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Sedimentary Depositional Environments:

Delta Systems


The following tables provide a brief description of the typical characteristics of depositional environments found in Upper Paleozoic rocks of southwestern Pennsylvania. it is summarized from Donaldson (1974b).

LOWER ALLUVIAL PLAIN TO UPPER DELTA PLAIN

DEPOSITIONAL ENVIRONMENT

LITHOLOGIC DESCRIPTION

Meandering Channels

Massive thick sandstones with point bar structures and wedge-to-trough type large scale cross beds with erosional basal contacts, chute-bar structures. Plant and wood fragments. Grain size and bedding thickness decrease upward. Lateral and basal contacts sharp. Shale plugs occasionally occur within the sandstone unit.

Levee and Crevasse Splay

Alternating thin beds of mudstone (variegated to brown) and fine-grained sandstones to siltstone with ripple bedding, root-disrupted parallel laminations, and plant fragments.

Swamp

Coal, Carbonaceous clay probably deposited in well drained swamps whereas peat in poorly drained swamps.

Lake

Limestones (dismicrites, pelmicrites, intramicrites) with shrinkage, laminated, or poorly developed algal structures. Limestones alternate with dark-gray calcareous shale. Spirorbis, fish parts, and ostracodes are most common fossils. Limestones are usually light colored and occur in rhythmical depositional sequences.Some lake facies consist of thin, discontinuous coals or dark laminated shales of limited extent.

 

LOWER DELTA PLAIN

DEPOSITIONAL ENVIRONMENT

LITHOLOGIC DESCRIPTION

Distributary Channels

Alternate-bar structures (minor lateral migration) with dune and ripple structures in massive sandstones; basal scour. Abandoned channel deposits of siltstone and dark-gray shale with abundant plant fragments. Siderite and ironstone concretions commonly in lag gravel deposit in channel.

Levee and Crevasse Splay

Alternating thin beds of mudstone and fine-grained sandstones to siltstone with ripple bedding, root-disrupted parallel laminations, and plant fragments. Iron oxide concretions above water table.

Swamp

Coal, pyrite, claystone-siltstone partings.

Lake, Interdistributary bay, and Interdelta Bay

Bay laminated shale, ironstone concretions, plant fragments and some ostracodes. Limestones with ostracodes, spirorbis, and fish scales alternating with dark gray shale.

Beach

Thin siltstone and fine grained sandstone beds. Transported shell fragments rarely preserved. Greater quartz content than equivalent grain size deposits of fluvial origin.

 

ADDITIONAL ENVIRONMENTS

DEPOSITIONAL ENVIRONMENT

LITHOLOGIC DESCRIPTION

Delta Front, Distributary Mouth Bars

Alternating thin beds of siltstone, fine-grained sandstone and shale (mudstone) with gradational basal contact. Slightly arched bedding reflecting bar shape. Ripple, dune, and parallel bedding with burrow mottled structures, especially at distal fringe.

Prodelta

Laminated silty shale, ironstone concretions, and plant fragments. Bedding can be burrow obliterated creating massive mudstone.

Bay

Mudstone and shale, dark gray to reddish gray. Limestones, massive or shaly with brackish fossils.

Destructive Delta Margin and Interdelta (including tidal flat)

Sandstones with slightly increased quartz content: ripple, dune, flaser bedding and burrow-mottled structures. Tidal flat mudstone (some variegated), flint clay, siliceous mudstone, nd iron carbonate. Tidal flat limestones (dismicrites, pelmicrites, intramicrudites) with shrinkage, laminated, or poorly developed algal structures. Limestone alternates with dark-gray calcareous shale. Spirorbis, fish scales, and ostracodes are the most common fossils.

The following diagrams are included here to provide the reader a guide to the kinds of depositional systems in which the Pennsylvanian and Permian rocks of southwestern Pennsylvania were deposited.

This diagram, from Fisher and others (1969), depicts the four main delta types and the kinds of depositional environments associated with each. Click on image for a larger version.

Environments of deposition in a deltaic sequence (from Horne and others, 1978).

Development of a delta sequence (from Frazier and Osanik, 1969).

Back Barrier typical section (from Horne and others, 1978).

Back Barrier Island depositional environments  (from Horne and others, 1978).

Distributary Environment  (from Horne and others, 1978).

Lower Delta Plain Environment (from Horne and others, 1978).

Typical Sections from the lower delta plain, bottom section with crevass splay (from Horne and others, 1978)..

Typical Sequence from lower delta plain (from Horne and others, 1978).

Upper Delta Plain Environment (from Horne and others, 1978).

Typical Section from Upper Delta plain (from Horne and others, 1978).

An example of facies relationships in deltaic sequences from the Virgin and Wolfcamp rocks of Stephens County, Texas (From Brown, 1969). Note the lateral variations in facies due to the shifting positions of the delta lobes. Pennsylvanian and Permian rocks in southwestern Pennsylvania exhibit similar lateral relationships, although the paucity of outcrops usually precludes such definition.

Model for the deposition of adjacent channel systems. New channels form in the interdstributary areas due to subsidence by compaction and dewatering. From Ferm and Cavaroc (1968).

Model for the deposition of adjacent channel systems. Note offsetting channels through time. From Ferm and Cavaroc (1968).

Here are the facies relationships in the Upper Pennsylvanian Uniontown Formation and the Permian Waynesburg Formation in southern Pennsylvanian and Northern West Virginia. Note the similarities to the Pennsylvanian of Texas (above). This is from an untitled late 1970's guidebook by A. C. Donaldson (West Virginia University, retired).

Section running NW - SE across the Pittburgh 15' quadrangle showing the relationships of sandstone bodies. Section covers the Upper Allegheny Group into the Lower Conemaugh Group. the Mahoning sandstone is at the top. Original data from Johnson (1928). From Ferm and Cavaroc (1968).

References

Beerbower, J. R., 1961, Origin of cyclothems of the Dunkard Group (Upper Pennsylvanian - Lower Permian) in Pennsylvania, West Virginia, and Ohio: Geological Society of America Bulletin, v.  p. 1029-1050.

Brown, S. L., 1969, Late Pennsylvanian paralic sediments, in Guidebook to the Late Pennsylvanian Shelf Sediments, North-Central Texas: Dallas Geological Society, p. 21-33.

Donaldson, A. C., 1974a, Pennsylvanian Sedimentation of central Appalachians: in Briggs, G., ed., Carboniferous of the Southeastern United States, Geological Society of America Special paper148, p. 47-78.

Donaldson, A. C., 1974b, Ancient deltaic depositional environments recognized in Pennsylvania rocks of northern Ohio River valley: in Doanhue, J., and Rollins, H. B., eds., Conemaugh (Glenshaw) Marine Events, Field Guidebook, Pittsburgh Geological Society, p. F-1 to F-11.

Donaldson, A. C., and Shumaker, R. C., 1981, Late Paleozoic molasse of the central Appalachians: in Miall, A. D. ed., Sedimentation and Tectonics in Alluvial Basins, Geological Association of Canada Special Paper 23, p. 99-124.

Ferm, J. C., and Cavaroc, V. V., Jr., 1968, A nonmarine sedimentary model for the Allegheny rocks of West Virginia in Klein, G. deVries Klein, ed., Late Paleozoic and Mesozoic Continental Sedimentation, Northeastern North America, Geological Society of America Special paper 106, p. 1-20.

Fisher, W. L., Brown, L. F., Jr., Scott, A. J., and McGowen, J. H., 1969, Delta systems in the exploration for oil and gas: A research colloquium: Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas, August 27-29, 1969.

Frazier, D. E., and Osanik, A., 1969, Recent peat deposits - Louisiana coastal plain: in Dapples, E. C., and Hopkins, M. E., eds., Environments of Coal Deposition, Geological Society of America Special paper 114, p. 63-86.

Horne, J. C., Ferm, J. C., Caruccio, F. T., and Baganz, B. P., 1978, Depositional models in coal exploration and mine planning in Appalachian region: American Association of Petroleum Geologists Bulletin, v. 62, p. 2379-2411.

Johnson, M. E., 1928, Geology and Mineral Resources of the Pittsburgh Quadrangle, Pennsylvania: Pennsylvania Geological Survey Bulletin A 27, 236 p.