Geology

doi: 10.1130/0091-7613(1986)14<577:DBOCSA>2.0.CO;2 1986;14;577-580Geology

 Jeffrey Ross Levine Sedimentation or tectonics?Deep burial of coal-bearing strata, Anthracite region, Pennsylvania:  

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Deep burial of coal-bearing strata, Anthracite region, Pennsylvania: Sedimentation or tectonics?

Jeffrey Ross Levine* Pennsylvania State University, University Park, Pennsylvania 16802

ABSTRACT Recent studies of vitrinite reflectance, sedimentary compaction, and fluid inclusions

indicate that coal-bearing strata of the Pennsylvania Anthacite region were once buried at depths ranging from 6 to as much as 9 km. Vitrinite reflectance anisotropy suggests that anthracite rank was attained by normal geothermal heating largely prior to and during Alleghanian folding. Paleomagnetic ages for the folding and paleobotanies) ages of the youngest extant sediments delimit the maximum burial and deformation to a relatively short interval (285 to 270 Ma). In comparison with typical molasse sequences, the implied thicknesses and rates of deposition are excessive, suggesting that much of the former overburden was emplaced tectonically or by extremely rapid sedimentation.

The total implied thickness (about 20 km) of Paleozoic strata in the region is abnormally large for an epicontinental basin. An inferred body of mafic rock embedded in the Precambrian basement, evidenced by the Scranton gravity high (SGH), may have influenced crustal subsidence, especially during the Alleghany orogeny, and produced a large downwarp accounting for the high coal rank. The fact that isorank contours, Paleozoic sediment isopach contours, and a large structural sag all lie roughly parallel to the boundaries of the SGH, oblique to the principal Alleghanian fold trend, lends support to this idea.

INTRODUCTION Erosion has removed most stratigraphic evi-

dence of the late Paleozoic Alleghany orogeny in the central Appalachians, but recent studies of diagenesis, paleomagnetism, and gravimetry have provided new, indirect evidence bearing on former overburden thicknesses and the se-quence of tectonic events. The strata compris-ing and surrounding the Pennsylvania Anthracite fields are the youngest pre-Alleghanian rocks preserved in eastern Penn-sylvania (Fig. 1), remnants of a multistage molasse sequence that accumulated in the Appalachian foreland basin throughout the late Paleozoic. Molasse sedimentation commenced with the Catskill deltaic sequence, shed from an Acadian source terrane situated to the southeast (Edmunds et al., 1979), followed by a period of diminished sedimentation around the middle Mississippian. A renewed influx of coarse-grained clastics occurred in the late Mississip-pian to Middle Pennsylvanian; the influx represented an incomplete coarsening-upward then fining-upward megasequence, consisting of the upper member of the Mauch Chunk For-mation, the conglomeratic Pottsville Formation, and the coal-rich Llewellyn Formation (Wood et al., 1969). It is problematical whether this second influx reflects renewed (incipient Al-leghanian?) tectonic activity or merely a change

to more humid climatic conditions affecting an Acadian source terrane.

Present-day coal rank and sedimentary com-paction indicate that a large thickness of addi-tional strata formerly overlay the existing section. It is rare that large regions of coal-

bearing molasse sediments are metamorphosed to anthracite rank as is the case in the Pennsyl-vania Anthracite fields. The high temperatures required to accomplish this might have been due to exceptionally large depths of burial or to unusually high geothermal gradients. Dam-berger (1974) proposed that elevated geother-mal gradients arose from deeply buried Mesozoic intrusives, but there is almost no cor-roborative evidence for their existence. More-over, the unusual occurrence of high-rank coal in the synclinal cores of the Western Middle Anthracite and Southern Anthracite fields, cited as evidence for magmatic heating, may also be explained by structural disruption of the re-gional rank pattern (Levine and Davis, 1982).

In contrast, sedimentary compaction, vitri-nite reflectance gradients, and fluid inclusion studies all suggest normal paleogeothermal gradients and large burial depths in the Anthracite region. The negligible porosity of Pennsylvanian-age sandstones, which micro-scopic features indicate was a consequence of pretectonic vertical loading, requires former

PENNSYL VANIA

KEY MAP A N T H R A C I T E D E P R E S S I O N

i [2

/ / A D '

/

R e g i o n s u n d e r l a i n by C o r b o n i f e r o u s c o a l - b e a r i n g r o c k s

A l l e g h e n i a n f o l d a x e s , showing d i rect -ion of p l u n g e

A n t h r a c i t e D e p r e s s i o n

Bouguer contours of t h e S c r a n t o n G r a v i t y H i g h , in m i l l i g a l

*Present address: School of Mines and Energy De-velopment, University of Alabama, Drawer AY, University, Alabama 35405. K I L O M E T E R S

Figure 1. Tectonic ele-ments of Anthracite re-gion, Pennsylvania (after Gwinn, 1964; Rodgers, 1970; Lavin, unpub.; Berg, 1980).

G E O L O G Y , v. 14, p. 5 7 7 - 5 8 0 , July 1986 577

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overburden thicknesses of at least 5 - 7 km, and possibly as much as 9 km (Paxton, 1983). The vertical vitrinite reflectance gradient from two boreholes in the Southern Anthracite field indi-cates a paleogeothermal gradient of about 33 °C/km (Levine, 1983), which is typical for a foreland basin (cf. Hitchon, 1984) and consist-ent with other estimates from the Allegheny Plateau. On the basis of commonly accepted coalification models, temperatures on the order of 200 to 250 °C are interpreted for the west-ern part of the Anthracite region, and they imply burial depths similar to those estimated from sandstone compaction. Methane-bearing fluid inclusions from the western end of the Western Middle Anthracite field entrapped dur-ing the Alleghanian tectonism indicate maxi-mum temperatures of about 205 °C and pressures of about 1150 bar (Orkan and Voight, 1983), consistent with the previous es-timates. If this represents lithostatic pressure gradient, Orkan and Voight estimate that the paleogeothermal gradient would not have been greater than 37 °C/km.

If a roughly constant geothermal gradient throughout the study region is assumed, the variations of coal rank would be primarily a function of former overburden thickness (Fig. 2), but the thickness of strata implied on this basis is unusually large. For example, in the re-gion of the Southern Anthracite field, the 7 km of former overburden, together with about 5 km of preserved upper Paleozoic molasse (Wood et al„ 1969), and the roughly 8 km of underlying shelf sediments (Berg, 1980), gi ve a

O 2 0 4 0 6 0 8 0 mi

cumulative Paleozoic section approaching 20 km. Such sediment thicknesses are not unusual at continental margins where they are deposited on oceanic or attenuated continental crust (Kinsman, 1975), but the Anthracite region is thought to have been part of the stable conti-nental margin of North America. Moreover, the proposed thickness of molasse sediments iar ex-ceeds that of typical molasse-type sequences, which are in the 5-6-km range at most (Spencer, 1974; Beaumont, 1981). Timing constraints introduce additional problems.

TIMING OF DEPOSITION A N D ALLEGHANIAN D E F O R M A T I O N

The age and sequence of events in the burial and tectonic history of the Anthracite region can be interpreted from optical reflectance an-isotropy of vitrinite, paleomagnetism, and pa-leobotany. Vitrinite anisotropy represents the cumulative molecular fabric imposed on coal by stresses ambient during coalification. The relative attitude of the vitrinite fabric and other structural elements can be used to interpret the relative timing of strain events and coalification (thermal history). Studies of vitrinite fabrics in coal from the Western Middle Anthracite field show the minimum principal reflectance axes to be scattered about a great circle, the pole to which is precisely coincident with the regional fold axis trend (Levine, 1983). When bedding is rotated back to horizontal, the minimum re-flectance axes converge near the vertical For the 26 samples analyzed, the "fold test" passes with 99% confidence, a result indicating that

Correlation between coal rank and former

thickness of overburden

Coal Rank (volatile matter, dry ash free)

Former Dopth of burial (km)=

Coal Rank (volatile matter, dry ash free) range= mean:

3 0 3 .0-4 .5 3 . 8 2 0 3 .5 -6 .0 5 . 0 1 O 5 . 8 - 7 0 6 . 5 5 7 . 0 - 9 . 0 7 . 8

the minimum reflectance axes become signifi-cantly less scattered when the beds are un-folded. This shows that maximum coal rank was attained before or during early stages of folding, and therefore required that the postu-lated overburden was also present at this stage. If heating of the coals (i.e., coalification) had taken place after, or had continued long after, cessation of folding, then a postfolding reflec-tance fabric would be expected rather than the prefolding to synfolding fabric observed. (Cf. Levine and Davis, 1984. The postfolding fab-rics observed in the Broad Top coal field in southern Pennsylvania apparently formed sub-sequent to Alleghanian folding, whereas fabrics in the Anthracite region were reoriented by folding. This is one of several indications of multiple folding episodes in the Anthracite region.)

A limiting age for Alleghanian folding in the Anthracite region has been derived from a sec-ondary (postdepositional) natural remanant magnetization (NRM) in the sediments (Van der Voo, 1979; Kent, 1979; Scotese et al., 1982; V. Schmidt, 1983, personal commun.). Unlike the vitrinite reflectance fabrics, the atti-tude of this secondary N R M is typically inde-pendent of bedding attitude, an indication that it is post-tectonic. The mean pole position indi-cates magnetization between Late Pennsylva-nian and Early Permian time (ca. 290 to 270 Ma); therefore, the folding would be earlier than this. Because the sediments are themselves of virtually the same age, there would be little time for accumulation of the proposed large thicknesses of overburden.

From paleobotanical evidence, the renewed clastic influx in the Anthracite region (Upper Mauch Chunk/Pottsville Formations) is thought to have begun near the Mississippian/ Pennsylvanian boundary, ca. 320 Ma (Wood et al., 1969, p. 70; Edmunds et al., 1979, p. 13-19; absolute age estimates from Palmer, 1984). The lower part of the Llewellyn Forma-tion is dated sis latest Middle Pennsylvanian or early Late Pennsylvanian (Read and Mamay, 1964; Oleksyshyn, 1982; ca. 296 Ma). In the Southern Anthracite field, the uppermost part of the Llewellyn Formation has been estimated as Stephanian C (Oleksyshyn, 1982; ca. 287 Ma).

A generous estimate of the maximum total thickness of Upper Mauch Chunk and younger strata in the Southern Anthracite field is about 1.5 km (Wood et al., 1969), representing less than 3 km of uncompacted original sediments. Using the above ages, the average rate of sedi-ment accumulation (3 k m / 3 3 m.y.) was less than 100 m/m.y. At this rate it would have re-quired at least 70 m.y. to produce an additional 7 km of overburden; however, the paleomag-netic ages suggest that no more than about 10-15 m.y. were available to emplace the miss-

EASTERN "MIDDLE ANTHRACITE

FIELD^

SOUTHERN ANTHRACITE

FIELD

Linos of «quill volati le mattor

COAL FIELDS OF (d o. f . ) PENNSYLVANIA tirreoulor interval)

Figure 2. Coal-rank patterns in eastern Pennsylvania have been interpreted as being a conse-quence of varying burial depths at constant, normal geothermal gradient. Therefore, isorank lines correspond to former depths of burial. Coal-rank contours modified after Edmunds et al. (1979) to show regional northwest to southeast rank increase, free of disruptive structural influ-ences within Western Middle Anthracite and Southern Anthracite fields. Depth estimates are based on compaction properties of clastic sediments (Paxton, 1983).

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ing overburden and elevate the coals to anthra-cite rank.

Two models could account for the proposed overburden and its rapid accumulation: (1) another, more massive influx of clastic sedi-ments into the foreland basin preceding or ac-companying Alleghanian deformation, or (2) development of a substantial portion of the overburden by tectonic processes—by either nappe emplacement or structural thickening by folding. Structural evidence for two phases of Alleghanian deformation in the Anthracite re-gion (Geiser and Engelder, 1983) suggests the possibility of an additional molasse cycle now removed by erosion, similar to the two mega-cycles seen in the Alps (Van Houten, 1974). However, even with a second sediment influx, for which there is no direct evidence, the sedi-mentation rates would be unusually high. Max-imum average preservation rates are estimated at 400 m/m.y. in the Alpine Molasse (Van Houten, 1974) and 330-500 m/m.y. in the sub-Himalayan Siwaliks (Gansser, 1964). For the Anthracite region (minimum 7 km of sedi-ments in maximum 10-15 m.y.), much higher sustained average rates of 470 to 700 m/m.y. are required. The problem is compounded by the return to drier climatic conditions during the Late Pennsylvanian through Permian (Phil-lips and Peppers, 1984) and the fining-upward nature of the Pottsville/Llewellyn sequence, which together indicate declining rates of sedi-mentation. This suggests that sedimentary proc-esses alone are inadequate to explain the thick, rapid molasse accumulation required.

Instead, overburden accumulation can be attributed to tectonic processes commencing during early stages of the Alleghany orogeny. Northwest-directed Alleghanian compression produced tectonic shortening of as much as 50% in the Anthracite region. At the strati-graphic level of the upper Paleozoic molasse, this shortening was accomplished primarily by flexural slip folding (Wood and Bergin, 1970), whereas lower Paleozoic and Precambrian rocks now exposed to the southeast in the Great Valley and Reading Prong (Fig. 1) are deformed into large-scale recumbent folds and thrust faults. Much of the thrust faulting in eastern Pennsylvania is Taconian in age (Root and MacLachlan, 1978), but overprinted Al-leghanian thrusts are observed at many locali-ties (Drake, 1980; Drake and Lyttle, 1980; MacLachlan, 1985). Overthrust sheets could have provided a virtually instantaneous increase in burial depth and may have formed prior to the latest stages of folding. Analogous nappe structures having estimated basinward velocities of 2 to 30 km/m.y. have evolved in many other foreland basins wherein proximal molasse has been overridden for distances up to several tens of kilometres (Hsu, 1969; Elliott, 1976; Clar, 1973; Van Houten, 1974; Stoneley,

1969). Large-scale overthrust sheets are also well documented in other parts of the Appala-chian orogen. Thrust sheets formerly overlying the Anthracite region could alternatively have been rooted in the Precambrian to lower Pa-leozoic rocks now exposed to the southeast in the Great Valley and Reading Prong (Fig. 1) or in the middle to upper Paleozoic strata that once overlay them. Existing structural evidence is not strong for a thrust sheet of sufficient magnitude to cover a large part of the Anthra-cite region, but even thrust sheets of more modest dimensions could have enhanced sedi-ment influx by providing a source terrane more proximal than the present Piedmont.

Alternatively, if some unknown thickness of overlying sedimentary cover were folded to the same tightness as the existing strata (i.e., greater than 30% shortening), the section would be-come thickened by as much as 50%. However, this model would not deal as effectively with the prefolding components of the vitrinite re-flectance and sandstone compaction fabrics, in-asmuch as the full overburden thickness would not have been emplaced until folding was completed.

An additional problem concerning em-placement of the overburden is the amount of time required for thermal response and re-equilibration of the system. Conductive heat flow from below is too slow to account for rapid heating of the coal-bearing section. Even conductive heating from the base of an incom-ing nappe (Oxburgh and Turcotte, 1974) is probably inadequate to produce a normal geothermal gradient within the time available. Fluid flow arising from compressive dewatering could, however, enhance the heating rate sev-eralfold and is a demonstrated mechanism of heat transfer in active foreland basins (Hitchon, 1984).

FORMATION OF THE ANTHRACITE DEEP BASIN

Regardless of how they were derived, the upper crustal thicknesses in the study area were abnormally large at the end of the Alleghany orogeny, an indication that some unusual tec-tonic controls acted here. Various other data indicate that the entire Pennsylvania Anthracite region behaved as a large-scale structural de-pression that was active throughout most of the Paleozoic: (1) Several lower Paleozoic units exhibit their maximum thickness along a northeast-trending axis in eastern Pennsylvania; they thin both to the east and west (Swartz, 1946; Root, 1973). (2) Contours of stratigraph-ic thickness and maximum pebble diameter for the Pottsville Formation (Meckel, 1967) exhibit maximum values along a northeast-trending axis centered in the west-central part of the Southern Anthracite field, and they diminish to the west, north, and east. Paleoflow directions

(Meckel, 1967) are slightly convergent on this axis, suggestive of a subsiding trough. (3) Re-gional coal-rank contours (Fig. 2) are aligned along a northeast-trending axis, interpreted here as indicative of former overburden thickness. (4) The overall outcrop pattern of the four An-thracite basins reflects a structural depression established during the Alleghany orogeny, the axis of which is oriented approximately N35°E (Wood et al., 1969; Rodgers, 1970, p. 67). This feature is nearly obscured by the superimposed east-northeast-trending Alleghanian folds and faults. Along the western margin of this depres-sion, the major Alleghanian fold trains devel-oped in the Silurian Tuscorora Formation plunge eastward and disappear beneath the younger upper Paleozoic rocks. Unpublished seismic sections indicate that this is accompa-nied by an eastward plunge of the basement.

The fundamental control of all these trends may be a basement feature underlying the western margin of the Anthracite region, mani-fested by a large, well-defined Bouguer grav-ity anomaly (Scranton gravity high: SGH), extending 400 km from Harrisburg, Pennsylva-nia, to Albany, New York, along an azimuth of approximately 045. The SGH has a maximum relief of about +70 mgal (Fig. 1) and is the most pronounced gravity feature of the Appa-lachian foreland. The only other occurrence of regionally metamorphosed anthracite in the Appalachian foreland basin, the Valley fields of southeastern Virginia, lies directly east of a sim-ilar foreland gravity anomaly, the Kentucky gravity high (KGH) (Haworth et al., 1980; Hawman, 1980). Although the KGH is neither as large nor as pronounced as the SGH, similar tectonic processes may have influenced both regions.

The SGH and KGH have been interpreted as caused by high-density mafic intrusions in the Precambrian basement, emplaced during the late Precambrian rifting of the Iapetus ocean (Rankin, 1976; Hawman, 1980). Loading of the crust by this rock mass may have produced abnormal flexural downwarp in the region (Hawman, 1980). This downwarp could have been augmented by (1) downward flexure by buckling arising from lateral tectonic compres-sion; (2) additional supracrustal loading by overthrusting and nappe emplacement (me-chanical models of fold-and-thrust belt evolu-tion [e.g., Beaumont, 1981] imply that a depression in the foreland would engender a greater than normal thickness of overthrust sheets); and (3) high-density phase transitions in the lower crust, proposed by Richardson and England (1979), which reflect the pressure-sensitive transition from granulite or amphibo-lite facies to eclogite facies (-10% change in volume) arising from sudden supracrustal load-ing. Because these supplementary mechanisms would not have become active until the Al-

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leghanian, nearly half of the total Paleozoic section was not deposited until this time. Additionally, all three mechanisms could ac-count for the post-tectonic removal of over-burden by (1) relief of elastic tectonic crustal flexure, (2) surficial erosion of the overlying sediments with isostatic uplift, and (3) lower crustal phase transitions ultimately being re-versed by post-tectonic thermal reequilibration. Process 3 might have followed the tectonism by several tens of millions of years (Oxburgh and Turcotte, 1974; Richardson and England, 1979).

CONCLUSIONS The late-stage sedimentary record of the Al-

leghany orogeny has been entirely removed by erosion in eastern Pennsylvania, but coal-rank patterns and sandstone compaction (Paxton, 1983) indicate that the paleogeothermal gra-dient was near normal in the region (about 33 °C/km) and that as much as 6 to 9 km of overburden formerly overlay the existing sec-tion. Optical anisotropy of the coal indicates that most of the coalification preceded the ces-sation of folding of the strata. Hence, maximum burial and maximum heating were roughly contemporaneous with the folding. Paleobotan-ical and paleomagnetic age dating delimit the late Alleghanian sedimentation and deforma-tion to a 10-15-m.y. interval (ca. 285-270 Ma). The sedimentary thicknesses and implied sedimentation rates are excessively high, which suggests that at least some of the former over-burden was emplaced tectonically rather than by sedimentation. Regardless of the process of emplacement, the proposed overburden thick-nesses are abnormally large for continental crust. It is proposed, therefore, that normal tec-tonic processes were modified in this region by the presence of a deep-seated body of high-density material, evidenced by the SGH. This feature produced greater than normal crustal downwarp in this region, especially during the Alleghany orogeny. The Bouguer anomaly con-tours lie oblique to the structural grain of the major Alleghanian folding but nearly parallel to regional iso-coal-rank trends and to the "Anthracite depression."

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ACKNOWLEDGMENTS The Pennsylvania State University Coal Research Section

provided facilities and support for this study. Research fund-ing was provided by the U.S. Geological Survey Branch of Coal Resources. A. Davis, R. Scholten, B. Voight, and E. G. Williams of Penn State; R. P. Nickelsen of Bucknell Univer-sity; and E. Mouritjoy of McGill University contributed ideas and discussion. Andrew Hynes of McGill offered many use-ful comments on the manuscript, as did reviewer Ed Beutner. This paper was written during my tenure as Research Asso-ciate at McGill University.

Manuscript received March 10, 1986 Manuscript accepted April 2, 1986

580 Printed in U.S.A. GEOLOGY, July 1986

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