be of signifi-cance. Site 88, as well as later sites, showed measurablequantities of methane, and minor amounts of ethane andhydrogen sulfide. Volumes of methane and ethane appearto reach a maximum at levels of approximately 100 me-ters, declining downwards. Hydrogen sulfide gas appearsto be most abundant in very shallow cores.
The presence of natural gas is intriguing, in that at leasttwo origins can be hypothesized. The first hypothesismight be that these hydrocarbons (methane and ethane)represent simple diffusion from the underlying caprockthrough the low permeability ooze or clay up to the sedi-ment surface. In such a case, however, one might expecta continuous increase in the amount of hydrocarbon gasdetected as well as a continuous increase in the proportionof heavier hydrocarbons. Such does not appear to be thecase. The second hypothesis, and the one favored here, is
715
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
that the natural gases represent the generation of in situnatural gas through organogenic processes, probably bac-terial. Evidence of reducing conditions is taken from thegreenish gray to gray pigmentation of the sediment as wellas the presence of fecal and FeS material throughout. Thedecline of ethane as well as the decline of total gas withdepth suggests that generation may also be inhibited atgreater depths of burial (with lesser permeability andporosity). The total volume of natural gas in these sedi-ments must be small in comparison with the volume ofsediment and fluid while the relatively low permeability ofenclosing sediment suggests that any vertical diffusion ofhydrocarbons is a slow process. We have thus concludedthat the gas in this case, is biogenic in origin.
REFERENCESBeall, A.O. and Fischer, A.G., 1969. Sedimentology. In Ewing,
W.M., Worzel, J.L. et al., 1969. Initial Reports of the DeepSea Drilling Project, Volume I. Washington (U.S. Govern-ment Printing Office). 521.
Beall, Arthur O., Jr., 1968. Deep sea drilling project throws newlight on Gulf sedimentation. Oil and Gas Jour., Dec. 1, 1969.94.
Beall, A.O., Jr., 1970. Textural differentiation within the finesand grade. J. Geol. 78, 77.
Behrens, E.W., 1965. Environment reconstruction for a part ofthe Glen Rose Limestone, central Texas. Sedimentology. 4,65.
Griffith, L.S., Pitcher, M.G. and Rice, G.W., 1969. Quantitativeenvironmental analysis of a Lower Cretaceous reef complex.Soc. Econ. Paleontologists and Mineralogists, Spec. Pub. 14.120.
Manheim, F.T. and Bischoff, J.L., 1969. Geochemistry of porewaters from Shell Oil Co. drill holes on the continental slopeof the Gulf of Mexico. In Geochemistry of subsurface brines(Symposium). E.E. Angino and G.K. Billings (Eds.). Chem.Geol.
Manheim, F.T., Sayles, F.L. and Friedman, I., 1969. Interstitialwater studies on small core samples, Deep Sea DrillingProject, Leg I. In Ewing, W.M., Worzel, J.L. et al., 1969.Initial Reports of the Deep Sea Drilling Project, Volume I.Washington (U.S. Government Printing Office). 403.
716
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
PLATE 1
Figure 1 Sample 85-1-6 (3 cm); plain light. Late Pleistocene,zeolite crystals, one twinned (in center), in a quart-zose marl silt.
Figure 2 Sample 85-5-2 (80 cm); plain light. Late Pleistocene,calcareous clayey medium sand. Included in slideare an abraded calcite prism from a pelecypod shell(in center of plate), orthoclase feldspar (0), plagi-ocalse feldspar (P), chert grain (C), micritic intra-clasts (large, rounded to subrounded, opaquegrains), and quartz (most of the remaining grains).The smaller of the two plagioclase grains is badlyaltered. Following figure from same slide.
Figure 3 Sample 85-5-2 (80 cm); plain light. Late Pleistocene,detrital calcite(C), plagioclase with rutile inclusions(P), microcline(M), green hornblende(H), and a sil-icic volcanic rock fragment in a calcareous turbiditesand.
Figure 4 Sample 85A-2(CC); plain light. Late Pleistocene,dolomite rhombs in terrigenous clay rich in mace-rated plant detritus.
Figures 5, 6 Sample 86-2-1 (65 cm); crossed-polarized (Figure 5)and plain (Figure 6) light. Early Pleistocene, angularglass shard (center), faecal pellet (P), single forami-nifer chamber (F), calcitic molluscan fragment (M),and larger coccoliths (C), in nannoplankton chalkooze. Clay-size detrital carbonate is abundant (Fig-ure 6).
Figure 7 Sample 86-3-2 (10 cm); plain light. Middle to LatePliocene, foram-bearing nannoplankton chalk ooze.Foraminifera, except for one on right, are mostlyfragmented. Six-rayed discoasters are common.Larger coccoliths are oval-shaped. Zeolite crystal inupper right (Z).
Figure 8 Sample 86-5-2 (71 cm); plain light. Late Oligocene,volcanic glass(g) present in coccolith chalk ooze.Triaxon sponge spicule and silt-size radiolarian frag-ments can be seen.
Figures 9, 10 Sample 86-7-5 (2 cm); plain (Figure 9) and crossed-polarized (Figure 10) light. Late Paleocene, calciticpentaliths of the coccolith Braarudosphaera (P) andfragments thereof are abundant consituents alongwith other coccoliths. Also present are euhedralcrystals of zeolites (Z) and dolomite (D). The sedi-ment is a nannoplankton chalk ooze.
Figure 11 Sample 87-1-1 (136 cm); plain light. Late Miocene,large, sub-angular, brown biotite grain in a marlsandy silt. A few silt-size zeolites (Z) are visible.
Figure 12 Sample 87-1-2 (14 cm); plain light. Large, fresh,green hornblende grain in a marl sandy silt (LateMiocene).
718
SEDIMENTOLOGY
PLATE 1
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719
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
PLATE 2
Figure 1 Sample 88-1-1 (135 cm); crossed-polarized light.Late Pleistocene, nannoplankton chalk ooze withlarge Carlsbad-twinned K-feldspar grain andsmaller chert grain (C). Detrital (probably biogenic)carbonate grains, highly birefringent, equant tospindle-shaped, are common in the 10-15 micronsize range.
Figure 2 Sample 88-2-5 (80 cm); plain light plus reflectedlight. Late Pleistocene, spherulitic pyrite filling twochambers of globigerinid foraminiferal test. Sedi-ment is a nannoplankton chalk ooze with fragmentsof foraminifera and, less frequently, pteropods (p).
Figure 3 Sample 89-1-2 (97 cm); plain light. Late Pleistocene,pumice (very vesicular) (center and upper right) andabundant other angular, glassy ash particles in ashynannoplankton chalk ooze.
Figures 4, 5 Sample 89-4-4 (75 cm); plain (Figure 4) and crossed-polarized (Figure 5) light. Middle Pliocene, un-twinned sanidine (S) and volcanic glass shard (G) innannoplankton chalk ooze. Discoasters and coc-coliths present; the latter dominate.
Figure 6 Sample 89-4-6 (111 cm); plain light. Middle Plio-cene, clear, brown, and very iron-rich (essentiallyopaque) volcanic glass shards. Angularity is high,sorting very poor; median diameter probably in fine-sand range.
Figure 7 Sample 90-1-4 (102 cm); plain light. Late Pleisto-cene, angular and frequently grooved glass shards inash-bearing nannoplankton chalk clay. Fragmentedor entire foraminifera (f) also present. Aggregates ofvery fine carbonate (p) are probably faecal pellets.
Figure 8 Sample 90-5-4 (100 cm); plain light. Late Miocene,terrigenous clay with common euhedral, silt-size py-rite; bladed, clear zeolite crystals (commonlytwinned); and occasional green glauconite pellets(G)
Figure 9 Sample 90-10-1 (50 cm); plain light. Late MiddleMiocene, quartzose marl sandy silt, poorly sortedand with an unstable grain assemblage includinggreen hornblende (h), plagioclase (P), garnet (G),rutile (R), zircon (Z), micritic clasts (m), maceratedplant matter (C), in addition to quartz, other feld-spar, detrital carbonate, rock fragments, and others.
Figure 10 Sample 90-12 (CC); plain light. Middle Miocene,brown biotite (b), quartz (Q), and abundant zeolites(commonly euhedral, 6-sided, subequant to bladed)in zeolitic, calcareous, sandy mud. Following figurefrom same slide.
Figure 11 Sample 90-12 (CC); crossed-polarized light. MiddleMiocene, fresh, normally zoned plagioclase crystal,found in a zeolite-bearing, calcareous sandy mud.
Figure 12 Sample 91-20-2 (80 cm); plain light. Middle Mio-cene, rounded sand grain in center is of volcanicglass (isotropic, negative relief); conchoidal chip inupper part of grain was proabably incurred duringturbidity flow transit. Orthoclase (O) and an uniden-tified volcanic rock fragment (V) are also present inthe field.
720
SEDIMENTOLOGY
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A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
PLATE 3Thin Sections
Figure 1 Sample 90-13-4 (35 cm); plain light. Middle Mio-cene, large, fresh, basaltic rock fragment with plagi-oclase laths, from turbidite sand.
Figure 2 Sample 90-13-4 (35 cm); crossed-polarized light.Middle Miocene, rock fragment of weakly metamor-phosed (chlorite grade), sandy quartzose siltstone.From poorly sorted, unstable, turbidite sand.
Figure 3 Sample 90-13-4 (35 cm); crossed-polarized light.Middle Miocene, exceedingly fresh, fairly calcicplagioclase grain from the turbidite sand.
Figure 4 Sample 90-13-4 (35 cm); plain light. Middle Mio-cene, large, subrounded pelecypod fragment show-ing preservation of crossed-lamellar microstructure(probably aragonitic) and extensive marginal boring(B) by endolithic blue green algae. Borings havebeen subsequently infilled with micritic carbonate,producing a micritic envelope around the grain. Nu-merous borings are seen to penetrate more deeplyinto the fragment. Algal borings indicate derivationof the mollusc fragment from shallow water, withinthe photic zone.
Figures 5, 6 Sample 90-13-4 (35 cm); plain (Figure 5) andcrossed-polarized (Figure 6) light. Middle Miocene,large spherulitic volcanic rock fragment in textur-ally very immature and compositionally very unsta-ble turbidite sand. Rock fragment, acidic tointermediate in composition, is relatively fresh.
722
PLATE 3
SEDIMENTOLOGY
723
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
PLATE 4
Figures 1, 2 Sample 92-9 (CC); plain (Figure 1) and crossed-polarized (Figure 2) light. Single-crystals, spheru-litic, of carbonate, probably siderite. These crystalsare also present in the terrigenous clay of sample92-7-1 (123 cm). Early Pleistocene.
Figure 3 Sample 94-1 -2 (80 cm); plain light. Late Pleistocene,aragonitic didemnid tunicate spicules (T), calciticmolluscan prism (M), foraminifera, glass shard (G),faecal (?) pellets (P), and a siliceous sponge spicule(S) in a foram-bearing nannoplankton chalk ooze.
Figure 4 Sample 94-11-6 (5 cm); plain light. Late Oligocene,opaline sponge spicules (S), diatom frustule (D), vol-canic glass (G), and foraminifera (F) in calcareousnannonplankton organic ooze.
Figure 5 Sample 94-35-1 (147 cm); crossed-polarized light.Late Paleocene, subhedral, zoned sanidine crystal inmontmorillonitic clay. The sanidine is apparentlythe remnant of a volcanic ash, the glass of which hasaltered to montmorillonite.
Figure 6 Sample 94-38 (CC); crossed-polarized light. Creta-ceous, Late Albian (?), detrital, silt-size carbonate(calcite) of unknown derivation. Possibly producedby breakdown (through bioturbation?) of pentalithcoccoliths? Relatively well sorted chalk mud. Couldalso be the product of shallow-water calcareous al-gol productivity. Considered a shallow-water sedi-ment.
Figure 7 Sample 95-5-4 (42 cm); plain light. Early Oligocene,predominantly foram-nannoplankton chalk ooze.Foraminifera commonly obscured by coating of veryfine coccoliths or discoasters. Siliceous sponge spi-cules (one antler-shaped), and large glass shard (G)also visible.
Figure 8 Sample 95-8-6 (100 cm); plain light. Middle Eocene,spicule-bearing foram nannoplankton radiolarianmarl ooze. Radiolaria, commonly coated with nan-noplankton and fragmented clay and fine-silt-sizeskeletal debris, are dominant in this picture. Forami-nifera and straight sponge spicules also visible.
Figure 9 Sample 95-12-1 (106 cm); plain light. Early Paleo-cene, small bladed zeolite crystals and larger, sube-quant volcanic glass in zeolite-bearing, ashy,calcareous sandy mud. The glass has a general cor-roded appearance and appears to be altering to clay.
Figure 10 Sample 95-13-3 (85 cm); crossed-polarized light.Cretaceous, late Campanian, spindle-shaped calcitegrains of unknown derivation in chalk mud. Grainsaverage 2-4 microns by 10-40 microns. Fair sorting.
Figure 11 Sample 96-3-5 (85 cm); plain light. Early Eocene,ash-bearing organic marl ooze, with a mixture ofradiolaria (R), silicoflagellates (S), volcanic shards(G), foraminifera (F), and calcareous coccoliths (C).
Figure 12 Sample 96-5-1 (20 cm); plain light. Late Paleocene,radiolarian fragments (R) and even some of the zeo-lite crystals (Z) in this slide appear to be corroded.Primarily a coccolith chalk ooze.
724
SEDIMENTOLOGY
PLATE 4
o µ. 50
725
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
PLATE 5
Thin Sections
Figure 1 Sample 96-4-1; plain light. Late Paleocene, chertreplacement of carbonate ooze. Globigerinid forami-nifer in matrix of opal-alpha cristobalite (o-a).Foram wall structure fairly well preserved eventhough replaced by chalcedony. Chalcedony (c) alsofills light-colored areas of chambers. Foram cham-bers are coated with a very fine mosaic of alphacristobalite (a), has large negative relief relative tochalcedony and has replaced opal. Light blotchesoutside the foram are recrystallized radiolaria and-/or sponge spicules.
Figures 2, 3 Sample 97-6-1 (40 cm); plain (Figure 2) and crossed-polarized (Figure 3) light. Late Cretaceous,Cenomanian, patchy, irregular silica front in foram-nannoplankton chalk. Silicified portion of rock is inupper half. Opal-cristobalite-rich matrix in silicifiedzone is still largely isotropic; hence its dark appear-ance in the cross-polarized photograph (B). Porosityin unsilicified zone is largely in form of unfilledforam chambers. Opal-cristobalite and sometimeschalcedony have filled these pores and destroyedporosity in silicified area. Patchy replacement prob-ably due to bioturbation of original chalk ooze.
Figure 4 Sample 97-12-1, 337 m (?) crossed-polarized light.Late Cretaceous, Cenomanian, light colored chert inlower half of picture changes upward, abruptly intoa dark, carbonaceous zone which marks the leadingedge of the silica front. The contact between thesilica front and the unaltered chalk is relativelysharp and parallel to internal sediment stratification.Radiolaria, foraminifera, and sponge spicules,shown as ghosts in the black organic-rich zone, havebeen completely replaced or recrystallized to alphaquartz. Many of the siliceous microorganisms in thechalk have been replaced by calcite.
Figures 5, 6 Sample 97-12-1, 333 m (?); crossed-polarized light.Late Cretaceous, Cenomanian, intensive silicifica-tion of thinly laminated chalk. Except for discon-tinuous organic-rich (carbonaceous) laminae andoccasional ghosts of foraminifera and radiolaria, thechert is a mosaic of microcrystalline alpha quartz.Foraminifera, especially the larger ones (B), arecommonly infilled with fibrous chalcedony (forminga thick layer near the outer chamber wall) andmosaic, subsequent alpha quartz (in the center of theforam). Internal wall structure of the foram has beendestroyed.
726
PLATE 5
SEDIMENTOLOGY
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727
A. O. BEALL, JR., R. LAURY, K. DICKSON, W. C. PUSEY, III
PLATE 6
Plain light.Figure 1 10-94-39 (31-47 cm); pelletoidal packstone (shallow
marine shelf). Grains are pelletoids (a) lithoclasts(b), miliolids (c), ostracods and mollusks. Microsparmud matrix is half altered to dolomite in 10 to 50micron rhombs. Mollusks are selectively leached,bulk composition of whole rock (by X-ray) is 71 percent calcite and 29 per cent dolomite.
Figure 2 10-94-39 (65-71 cm); Plain light. Miliolid wacke-stone (low energy, semi-restricted marine shelf).Grains are miliolids (a), ostracods (b), orbitolinidforams (c), and pelecypods. Matrix is micrite. Bulkcomposition is 100 per cent calcite.
Figure 3 10-94-39 (74-83 cm); plain light. Orbitolinid pack-stone (very shallow, restricted marine shelf). Grainsare large dicyclinid and orbitolinid forams (a), mili-olids, pelecypods, red algae, ostracods, and othersmall forams (b). Matrix consists of micrite withnumerous grain-sheltered voids (now spar-filled).
Figure 4 10-94-39 (74-83 cm); plain light. Red algal nodule.High magnification of coralline alga showing cellu-lar structure.
Figure 5 10-94-39 (112-117 cm); plain light. Stromatoliticmudstone (supraüdal mudflat). Laminae of miliolid-rich wackestone alternating with dark micrite lami-nae. Near-vertical desiccation cracks concentratedin specific layers and terminated abruptly by con-tinuous micrite layers. The miliolids have been pref-erentially leached relative to hyaline forams,mollusks, and ostracods. Bulk composition is 95 percent dolomite and 5 per cent calcite (the mollusksare calcite).
Figure 6 10-94-39 (139-146 cm); plain light. Miliolid grain-stone (shallow marine, moderate energy). Consists oflarge miliolids of many types, especially Num-moloculina (a), other small forams (b), and pelecy-pods. Spar matrix. Bulk composition is 100 per centcalcite.
Figure 7 10-94-40 (CC); crossed-polarized light. Miliolidgrainstone (shallow marine, moderate energy). Notedolomite rhombs as partial cement. Porosity is veryhigh (approximately 25%).
Figure 8 10-95-21 (134-142 cm); crossed-polarized light.Dolomite (? low energy). Variable rhomb sizes andiron staining suggest that the original rock was amottled mud or wackestone. No identifiable grainsobserved.
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SEDIMENTOLOGY
PLATE 6
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