Edward S. Belt

STRANDED IN UPSTATE NEW YORK: CAMBRIAN SCYPHOMEDUSAE FROM THE POTSDAM SANDSTONE

Abstract The Cambrian portion of the Potsdam Sandstone contains a suite of scyphomedusae impressions in fine-grained to medium-grained quartz arenites that outcrop on the periphery of the Adirondack Mountains, New York. The fossils are similar taphonomically and morphologically to coeval scyphomedusae from the Elk Mound Group of Wisconsin and were likely stranded on a sand flat. Soft-tissue preservation in such sandstones is rare, except in Ediacaran Konservat-Lagerstätten. Although subtidal facies are abundant and continental facies are present in the Potsdam, soft-bodied fossils are found only in emergent coastal facies. These units are characterized by microbial structures including domal sand buildups, sand shadows, and breached ripples and by such horizontal trace fossils as Climactichnites and Protichnites. Domal sand buildups mantle some medusa carcasses and suggest that carcasses were exposed at the sediment-water or sediment-air interface for significant intervals of time prior to burial. It is unknown if microbial binding mediated preservation of these carcasses, but evidence for rapid flow regime changes in the section suggest stranded medusae were resistant to the upper-flow regime deposition that buried them. In many Laurentian Cambrian sandstones, microbial binding is common, and metazoan bioturbation is minimal in intertidal and emergent facies. The Potsdam Sandstone, thus, exemplifies how the Ediacara-style taphonomic window persisted in emergent Cambrian settings. This preservational regime may persist because bioturbating metazoans did not fully colonize tidal flats until the Middle Ordovician, which allowed soft-bodied animal tracks and carcasses to be preserved without scavenging or disturbance.

Evidence for marine influence on a low-gradient coastal plain: Ichnology and invertebrate paleontology of the lower Tongue River Member (Fort Union Formation, middle Paleocene), western Williston Basin, U.S.A.

The Paleocene Tongue River Member of the Fort Union Formation contains trace-fossil associations indicative of marine influence in otherwise freshwater facies. The identified ichnogenera include: Arenicolites, Diplocraterion, Monocraterion, Ophiomorpha, Rhizocorallium, Skolithos linearis, Teichichnus, Thalassinoides , and one form of uncertain affinity. Two species of the marine diatom Coscinodiscus occur a few meters above the base of the member. The burrows occur in at least five discrete, thin, rippled, fine-grained sandstone beds within the lower 85 m of the member west of the Cedar Creek anticline (CCA) in the Signal Butte, Terry Badlands, and Pine Hills areas. Two discrete burrowed beds are found in the lower 10 m of the member east of the CCA in the little Missouri River area. Abundant freshwater ostracodes include Bisulcocypridea arvadensis, Candona, and Cypridopsis . Freshwater bivalves include Plesielliptio and Pachydon mactriformis . We recognize four fossil assemblages that represent fluvio-lacustrine, proximal estuarine, central estuarine, and distal estuarine environments. Biostratal alternations between fresh- and brackish-water assemblages indicate that the Tongue River Member was deposited along a low-gradient coastal plain that was repeatedly inundated from the east by the Cannonball Sea. The existence of marine-influenced beds in the Tongue River Member invalidates the basis for the Slope Formation.

EOLIAN-AQUATIC DEPOSITS AND FAUNAS OF THE MIDDLE CAMBRIAN POTSDAM GROUP

Abstract Exposures of the middle Cambrian Potsdam Group of northern New York, including the type section, represent a suite of interfingering eolian dune and aquatic deposits that record the activities of early land-going arthropods. Quartz arenites at these exposures are dominantly fine to medium grained, well sorted, and have hematite-coated well-rounded, high-sphericity quartz grains characterized by secondary optically continuous quartz cement overgrowths. Eolian beds are laminated and dominated by m- to dm-scale foresets characterized by large-scale, laterally extensive cross-beds that dip >∼15°–34° and contain reverse-graded lamination, en-echelon microfaulted slumps, adhesion structures, and very high ripple-index asymmetric ripples that lie on the foresets with crests that trend downdip. Lower parts of foresets are interpreted as toesets, and contain unusual Diplichnites and Protichnites trackways, which record the uphill, downhill, crest-parallel, and switchback-style movement of arthropods in dry or damp sand. Trackways that ascend dune faces do not possess a medial tail drag, whereas trackways that descend do, and turns of trackways often have deep continuous medial impressions; these features suggest arthropods leaned into the slope when turning downhill in dry sand. More shallowly dipping (∼4°–15°) cross-beds are interpreted as bottomsets. Like nearby intertidal Potsdam deposits, these beds contain Protichnites trackways and Arenicolites burrows. Paleocurrent analyses imply a coastline in which offshore and mixed-direction winds moved dunes seaward. Considered together, these strata record migration of coastal dunes into aquatic environments and flooding and reworking of distal dune bottomsets. In this setting, the same suite of epifaunal arthropods inhabited dry, damp, and aquatic marine environments.

Thrust-ridge paleodepositional model for the Upper Freeport coal bed and associated clastic facies, Upper Potomac coal field, Appalachian Basin, U.S.A.

Abstract A blind-thrust-ridge model is proposed to explain the lack of coarse clastic material in the vast minable Upper Freeport coal bed (UF). This coal bed contains only fine elastic partings and is overlain by regionally extensive, closely spaced channel-belt deposits in the Upper Potomac coal field of the Appalachian basin. A blind-thrust ridge may have formed a sediment trap and prevented c coarse fluvial sediments from entering the swamp during a period (Westphalian D) when the thick Upper Freeport peat accumulated. Anticlinal thrust ridges and associated depressions may have existed uninterrupted for about 40 km parallel to the Appalachian orogen. Sediment shed from the breached anticlinal ridges accumulated in the sediment trap and was carried out of the ends of the trap by streams that occupied the shear zone at the ends of the blind-thrust ridge. The extent, parallel to the orogen, of thick, areally extensive UF is related to the length of the blind-thrust ridge that, in turn, controlled the spacing of the river-derived coarse clastic sediments that entered the main basin from the east. The thrust plane eventually emerged to the surface of the blind-thrust ridge and peat accumulation was terminated when the ridge became eroded and the sediment trapped behind it was released. The peat was buried by abundant coarse clastic sediment, which formed closely spaced channel belts and intervening flood basins. This model has implications for widespread peat deposits (now coal) that developed in tropical regions a few hundred kilometers from the sea in a tectonically active foreland basin.

Significance of fossiliferous Middle Cambrian rocks of Rhode Island to the history of the Avalonian microcontinent

Middle Cambrian trilobites of Acado-Baltic affinities have been found in southern Narragansett Bay, Rhode Island, in phyllites previously mapped as part of the Pennsylvanian stratigraphy of the Narragansett Basin. The trilobite-bearing phyllites form the basal unit of an approximately 1-km-thick sequence that has undergone four episodes of folding and cleavage formation. Three different trilobites are represented. Badulesia tenera (Hartt), which gives a diagnostic medial Middle Cambrian age, is also known from New Brunswick, eastern Newfoundland, southern Germany, northern Spain, and eastern Turkey. This species in northern Spain characterizes a subzone within the Middle Cambrian Badulesia zone correlating approximately with the lower part of the Paradoxides paradoxissimus zone of northern Europe. Nearby in Newport, Rhode Island, a sequence of maroon and green clastic sediments rests unconformably on Precambrian igneous rocks and sediments. If these should be proven to be Lower Cambrian, this succession in southern Narragansett Bay would be the most complete Cambrian succession yet recognized in southern New England and possibly a nearly complete record of sedimentation for this area for the entire Cambrian Period. These well-exposed rocks are part of a distinctive succession of upper Precambrian and lower Paleozoic rocks and are interpreted as a fragment of the Avalonian platform or microcontinent, closely related to rocks of the eastern margin of the northern Appalachians and the western margin of western Europe.

Sedimentology and paleohydraulics of the meandering river systems of the Fort Union Formation, southeastern Montana

Abstract The Fort Union Formation in southeastern Montana is composed of four lithofacies associations: sandstone bodies, sandstone-mudstone interbeds, carbonaceous shales to coals, and sheet sandstones. The sandstone bodies are meters thick, fine upward and comprise cross-stratified, horizontally stratified, and ripple cross-laminated sandstones. Bedsets are typically arranged into erosively based storeys with lateral accretion bedding. Basal breccias above erosion surfaces locally contain turtle, fish and reptile bone fragments of Torrejonian age. Most sandstone bodies are laterally extensive but channel initiation and abandonment margins occur locally. Sandstone bodies are either single- or multi-storeyed and are generally tabular. Some sandstone bodies are locally thickened where the basal erosion surface cuts down several meters (commonly through an immediately underlying coal seam to a subjacent coal seam). The laterally extensive sandstone bodies are interpreted as point-bar deposits of laterally migrating and aggrading single-channel rivers. Single-storey sandstone bodies record deposition from single lateral migration events. Most multi-storey sandstone bodies probably record downvalley migration of meander bends and accumulation of point-bar deposits within an aggrading channel belt prior to an avulsion event. A few multi-storey sandstone bodies are likely the product of superimposed channel belts which returned to an area of the alluvial plain as a result of avulsion events. The locally thickened portions of sandstone bodies are interpreted as the product of scouring during episodic major flood events. The sandstone-mudstone interbeds, carbonaceous shales to coals, and sheet sandstones are interpreted as overbank, swamp and possibly lacustrine deposits accumulating next to the channels on an alluvial plain. Paleohydraulic reconstructions of point-bar deposits suggest three distinct channel sizes. Large channels (Lebo Member only) flowed easterly and had sinuosities, discharges and slopes of about 1.22 to 1.35, 460 to 600 cm, and 0.000026 to 0.000051, respectively. Intermediate channels show an upsection change in paleoflow from southeasterly in the Tullock Member, to easterly in the Lebo Member, to northerly in the lowest Tongue River Member, possibly influenced by the contemporary uplift of the Black Hills. Intermediate channels had sinuosities, discharges and slopes of 1.25 to 1.38, 208 to 351 cm, and 0.000038 to 0.000057, respectively. Small channels flowed in a variety of easterly directions (ranging from N 350° to N 110°), and had sinuosities, discharges and slopes of about 1.2 to 1.35, 140 to 240 cm, and 0.000038 to 0.000057, respectively. Outcrop relationships suggest the smaller channels were tributaries and/or distributaries of the larger channels. Regional paleodrainage studies also indicate a shift in drainage direction from southeast to northeast probably due to uplift of the Black Hills. The change in paleodrainage direction was accompanied by a shift in depocenters from the Black Hills region to the Williston Basin. In middle Tongue River times, the depocenter shifted to the Powder River Basin causing paleodrainage directions to change to a southwesterly direction. During times when paleodrainages were changing orientation, widespread coal swamps developed.

Unconformities and age relationships, Tongue River and older members of the Fort Union Formation (Paleocene), western Williston Basin, U.S.A.

An unconformable relationship is observed within the Paleocene Fort Union Formation in the western Williston Basin at the contact between the Tongue River Member and the underlying Lebo and Ludlow Members. Isotopic dates and pollen biozone data reported here are integrated with previously published data. A new correlation of these facies results in a revised history of localized depositional and tectonic events. One unconformity occurs at this lithological contact in the Pine Hills (PH), Terry Badlands (TB), and Ekalaka (E) areas west of the Cedar Creek anticline (CCA), and another unconformity occurs at the same lithological contact in the Little Missouri River (LMR) area east of the CCA. The two unconformities differ in age by about two million years. The older is the U 2 and the younger is the U 3 , which initially were recognized in the Ekalaka area of southeastern Montana (Belt et al., 2002). The U 2 crops out in the TB, PH, and E areas, where at least 85 m of Tongue River strata bearing palynomorphs characteristic of biozone P-3 are found above the unconformity. Radiometric dates from strata (bearing palynomorphs characteristic of biozone P-2) below the U 2 range in age from 64.0 to 64.73 Ma. The U 2 unconformity west of the CCA thus occurs in strata near the base of the lower P-3 biozone. The U 3 crops out in the LMR area (east of the CCA), where only 13 m of strata characterized by the P-3 pollen biozone occur above it. Radiometric dates from an ash <1 m above the U 3 in that area range in age from 61.03 to 61.23 Ma, and the P-3/P-4 pollen biozone boundary is located 13 m above the ashes. The U 3 thus occurs in strata characterized by upper parts of the P-3 pollen biozone east of the CCA. The U 3 is also identifiable in the middle of the ca. 200 m-thick Tongue River Member west of the CCA, where mammal sites 40 to 80 m above it are Tiffanian-3 in age. The strata below this unconformity are tilted gently to the northwest; strata above the unconformity are flat lying. This mid Tongue River unconformity probably correlates with the unconformity at the base of the Tongue River Member in the LMR area east of the CCA, where a Ti-2 mammal site (the “X-X” locality) occurs <10 m above it. Depositional and tectonic events can be summarized using North American Mammal Age nomenclature as a relative time scale. From latest Cretaceous through Puercan time, paleodrainage was toward the east or southeast, in the direction of the Cannonball Sea. The Black Hills did not serve as an obstruction at that time. During early Torrejonian time, the Miles City arch (MCA) and Black Hills were uplifted and partially eroded, leading to the U 2 unconformity. When deposition resumed, paleodrainages shifted to a northeasterly course. During middle and late Torrejonian time, facies of the lower Tongue River (“Dominy”) sequence and the Ekalaka Member of the Fort Union Formation were

Clastic diversion by fold salients and blind thrust ridges in coal-swamp development

Abrupt shifts from single widespread coal swamps to coarse siliciclastic alluvial channel deposits occur in at least five coal beds and zones within the Pennsylvania Allegheny Formation. One of these, the Upper Freeport coal zone, was deposited over and area at least 200 {times} 200 km with a spacing of alluvial channels one-half to possibly one-fifth that of the immediately overlying coarse clastics. All these shifts occured next to the rising Appalachian orogen, far from the eustatic effects of a marine shoreline. Recent models relating coal-swamp formation to isostatic warping of orogenic forelands by tectonic loads surely apply to this environment, but they seem to need an additional, more delicate mechanism to produce such abrupt but widespread switches in grain size and drainage spacing. The authors propose that irregularities in the advancing front folds and blind thrusts caused temporary geomorphic diversions into the recessed areas and allowed a widespread coal swamp to form in the sedimentary shadow of the salients, a shadowing process that is occurring today in the central Zagros Mountains of Iraq and Iran.