151-18 new dynamic classification of lake systems and ...polsen/nbcp/olsen_gsa_poster_08_5... · i...
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151-18
MLDAWB
Richmond
PMD
MLD
PMDAWB
Lake Malawi
0
20
40
60
80
100
120
140
10
20
30
40
50
60
70
80 *
Paleosol
-1 0 1 2 3
*
*
**
* Shoreline ElevationConstraints
0 m -350m-100 -593m
5pt-Smoothed PC1wetter drierAGE
(ky) MBLF
0
5
10
15
20
25
30
35
40
MBLF
Density(gm/cm3)
1.0 2.2
0 6TOC %
Well-laminatedSilty Clay
Sparsely-laminatedsilty-clay
Grey, mottled,carbonate-richsilty clay
Sand
MAL 05-2AMAL 05-1CLake Malawi Synthetic
Stratigraphy
MLDAWB
MLD
AWB
MLD
AWB
Figure 9A: Wilkins Peak
PMD
SyntheticStratigraphy
MLDPMDAWB
Figure 10A: Laney 1
MLD
PMDAWB
Figure 10B: Laney 2
PMD
PMD
I propose a classification of lakes and their records based on three variables (Fig. 1, left):
MLD, maximum lake depth from floor to outlet.PMD, potential mean depth of the lake relative to MLD unconstrained by the outlet or lake floor; AWB, amplitude of water balance variability deviating from PMD
For illustrative purposes I have depicted AWB as a precession-related signal representing water input into the lake basin (with mpv being a time ofmaxiuum precessional variability), but it could have any form or not vary at all. If AWB = 0, this system yields the classification of Carroll &Bohacs (1999) lake types (1): “overfilled”, with PMD > MLD; “balanced fill”, with PMD = MLD; and “underfilled”, with PMD < MLD (Fig. 2, right).
However, AWB ≠ 0 in general, but fluctuates in frequency and amplitude, leading to a huge range of lake behavior and sediment sequence types,some examples of which are shown below. The first 3 examples are of early Mesozoic age and illustrate the classification with the three styles ofsequences seen in the rifts of Eastern North America (Olsen, 1990). Applications to a modern lake (Malawi) and the Eocene Green River Fm. showthe versatility of the method. This method has some similarities to that of Keighley et al. (2003).
New Dynamic Classification of Lake Systems and Their Geological RecordsPaul E. Olsen (LDEO, Palisades, NY: polsen@LDEO.columbia.edu)
MLD
AWB
mpv
PMD
height of outlet
lake floor
MLD
PPLLDDAWB = 0
Balanced Fill lake type of Carroll & Bohacs (1999)
PMD = MLD = outlet
MLD
AWB = 0 PPLLDD
PMD > MLD
Overfilled lake type of Carroll & Bohacs (1999)
MLDPPLLDDAWB = 0
PMD < MLD
Underfilled lake type of Carroll & Bohacs (1999)
Figure 1: Basic definitions (see right).Figure 2: Lake types of Carroll & Bohacs (1999) in terms of new classification (see left).
Md An Cms Bs Sy Ca
545.8
1m
Basalt
Md, mud; An, anhydrite; Cms, chaotic mudstone-salt;Bs, banded salt; Sy, Sylvite; Ca, Carnellite
Core AV-C-1-4Fundy basin, Nova Scotia
Core Pz-18Kehmisset basin, Morocco
AV-C-1-4 (730-780 ft) spanning basal North Moun-tain Basalt (base at arrow) and the uppermostBlomidon Fm. Core is youngest in upper right.
SyntheticStratigraphy
Figure 3: Fundy-type lacustrine sequences: A, (below), relationship between vari-ables; B, (left-top) salt dissolution autobreccia bowls, Fundy basin, Nova Scotia, Can.;C, (left-bottom) sand patch cycles, also from the Fundy basin.
Figure 3AFigure 3B
Figure 3C
49.62±0.10
49.79±0.09
49.96±0.08
50.39±0.13
50.61±0.26
50.83±0.13
51.30±0.30
49.5
50.0
50.5
51.0
512
256
128
64 32 16 8 4 0 10 20 30 40
40Ar/39Ar AgesSmith et al.,
2008
Cycle AgeMachlus et al.,
2008PERIOD (ky) OIL YIELD (gal/ton)
El Paso 44-3
400 100 20 kySynthetic
Stratigraphy
SyntheticStratigraphy
In the case of Fundy, type sequences PMD was near or below the base of MLD, and deepening excursions of the AWB is less than the top of MLD. As a consequence, thelake and its record may be dominated by short periods of high-stands at mpv, and fre-quent periods of evaporite production, and long periods of eolian deposition on salineflats (sand-patch cycles: Fig. 3C). In many outcrops and at the basin margins the evap-orite layers may be represented by hiatuses or solution autobreccias (Fig. 3B). Exam-ples of the lateral transitions from mud-dominated cycles at the fringes of basins toevaporite-dominated sequences are common in the eastern North America and Moroc-co. These closed saline basins, because of their nearly continuously negative waterbalance can receive seepage from brines of marine origin if they are available.
Figure 3D
Figure 4: Cores of Fundy-type sequences below the oldest basalt in theFundy and Kehmisset basins. Marginal facies of the salt-rich deep parts of thelatter resemble the former.Newark
Fundy
Figure 5: Newark-type sequences: A, (above-right),relationship between variables; B,~ 6 m thick cycles(black to black) tracking climatic precession in theLate Triassic, lower Lockatong Fm, Newark basin,PA; C, ~11 m cycles, (black to black layers, track-ing precession), Early Jurassic East Berlin Fm.,Hartford basin, CT. Black layers are organic-richand microlaminated with whole fish.
Figure 5A
Figure 5B Figure 5C
In Newark-type sequences PMD was slightly above the base of MLD, and deep excursions of the AWB higher than the top of MLD during mpv (Fig. 5A). Deepestlakes occur at mpv flushing solutes out, but then drying up. Extreme dry times do notleave a distinct record compared to times of low precessional variability, and can becharacterized by non-deposition or deflation. High eccentricity (mpv) leave a recordof bundles of high-TOC cycles separated by low-TOC cycles. Dispersed evaporitested to occur in high amplitude cycles that do not reach the top of MLD (Fig. 5D).
Figure 7B
Figure 6: Cores of Newark-type sequences from the Newark Basin. Left,Lockatong Fm. (1620-1644 ft, Nursury no. 1); lake-level (precession) cycle isfrom black to black layer. Right, Passaic Fm. (808-835 ft, Somerset no. 1);lake-level cycle is banded silts (~815.5 ft) to banded silts (~823 ft) .
PMD was well above the base of MLD, in Rich-mond-type sequences, with deepening excur-sions of the AWB higher than the top of MLDand lower than MLD at times of mpv (Fig. 7A,left). In this mode, both deepest lake and shal-lowest lake sequences occur at mpv, with thelake rarely drying out. If the deepest-water unitsare similar in facies to the mean facies, it is easyto get the phase of the eccentricity cycle wrongrelative to Newark-type sequences.
Figure 5D
Figure 7: (Right) Richmond-type sequences: A,(above-right), relationship between variables; B,exposure of lower Vinita Fm., Richmond basinwith one cycle of microlaminated black shaleand gray sandstone; C, Vago no. 1 core showingalternating better laminted and more poorly lam-inated intervals, all with high TOC (>2%).
Figure 7A
Figure 7C
Figure 8: Lake Malawi and its sedimentary record: A, model showing relationship ofthe classification variables; B, synthetic stratigraphy derived from A; C, principle com-ponent 1 (PC1) of various biotic and physical properties of core MAL 05-1C that areproxies of climate and hence water depth (modified from Cohen et al., 2007) correlatedto the synthetic stratigraphy (B); D, core MAL 05-2A from a more up-dip position, withan unconformity corresponding to one of the shallower excursions in C and showingexample of sedimentary features of the core (modified from Scholz, et al., 2007).
(Left) In terms of this new classification, Lake Malawi is characterized by having PMD above the top of MLD and an AWB that is larger than MDL. The dry excursions of AWL, infrequently pass below the base of MLD (Fig. 8A). Thus despite a large amplitude climate forcing, because PMD is above the top of MLD, the lake is usually hydrologically open. However, during times of high mpv, there are large lake level drops, resulting in erosion shoreward and carbon-ate deposition, and rare exposure and paleosols develop-ment, basinward (Fig. 8C). As a consequence the sedimen-tary record consists mostly of deeper-water deposits, punctuated by shallow water excursions (Fig. 8B-8D). These shallow-water excursions are the megadroughts of Scholz et al. (2007) and Cohen et al. (2007).
In many ways, the Malawi sequence resembles the Rich-mond-type sequences, except that Malawi is even more per-sistently deep as indicated by PMD being above the top of MLD.
A counter intuitive (at first) consequence of this relation-ship is that the intervals of the core with the most indica-tions of aridity and major drops in lake level, are possiblealso those that experienced some the the most extremehumid events, uncoupled however from increases in lakedepth.
The Green River Fm. arguably constitutes the most famous lacustrine deposit inthe world. The Wilkins Peak Mb. of the formation in Wyoming has in addition tooil shale, cyclicaly disposed (Fig. 9C), significant beds of evaporites (e.g., trona)and many clear surfaces of exposure. The overlying Laney Mb, has a much higherconcentration of oil shale and few or no surfaces of exposure. The sequence isprofoundly cyclical and astronomically tuned records of oil shale yield (Machluset al., 2008) produce a chronology remarkably close to 40Ar/39Ar chronology ofSmith et al. (2008) (Fig. 9C).
In this classification the Wilkins Peak has PMD very close to the base of MLD with wet excursions of AWB never reaching MLD (Fig. 9A), and hence, there isno solute flushing.
The new classification suggest two ways the transition from Wilkins Peak to Laney may have occurred. One is by effectively lowering the outlet (Fig. 10A). The other is by raising PMD above the top of MLD (outlet) by either climate change or watershed capture (Fig. 10B). The two mechanisms suggest rather dif-ferent synthetic stratigraphies. Which is a better fit to reality?
Blacks Fork no. 11 m
538-553
Figure 9: Wilkins Peak Mb. of the Green River Fm.: A, rela-tionship of the variables; B, Wilkins Peak overlain by LaneyMb.; C. Synthetic stratigraphy; D, Wavelet Spectrum of theWilkins Peak from Machlus et al. (2008: cycle age at 0 onMachlus, reassigned a value corresponding to the uppermostdate of Smith) with the ages of Smith et al. (2008) superim-posed; E, portion of Blacks Fork no. 1 core, showing alterna-tions of high (1) and low (2) TOC interals.
SyntheticStratigraphy
SyntheticStratigraphy
References: Carroll AR & Bohacs KM. 1999. Geology 27:99; Cohen AS et al. 2007. PNAS 104:16422-16427; Keighley D. et al. 2003. Jour. Sed. Res. 73(6):987-1006; Olsen PE. 1990, in Katz B (ed), LacustrineBasin Exploration, AAPG Memoir 50:209-224; Machlus ML. et al. 2008. EPSL 268:64-75; Scholz CA et al.2007. PNAS 104(42):16416-16421; Smith ME et al. 2008 Nature Geoscience 1(6):370-374.
My Thanks to Malka Machlus for Fig. 9E and Mohammed Et-Touhami for Fig. 4(right).
Figure 9B
Figure 9CFigure 9D Figure 9E
Red mudstone
Gray mudstone
Dark Graymudstone
Black mudstone
Range of mudstonecolors with evaporitecrystals
Bedded evaporites
Key to SyntheticStratigraphies
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