Edward Cotter

Shelf, Paralic, and Fluvial Environments and Eustatic Sea-Level Fluctuations in the Origin of the Tuscarora Formation (Lower Silurian) of Central Pennsylvania

ABSTRACT The Tuscarora Formation (Lower Silurian) of central Pennsylvania owes its origin to a combination of tectonic, eustatic, and depositional events. Tuscarora deposition began at the beginning of the Silurian Period at the time of both glacioeustatic sea-level rise and renewed tectonic elevation of the Taconic source terrain. Braided alluvial systems transported coarse sediment northwestward to a wave-dominated coast, yet sea-level rise was so rapid that the shoreline retrograded from western to eastern Pennsylvania. At the time of transgression, shoreline characteristics varied from high-energy beaches in central Pennsylvania to estuaries along the south-central border region of Pennsylvania. Seaward of the shoreline, sand was fashioned into shelf sand-wave complexes, analogous to moder shoreface-connected sand ridges. Much of the Tuscarora Formation west of the Susquehanna River accumulated in this shallow-marine shelf environment. Later in the Early Silurian, eustatic sea-level fall resulted in progradation of lower energy coastal sand/mud flats northwestward over the former shelf, shortly before a subsequent sea-level rise terminated Tuscarora development with the return of the sea in which the Rose Hill Formation was deposited. The basis for these interpretations is the analysis of five major lithofacies in the exposed Tuscarora Formation of central Pennsylvania. The basal part of the Tuscarora consists of two lithofacies of coastal origin: the horizontally laminated lithofacies (high-energy beach) and the pink transitional lithofacies (estuary). The main body of the Tuscarora comprises the eastern cross-laminated lithofacies (braided fluvial) and the western cross-laminated lithofacies (shelf sand-wave complexes). Capping the Tuscarora is the red, Skolithos-burrowed lithofacies (coastal sand/mud flats).

Coastal plain sedimentation in the late Devonian of Southern Ireland; hummocky cross-stratification in fluvial deposits?

Abstract The late Devonian Toe Head Formation crops out extensively in southwest Ireland and has been considered previously as the topmost Old Red Sandstone (terrestrial) formation in a conformable non-marine to marine sequence. The most characteristic lithofacies, the flat laminated and inclined parallel laminated sandstones, displays a range of structures from flat to gently inclined laminae and includes sets which meet the four criteria of Harms et al. (1982) for hummocky cross-strata. Associated facies include desiccated mudrocks, palaeosols and ripple cross-laminated sandstones which lack the flaser-linsen dominated bedsets characteristic of the overlying marine strata. Only non-marine fossils are recorded and numerous palynological preparations lack the marine indicators which appear at the top of the formation. Palaeocurrents show considerable spread with indications of overall easterly transport of sand. There is no preferred vertical sequence of lithofacies. The balance of evidence strongly favours a non-marine environment with the implication that hummocky cross-strata, as presently defined, are not good environmental or even process indicators. The fine to very fine sand sizes and possibly the amount of suspended load were important controls on the resultant structures. A depositional model of essentially fluvial deposition on a low-gradient coastal plain is proposed.

Paleoflow Characteristics of a Late Cretaceous River in Utah from Analysis of Sedimentary Structures in the Ferron Sandstone

ABSTRACT Interpretation of sedimentary structures and detailed stratigraphic relations of the fluvial facies of the Upper Cretaceous Ferron Sandstone in the Castle Valley in east-central Utah permits reasonable reconstruction of many parameters of a Late Cretaceous alluvial system. After the width and depth of flow of the ancient Ferron river have been estimated from the geometry of preserved sedimentary structures, and the type of sediment transported by the river has been determined by study of the sandstone texture, various relationships of modern streams empirically derived by Schumm are used to estimate channel sinuosity, meander length, mean annual discharge, mean annual flood, channel slope, and flow velocity. Values derived for the slope and velocity are supported by other indirect met ods based on the Manning equation. It appears likely that the Late Cretaceous Ferron river was about 300 feet wide and 25 feet deep, and that it was highly sinuous, with meander lengths of 2,500 to 4,100 feet. As the 200-mile-long river drained an area to the southwest of 6,000 to 8,000 square miles, it had a mean annual discharge of approximately 6,000 to 7,000 cubic feet per second and a mean annual flood of about 22,000 cubic feet per second. Although only 2 percent of the total river load was bedload, the flow velocity of between 2.0 and 4.6 feet per second in the upper part of the lower flow regime caused the fine- to medium-grained sand to be in a dune bed configuration. Some caution is advised in using this approach because of unresolved questions of applying modern stream relations to pre-Quaternary deposits and because of uncertainties in determining the cross-sectional shape and sediment texture of the ancient Ferron river.

Hierarchy of sea-level cycles in the medial Silurian siliciclastic succession of Pennsylvania

The medial Silurian siliciclastic succession in central Pennsylvania records two hierarchically superimposed cycles of sea-level fluctuations. The larger of these cycles controlled lithofacies architecture at the magnitude of formations and members. Five alternations of transgression and regression, with an average cycle period of about 2.5 m.y., produced wedges of coarser, shallow-water facies pointing basinward (northwest) and wedges of deeper water, basin-center mudrock facies extending marginward (southeast). Superimposed on these are smaller scale, shallowing-upward cycles, typically 1-3 m thick, that occur in three different, contemporaneous facies: basin-margin tidal flats, mid-shelf offshore bars, and below wave base in the storm-influenced transition to the basin center. Average recurrence interval for such smaller cycles was about 100 ka; their period was regular and was caused by sea-level fluctuations of about 1-3 m.

Late Cretaceous Sedimentation in a Low-Energy Coastal Zone: The Ferron Sandstone of Utah

ABSTRACT Thin sandstone units that comprise the Ferron Sandstone Member of the Mancos Shale (Upper Cretaceous) in the northern part of Castle Valley, Utah, were deposited along a low-energy epeiric sea margin. Interpretation of these units by uniformitarian comparison with modern low-energy coastal zones, in particular the Sapelo Island coast of Georgia, reveals an interplay between sediment-distributive processes that accompany storms and the sediment-disruptive activities of burrowing organisms in fairweather periods between storms. During storms, nearshore sediment is thrown into suspension and moved seaward where, upon storm abatement, it settles out. A return to fair weather brings renewed biogenic activity, completely homogenizing thinner storm-deposited beds, but reworking only the uppe parts of thicker beds. Storms also accentuate sedimentation processes in tidal inlets and on offshore bars seaward of the shoreface. Sediment now in these thin Ferron units was derived from the large Vernal Delta, located north and west of the Castle Valley outcrops, and was transported generally southwestward, parallel with the coast. Specific interpretations of depositional environment have been made for four informally named sandstone units. The Clawson unit was deposited in the offshore environment; it consists of bioturbated silty sandstone with numerous large concretions. The Washboard unit has, in addition to bioturbated, concretion-rich sandstone, intercalated thin sandstone beds that are even parallel laminated and have burrowed upper parts; this unit was deposited in the lower shoreface environment. In a limited part of the outcrop belt the Washboard unit is replaced by a tidal inlet deposit termed the Farnham unit, that comprises shelly, little-burrowed sandstone with bipolar, coast-normal, trough cross lamination. And the Woodside unit is a sequence of thin, medium- to coarse-grained sandstone beds that have bipolar, coast-parallel, trough cross lamination; these beds accumulated as submerged offshore sand bars in a shallow shelf sea.

Depositional setting and cyclic development of the lower part of the Witteberg Group (Mid- to Upper Devonian), Cape Supergroup, Western Cape, South Africa

Mid- to Upper Devonian strata of the lower Witteberg Group in the Cape Fold Belt of South Africa comprise three principal facies deposited in the shoreface and contiguous inner shelfalong a storm-dominated coastal zone. Weakly burrowed mudrock with variable minor proportions of wave-rippled sandstone (linsen, flasers, hummocky cross stratification [HCSJ) (facies 1) accumulated in the inner shelf below, near, and just above storm wave base. Very fine- and fine-grained sandstone that displays flat and wavy lamination and HCS, as well as compound wave-ripple lamination (facies 2), was deposited in the lower shoreface above fair-weather wave base. Fine- to coarse-grained sandstone containing cross-lamination, anisotropic HCS, and compound wave-ripple lamination (facies 3) formed in the mid to upper shoreface. Regionally, lower Witteberg strata change from proportionally more facies 2 and 3 at proximal (northern) sites to mostly facies 1 at distal (southern) sites. The lower Witteberg Group consists of approximately 40 shallowing-upward facies sequences (parasequences) whose average thickness is approximately 12 m. These are grouped into progradational parasequence sets of up to 10 parasequences each for a total of approximately 3.5 parasequence sets in the lower Witteberg. The lowest parasequence set extends from the base of the Karoopoort Formation (top of underlying Bokkeveld Group) and culminates at the top of the Blinkberg Formation in the Witteberg Group, and approximately 2.5 parasequence sets make up the overlying Swartruggens Formation. The characteristics and stratigraphic architecture of the lower Witteberg Group are consequences of the interaction oftwo different orders of global sea-level fluctuation. The lower Witteberg parasequences had a periodicity between 250 and 300 ka, within the Milankovitch range, and their character and mean thickness are very similar to cyclic patterns recorded in contemporaneous Givetian—Frasnian siliciclastic and carbonate strata on four other continents. The progradational parasequence sets had a mean recurrence interval in the order of 3.0 Ma and can be correlated with specific third-order fluctuations of global sea-level proposed for contemporaneous Upper Devonian strata in Euramerica. This reinforces assignment of the age of the lower Witteberg as extending from the mid-Givetian Stage to the end of the Frasnian Stage.

Deposition and diagenesis of Clinton ironstones (Silurian) in the Appalachian Foreland Basin of Pennsylvania

Clinton ironstones (mid-Silurian) on the southeastern margin of the Appalachian Foreland Basin in central Pennsylvania consist largely of skeletal grainstone storm beds whose skeletal fragments are extensively impregnated and replaced by ferric oxide. Other forms of iron mineralization consist of ooids, superficial ooids, cavity fillings and cements composed of ferric oxide and/or chamositic clay, and two generations of iron-rich carbonate spar. True oolite is rare. Small proportions of ferruginized grains are present in coarse lags at tops of some small-scale, 1- to 5-m coarsening-upward sequences, which are the record of mid-shelf shoals from which the skeletal grainstone storm beds were derived. Ooids and superficial ooids are likely to have formed in dysaerobic conditions where the pycnocline of a highly stratified sea intersected shoal flanks. Ferrous iron from anaerobic deeper waters precipitated on quartz and skeletal fragment nuclei in weakly reducing conditions with episodic gentle agitation. Several stages of early diagenetic ferruginization affected skeletal grainstone storm beds and sequence-capping coarse lags: chamositic clay precipitated in pores and cavities of skeletal fragments, within quartz sand adjacent to crinoid ossicles, and as bed-parallel connectors of large skeletal fragments. As diagenesis progressed with further burial, many skeletal fragments were extensively replaced by ferric oxide, most chamositic clay altered to ferric oxide, other chamositic clay matured from berthierine to chamosite, and clusters of bladed specularite replaced parts of primary grains and secondary carbonate cements. Paleomagnetic and fluid-inclusion analysis indicates that many late diagenetic changes took place at elevated temperatures during deen burial, in an early phase of the Alleghanian orogeny.

Storm effects on siliciclastic and carbonate shelf sediments in the medial Silurian succession of Pennsylvania

Abstract Upper Llandoverian through Wenlockian strata in the Pennsylvania part of the northern Appalachian Basin formed on a gentle epeiric ramp that extended from marginal tidal flats through coarser-grained mid-shelf shoals to a muddy basin center. Over approximately 10 million years, the composition of the accumulating sediment evolved from siliciclastic to carbonate, while the ramp maintained a relatively similar configuration and position below sea level, and while storm-generated processes continued as the major producers of sediment characteristics. In both siliciclastic and carbonate parts of the succession, sedimentary structures and their sequences are generally similar, with only a small number of dissimilar features resulting from the differences in composition. Small-scale coarsening- and shallowing-upward sequences are present in both mid-shelf bar and basin-transition deposits, although specific sequence characteristics are different in siliciclastic and carbonate parts. Such sequences record cyclic fluctuations of relative sea level. While the composition of the sediment evolved over this extended period, the gentle ramp gradient was maintained largely by the processes generated by storms. Such processes were more effective because of the fluctuations of relative sea level.

Limestone diagenesis and dolomitization in Mississippian carbonate banks in Montana

ABSTRACT Waulsortian type carbonate banks in central Montana were diagenetically altered in three major stages: before, during, and after subaerial exposure of the tops of the banks. Before subaerial exposure, cloudy radiaxial fibrous drusy mosaic was precipitated as aragonite on all free internal surfaces. Opaline silica was dissolved, and small amounts of chalcedonic chert replaced parts of fossils and the fibrous mosaic. With partial emergence of the banks, the lime mud and fossils were altered, clear equant-grained drusy mosaic calcite and euhedra of dolomite were formed in centers of cavities lined with fibrous crusts, and seepage refluxion of hypersaline water formed an irregular zone of dolostone at the tops of the banks. Other changes involved weathering of the exposed surface, dilatational fracturing, dissolution of cavelike openings within the banks, and filling of the fractures and openings with crinoid debris. After resubmergence, the banks were engulfed by continued Lodgepole limestone deposition. Late in diagenesis a minor amount of dedolomitization occurred, and, perhaps, microspar (Folk, 1965) formed neomorphically.