ABSTRACT A laterally discontinuous sandstone at the south end of the Tellico-Sevier syncline in Blount County, Tennessee, was mapped in 1955 by Robert Neuman and in 1965 by Neuman and Willis Nelson of the U.S. Geological Survey as a “quartzite” that they considered to be the uppermost bed of the Bays Formation (Ordovician). On the basis of new mapping and conodont biostratigraphy, lithostratigraphy, and regional K-bentonite correlations, this sandstone, a distinctive quartz arenite, is reassigned to and correlated with the Clinch Sandstone (Silurian). At the Harrison Branch section (HBRA) in Blount County, in an exposure near the confluence of Harrison Branch and the Little Tennessee River, this sandstone underlies the Devonian Chattanooga Shale, and it overlies ~43 m of gray limestones and shales that are themselves above the red clastic and minor carbonate rocks of the Bays Formation. The limestones and shales between the Bays Formation and this sandstone crop out on a wooded hillside and were apparently not observed by Neuman and Nelson during their mapping of the region. We measured the HBRA section, collected 20 samples from the limestone interval, and processed them for conodonts. These limestones contain a definitive Late Ordovician (Katian) conodont fauna that includes Drepanoistodus suberectus , Plectodina tenuis , Panderodus gracilis , and Phragmodus undatus . On the basis of this fauna, the 40+ m of limestone between the youngest red beds of unequivocal Bays Formation (below) and the quartz arenite (above) can be assigned to the Ordovician P. tenuis zone or younger, making them correlative regionally with limestones of the Trenton Group. Using these new biostratigraphic data combined with existing tephrostratigraphic relations of Ordovician K-bentonites, we identify the overlying sandstone at the HBRA section as an erosional outlier of the Silurian Clinch Sandstone, and we correlate the Ordovician-Silurian-Devonian unconformities at these two localities, which are now better constrained, with unconformities A through F in the Silurian and Devonian of this region, as identified and described in detail at several exposures north and northwest of the Tellico-Sevier syncline, most prominently at outcrops near Wytheville, Seven Mile Ford, and Max Meadows in southwest Virginia, where Devonian strata unconformably overlie Ordovician strata.

ABSTRACT More than 100 air-fall volcanic tephra beds are currently documented from Devonian strata in the eastern United States. These beds act as key sources of various geological data. These include within-basin to basin-to-basin correlation, globally useful geochronologic age dates, and a relatively detailed, if incomplete, record of Acadian–Neoacadian silicic volcanism. The tephras occur irregularly through the vertical Devonian succession, in clusters of several beds, or scattered as a few to single beds. In this contribution, their vertical and lateral distribution and recent radiometric dates are reviewed. Current unresolved issues include correlation of the classic Eifelian-age (lower Middle Devonian) Tioga tephras and dates related to the age of the Onondaga-Marcellus contact in the Appalachian Basin. Here, we used two approaches to examine the paleovolcanic record of Acadian–Neoacadian silicic magmatism and volcanism. Reexamination of volcanic phenocryst distribution maps from the Tioga tephras indicates not one but four or more volcanic sources along the orogen, between southeastern Pennsylvania and northern North Carolina. Finally, radiometric and relative ages of the sedimentary basin tephras are compared and contrasted with current radiometric ages of igneous rocks from New England. Despite data gaps and biases in both records, their comparisons provide insights into Devonian silicic igneous activity in the eastern United States, and into various issues of recognition, deposition, and preservation of tephras in the sedimentary rock record.

ABSTRACT A local clay lens up to 60 cm thick in the Eocene Castle Hayne Limestone at the abandoned Fussell Quarry, Duplin County, North Carolina, is identified as a bentonite. It is composed of authigenic smectite with sparse euhedral biotite and apatite. Scanning electron microscope examination shows that the bentonite consists of relic bubble-wall shards altered to smectite. Smectitic columnules, rod-shaped casts of elongate pipe vesicles in pumice fragments derived from early dissolution of nearby small glass shards, also occur. This association is considered diagnostic of a silicic air-fall ash. K-Ar and Rb-Sr biotite dates from the bentonite are 46.2 ± 1.8 Ma and 45.7 ± 0.7 Ma, respectively, and a fission-track age of apatite is 51.0 ± 2.0 Ma; this later date is considered to be incorrect. Biotite compositions determined from electron microprobe analyses on 100 crystals suggest derivation from a single volcanic source no more than 4000 km from the bentonite. Possible sources of the ash include Bermuda; Highland County, Virginia; and the Caribbean; however, because of distance, prevailing wind direction, and similarity in age and composition, the volcanic swarm in Highland County, Virginia, is the suggested source.

ABSTRACT In a well-defined subrecess in the Appalachian thrust belt in northwestern Georgia, two distinct fold trains intersect at ~50° in the down-plunge depression of the Floyd synclinorium. A mushwad (ductile duplex) of tectonically thickened weak-layer rocks (primarily the shale-dominated Cambrian Conasauga Formation) filled the space beneath folds and faults of the overlying Cambrian–Ordovician regional stiff layer (mushwad roof). Measurements of the mushwad thickness from balanced cross sections provide the basis for three-dimensional (3-D) models. Tectonically thickened weak-layer shales in a model using a simple line-length balance of the stiff layer have a volume of ~64% of the volume in the deformed-state model, indicating that this balanced reconstruction is not appropriate. Previous work demonstrated deposition of a thick mud-dominated succession in a basement graben to balance the volume. A 3-D model incorporating a thick Conasauga Formation shale succession deposited in a basement graben yields good correspondence to the deformed-state mushwad volume. That model requires vertical separation on the graben boundary faults greater than the present small-magnitude separation; unconformable truncation of the upper part of the Cambrian–Ordovician carbonate succession documents Ordovician inversion of the graben boundary faults. In the 3-D models, the distribution of thickness in the deformed state suggests movement of weak-layer shale out of the planes of cross sections and up plunge away from the structural depression of the Floyd synclinorium. Out-of-plane tectonic translation is consistent with a relatively uniform depositional thickness of ~800 m, which allows calculation of the magnitude of vertical separation on basement faults during Conasauga Formation deposition.

ABSTRACT A 3 ton (2.7 metric tonnes [t]), granitoid lonestone with Appalachian provenance was found in situ in offshore Devonian black shale in northeastern Kentucky, United States, and is denoted herein as the Robinson boulder, or lonestone, after its discoverer, Michael J. Robinson. This large boulder appears to have been displaced nearly 500 km from its source on the opposite margin of the Acadian/Neoacadian Appalachian foreland basin. While previous identifications of possible lonestones have been attributed to Pleistocene glacial events, scrutiny of this lonestone’s origin suggests that the boulder, which was embedded in the Upper Devonian Cleveland Shale Member of the Ohio Shale in northeastern Kentucky, is most likely a Devonian ice-rafted glacial dropstone. Notably, palynologic correlation with reported glacial diamictites elsewhere in the basin indicates such a source. Together, the dropstone and diamictites, separated by ~500 km, provide evidence for alpine glaciation in the ancient Acadian/Neoacadian orogen and for tidewater glaciers in the adjacent, eastern margin of the foreland basin. The latest Devonian marine transgression and Neoacadian foreland subsidence are interpreted to have been associated with tidewater glacial connections to the open sea. Importantly, the existence of this dropstone and its likely glacial precursor events require new considerations about contemporary black-shale sedimentation and the influence of tectonics on the delivery of glacial sediments to foreland basins.

ABSTRACT Data from 33 locations were utilized in a stratigraphic study of the Early Mississippian Grainger Formation and related units in northeast Tennessee. Isopach maps, stratigraphic cross sections, and lithologic trends indicate the Grainger Formation was deposited in four deltaic lobes: Monroe, Rock Haven, Hancock, and Grainger-Borden. Each is in a separate outcrop belt: Chilhowie Mountain, Clinch Mountain, Newman Ridge, and Cumberland Mountain. The Monroe lobe is the eastern and southernmost of the lobes. Within it, the Grainger Formation is thicker and coarser than in the other locales. It is underlain by gray and black shale; the gray shale is a probable nearshore gray version of the usually greenish Maury Formation. The Greasy Cove Formation, a heterogeneous unit of sandstone, shale, red beds, and limestone, overlies the Grainger Formation and occupies the stratigraphic position of the Maccrady Formation and Newman Limestone in outcrop belts to the northwest. The Greasy Cove Formation is recognized only in the Monroe lobe. In the Rock Haven lobe, both the Grainger Formation and Chattanooga Shale are divisible into mappable members. The Chattanooga Shale consists of an upper Big Stone Gap Member, a middle Brallier Member, and a lower Millboro Member. The Chattanooga Shale locally is 600+ m thick. The Grainger Formation in the Rock Haven lobe is divisible into three newly named members: an upper Hayters Sandstone member, a middle Greendale member, and a basal Bean Station member. The Alumwell glauconite zone, within the upper part of the Greendale member, is also new. The center of the zone approximates a time line and is a key stratigraphic horizon. All Grainger members and the Alumwell glauconite are traceable into the Price Formation of southwest Virginia. In the Rock Haven lobe, the Chattanooga Shale, Grainger Formation, and Maccrady Formation were deposited in a subsiding trough; subsidence began in the Givetian and perhaps in the Eifelian, caused by a migrating peripheral bulge generated by Neoacadian deformation in the Carolina Piedmont. Highlands created by the deformation were the eastern sediment source for the Chattanooga, Grainger, and Maccrady formations in this lobe. Sediment for the Hancock and Grainger-Borden lobes originated from northerly sources. In the Hancock lobe, the Chattanooga Shale and Grainger Formation are thinner, and the Grainger Formation has increased shale content to the south. Paleocurrent data indicate a north-south current flow. The Hancock lobe is likely a southern extension of the Price delta system in southwest Virginia. The Grainger-Borden lobe is the southern terminus of the Borden delta system of Kentucky. Both the Chattanooga Shale and Grainger Formation thin to the south and southeast. The Floyds Knob glauconite bed was deposited during a pause in sediment delivery and separates the Fort Payne Chert from the underlying Grainger Formation as a distinct sedimentary unit. The Fort Payne Chert overlaps the Grainger Formation from a deeper southern basin where the dolostone and chert have little or no interbedded shale. The overlap does not interfinger with the Grainger Formation. The Fort Payne Chert becomes thinner as it progresses northward, finally passing into the Muldraugh Formation in Kentucky. It also made a minor incursion eastward into the western margin of the Hancock lobe, where some chert(y) beds occur at the Maccrady position.

ABSTRACT The Upper Devonian Foreknobs Formation is a series of sandstones and siltstones that outcrops in the central Appalachian Basin from Pennsylvania south into Virginia and West Virginia. The Foreknobs Formation is generally regarded as the delta slope portion of the Catskill clastic wedge (= Catskill delta) occurring between the dark marine lithologies of the underlying Brallier and Scherr Formations and the younger fluvial red beds and conglomerates of the overlying Hampshire Formation. Progradational and retrogradational pulses of the clastic wedge are recorded in the alternating siltstones and sandstones before grading into nonmarine lithologies at its top. Well-preserved fossils are very abundant within the Foreknobs Formation. They have aided in locating the Frasnian-Famennian Stage boundary and have provided important information regarding its namesake extinction event. The progradation of the clastic wedge during Foreknobs Formation deposition allowed for both shallow- and deep-water environments to exist up to and across the Frasnian-Famennian boundary, which may have contributed to some uncertainty in correlating the lithostratigraphic position of the boundary with the biostratigraphic signature of the extinction event.

ABSTRACT The Upper Ordovician Juniata Formation, Central Appalachian Basin, USA, is a thick succession of cyclically bedded arenites, wackes, and mudrocks. Sedimentary facies of the formation in West Virginia, Virginia, and Maryland indicate cyclic peritidal deposition along the northern shoreline of the basin. The subsurface Juniata Formation has been drilled throughout the basin, and long, continuous well logs are available for analysis of the cyclic deposition. A 2400-ft-long (731.52-m-long) gamma-ray (GR) log from the Preston 119 well, northern West Virginia, provides a proxy of terrigenous siliciclastic fluxes originating from the Taconic highlands, from the early Ashgillian to the Ordovician–Silurian transition. Strong cycling in the GR log shows evidence for Milankovitch-forced sea-level oscillations, hypothesized to have been produced by dynamic Late Ordovician glaciation in polar (southern) Gondwana. Juniata cycle frequencies are different from those of Quaternary Milankovitch cycles, with significantly higher obliquity and precession index frequencies, consistent with a 21.5 h Ordovician day and an Earth-Moon distance that was 95% of present day. These results support John Dennison’s long-held view that Milankovitch forcing of sedimentation took place in the early Paleozoic Appalachian Basin by action of remotely generated glacio-eustatic oscillations powered by glaciation on southern Gondwana, and that this sedimentary record has tracked “Earth’s movement through space.”

ABSTRACT Zone-diagnostic Middle to Upper Devonian ammonoids and conodonts occur in a sequence of thin carbonate beds in siliciclastic sections along the Allegheny front and northern fold belt of central Pennsylvania. The lowest, in the upper Tully Formation, and the succeeding four in the Harrell Shale, are southward continuations of marker beds long known in the Tully Limestone and overlying basin, slope, and shelf facies of the Genesee Group of New York State. These pelagic beds are Appalachian Basin signals of transgressive condensed intervals within global high-sea-level episodes and are associated with a worldwide biotic overturn in the tropical marine realm. The Upper Givetian Tully Pharciceras amplexum bed has close lithic and faunal equivalents in the Taghanic biocrisis interval in Morocco and southern Europe. The Lower Frasnian styliolinid upper Genundewa Limestone, with the entry of Manticoceras , and, in the succeeding dark West River Shale, the Bluff Point Bed, with the entry of late Koenenites and early palmatolepid conodonts, are Appalachian highstand signals of the worldwide Timan bioevent transgression recognized in Western Australia, the Russian Platform, southern Europe, Morocco, western Canada, and now Pennsylvania. The widespread occurrences of discrete ammonoid beds, even in the more proximal or shoreward sedimentary settings of the Catskill Delta, reinforce the view that they accumulated under conditions of sequestered sediment supply when transgressions flooded the newly vegetated and deep-root-forested delta flats. As such, they overprinted or interrupted the westward progradation of siliciclastic sediments generated by mountain-building tectophases of the Acadian orogeny. The Lower Frasnian ammonoid Epitornoceras dennisoni n. sp., described herein, from the Crosby Beds in the Harrell Shale at Milesburg, Pennsylvania, is named in honor of John Dennison.

ABSTRACT This study examines the usefulness of accommodation plots (Fischer plots) as a means of correlating mixed carbonate-siliciclastic strata in the subsurface. Fischer plots have been widely used to extract accommodation changes from carbonate platforms, but there are few published studies of siliciclastic or mixed carbonate-siliciclastic environments. The Middle Devonian of the Appalachian Basin is penetrated by thousands of wells, is exposed in numerous exceptional outcrops, and is an excellent place to test the usefulness of accommodation history plots as correlation tools. In the past, researchers have used cores, well cuttings, well logs, and outcrop gamma-ray profiles to correlate between outcrop and subsurface data, but all these methods have their limitations. Gamma-ray logs for wells penetrating the Middle Devonian from eight locations, from Preston County in the east to Wetzel County, West Virginia, in the west, were used in this study. Accommodation cycle thicknesses were measured from gamma-ray logs, printed at a vertical scale of one inch per ten feet (2.5 cm/3 m). The accommodation cycle thickness data were entered into Antun Husinec’s FISCHERPLOTS program to produce accommodation plots. Next, well-documented, outcrop-based sequence stratigraphy was used to help interpret the results of the accommodation plots. This study demonstrates that using accommodation plots is a novel way of overcoming the uncertainties and biases of other methods. The use of this approach in other mixed carbonate-siliciclastic successions with abundant subsurface data would help to demonstrate that Fischer plots are a novel and useful approach that can help remove many of the uncertainties and biases encountered in stratigraphic correlation.

ABSTRACT During latest Devonian to Middle Mississippian parts of the Neoacadian and Ouachita orogenies, the Appalachian Basin and parts of the Illinois Basin were filled with clastic debris derived from the westward-prograding Borden-Grainger-Price-Pocono clastic wedge. This delta complex is overlain by the widespread shallow-water Newman–Greenbrier–Slade–St. Louis–Warsaw–Salem–Harrodsburg carbonate interval across sediment-starved surfaces, comprising the Floyds Knob bed or interval. The Middle Mississippian (late Osagean; early Viséan) Floyds Knob interval is less than a meter to several meters thick and is composed of multiple zones of pelletal glauconite, finely divided glauconitic shales, glauconitic carbonates, and locally derived carbonate mud mounds. The interval occurs across most of the Borden-Grainger delta platform, delta front, prodelta, and within the starved-basin area seaward of the delta complex, which was then filled with the Fort Payne Formation. This study reports herein the first occurrence of the Floyds Knob interval within the Fort Payne Formation. Glauconite deposition in this interval apparently occurred in mildly oxic to dysoxic, sediment-starved, shallow-marine settings and is believed to represent termination of major clastic influx in more proximal parts of the Neoacadian foreland basin during lowstand conditions. Moreover, these starved-basin conditions can be correlated with delta diversion following bulge migration during flexural loading–type relaxation. During these sediment-starved, lowstand conditions, glauconite was deposited across deltaic and basinal settings in central and distal parts of the Neoacadian foreland basin, as well as in eastern parts of the present-day Illinois intracratonic basin. The cessation of deltaic clastic sedimentation permitted development of carbonate mud mounds and associated glauconitic shales on and near reactivated structures in central parts of the Fort Payne starved basin and set the stage for the widespread deposition of thick, Meramecian–Chesterian carbonates throughout the basins during succeeding subtropical and lowstand conditions. Whether less-than-a-meter or tens-of-meters thick, the Floyds Knob interval is a widespread Middle Mississippian chronostratigraphic interval in the east-central United States that reflects a change in tectonic regime, which is recorded in the shift from predominantly clastic to carbonate sedimentation across a broad region. Aside from its correlative value, the unit demonstrates consequent sedimentary responses to the interplay among tectonism, paleoclimate, and paleogeography.

ABSTRACT Beautifully fossiliferous strata in the Hamilton Group (Middle Devonian, central New York) constitute a rich “ecological archive” sufficient to probe and test foundational concepts in paleontology. The evident community stability of Hamilton faunas over 4–6 m.y.—including two proposed mechanisms for coordinated stasis—has ignited controversy. Resolving community structure and both taxonomic and ecological temporal persistence within the Hamilton Group thus becomes critical to testing whether these Hamilton communities are stable and whether they are ecologically “locked.” Toward this end, we conducted multivariate analyses (cluster and correspondence analysis) of marine faunas in 81 large samples (~300 specimens each) in shallowing-upward sequences of the Cardiff and Pecksport Members (Marcellus Subgroup, Oatka Creek Formation) of the Hamilton Group. Eight statistically and ecologically distinctive benthic communities characterize the vertical gradient, from depauperate, deeper-water dark shales below to species-rich shelf siltstones above. These communities correlate strongly with grain size, bioturbation intensity, bedding thickness, density of fossils, and faunal and ecological diversity. Species richness varies inversely with weight percent organic matter. We characterized taxonomic distributions using multivariate statistics; these statistical analyses were based on percentages of 50 taxa. In order of decreasing depth, the communities are: Cephalopod- Pterochaenia , Pterochaenia-Eumetabolotoechia , Eumetabolotoechia , Emanuella , Eumetabolotoechia-Ambocoelia , Arcuaminetes-Eumetabolotoechia , Arcuaminetes-Ambocoelia , and Mucrospirifer- Ambocoelia . The Cephalopod- Pterochaenia community represents a mixed benthic-pelagic fauna associated with the deepest and finest-grained facies. The Pterochaenia-Eumetabolotoechia , Eumetabolotoechia , and Emanuella communities have low to moderate species richness and are dominated by epifaunal, active suspension feeders, especially the small epibyssate bivalve Pterochaenia fragilis , and the pedunculate brachiopods Eumetabolotoechia multicostata and Emanuella subumbona . The Pterochaenia-Eumetabolotoechia community is an opportunistic fauna that developed when the substrate first became favorable for colonization by benthic organisms. To a lesser extent, this probably also holds true for the Eumetabolotoechia assemblage. Communities near the top of the shallowing-upward cycle— Eumetabolotoechia- Ambocoelia , Arcuaminetes-Eumetabolotoechia , Arcuaminetes-Ambocoelia , and Mucrospirifer-Ambocoelia —have higher taxonomic and ecological heterogeneity, with a more diverse array of trophic and locomotory groups than their counterparts in the finer-grained, and inferred deeper, facies. Cluster significance tests applied to all pairs of communities known from adequate numbers of samples demonstrated that the communities are statistically valid and distinctive. Multivariate means of all communities were significantly different; furthermore, most pairs of communities were drawn from populations that showed no overlap in terms of rectangular distributions. The community sequence and an ordination derived from the first two axes of the correspondence analysis provided relative depth curves. Our communities, with two exceptions, do not have clear counterparts among upper Hamilton Group faunas. The ecological locking model proposed to explain the stability of Hamilton faunas is not supported by our quantitative tests to date.

Dr. John M. Dennison spent his career studying the Appalachians; teaching and mentoring his students and professional colleagues; publishing papers; leading field trips; and presenting ideas at regional, national, and international conferences. This volume is a collection of papers contributed by former students and colleagues to honor his memory. Topics include stratigraphy and paleontology ranging in age from Ordovician to Mississippian in Kentucky, New York, Tennessee, Virginia, and West Virginia; Devonian airfall tephras throughout the eastern United States; a Devonian lonestone; a Middle Eocene bentonite in North Carolina and its relationship to a volcanic swarm in western Virginia; and a 3D model of a ductile duplex in northwestern Georgia. The stratigraphic and geologic diversity of the papers reflects Dennison's many interests and collaborative relationships.

Abstract Shelf deposition following lowstand delta building at the shelf edge has been documented for the northeast Gulf of Mexico between the Mississippi River and Apalachicola River deltas. Fifty-two vibracores, foraminiferal data, and bathymetry data have been used to detail the quartz-rich terrigenous clastic sediments that dominate the entire coastal to shelf depositional system. Because of low subsidence rates that characterize most of the study area, reworking and hydrodynamic winnowing occurred during repeated cycles of sea-level rise and fall in response to Pleistocene glaciation and deglaciation, producing sandy coastal-plain and continental-shelf deposits. Moreover, during the postglacial rise and present highstand in sea level, the eastern two-thirds of the shelf has been sediment starved, enabling additional reworking of the shelf sands during the passage of strong cold fronts and hurricanes, thus concentrating there a nearly uniform thickness of clean, multicyclic quartz sand known as the Mississippi–Alabama–Florida (MAFLA) shelf sand sheet. In the northeast Gulf of Mexico, variations in shelf width and morphology (e.g., shoals and shelf-edge deltas) are a function of glacio-eustatic changes in sea level, relative river discharge, size of drainage-basin area, and river location and frequency. Four surficial sediment types characterize the shelf: the MAFLA sand sheet, the St. Bernard prodelta deposit, the Chandeleur sand deposit, and outer-shelf carbonates. The MAFLA sand sheet dominates, covering about 75% of the shelf surface, and consists of a fine- to medium-grained quartz sand. Although relatively thin (3.5 to 5.5 m thick), the areal extent of the sand sheet is continuous and extensive (at least 400 km along strike; 60 km along dip), producing a sand volume of ∼ 7.2 × 10 10 m 3 . Furthermore, five major shoal complexes are located in the study area: Cape St. George shoals, Cape San Blas shoals, South Perdido shoal trend, North Perdido shoal trend, and St. Bernard shoals. The St. Bernard shoals consist of the Chandeleur sand deposit, whereas the other four formed within and consist of the MAFLA sand sheet. A composite stratigraphic column was compiled for the late Quaternary geology of the northeastern Gulf of Mexico shelf, which is up to 35 m thick and has seven depositional environments (Units 1–7) and five erosional surfaces. This composite stratigraphic column synthesizes the modern transgressive and highstand systems tracts preserved on the shelf. Unit 1 is a Pleistocene strandline deposit capped by a well-developed soil horizon, and represents the top of the last highstand and/or falling-stage systems tract. This unit is truncated by an erosional unconformity (Type 1 sequence boundary) produced when the entire shelf was subaerially exposed during the last sea-level lowstand, at about 18 ka. This unconformity was further eroded and reworked during the ensuing transgression to form a flooding surface (bay ravinement), thus creating a combined erosional surface (SB–FS). Overlying the sequence boundary is Unit 2, a fine-grained estuarine unit with occasional rip-up clasts and shell layers. The estuarine unit was planed off to form a regional transgressive surface of erosion (shoreface ravinement). Unit 3, a shelf sand sheet (MAFLA), sits on top of the transgressive surface of erosion and is dominated by fine-to-medium quartz sand up to 5.5 m thick with a distinctive shell bed and quartz pebbles at its base. The maximum flooding surface (MFS) overlies the MAFLA sand sheet and represents the boundary between the transgressive systems tract below (Units 2 and 3) and the highstand systems tract above (Units 4–7). Unit 4 is a prodelta deposit dominated by laminated silty clay that ranges in thickness from 12 to 16 m. The prodelta deposit grades upward into delta-front sediments (Unit 5) that are characterized by interlaminated silty sand and silty clay averaging 8 to 10 m thick. The delta-front unit is cut by channel-base diastems caused by the erosional scour of distributaries. Distributary sands (Unit 6) are 4 to 7 m thick and tend to be oriented shore-normal. Together, Units 4, 5, and 6 are coarsening-upward, shallow-water deltas, and represent parasequences within the modern highstand systems tract. In the western part of the study area, the shallow-water deltas overlie the MAFLA sand sheet, the result of the eastward progradation of the St. Bernard delta complex of the Mississippi River. The deltaic avulsion process (autocyclic) caused the St. Bernard delta complex to become abandoned, thus creating a local transgressive surface of erosion (parasequence boundary). The composite section is capped by a shelf sand shoal (Unit 7), which is a retrogradational parasequence up to 3.5 m thick within the modern highstand systems tract. The MAFLA sand sheet serves as an actualistic modern-day analog for shallow marine sandstones deposited under regional transgression in the ancient sedimentary record. These sandstones—commonly known as “transgressive lags” or “sheet sands”—are poorly documented with respect to sedimentary characteristics of recognition, stratigraphic framework, and reservoir architecture. This study provides additional insight to the geologic characterization of shelf sand sheets. Transgressive shelf sandstones can be significant hydrocarbon reservoirs in certain sedimentary basins of North America and elsewhere. This study offers examples that suggest that the preservation and resource potential of transgressive shelf sandstones commonly are misinterpreted in reconstructions of ancient sedimentary successions.

Abstract Plots of the cumulative aggradation of cyclically repetitive strata were generated by a method similar to that for Fischer plots. Plots were generated for three outcrop sections of the upper part of the Upper Ordovician Juniata Formation, eleven wells penetrating the Juniata Formation, and four wells penetrating the entire Ordovician System. The resultant plots are inferred to represent relative changes in sea level for each locality over the time interval represented. Cycles of relative sea-level change with a periodicity of approximately 5 my are apparent throughout the Ordovician. These cycles are superimposed upon a larger scale relative sea-level curve that reflects the sea-level pattern associated with the Creek holostrome (Tippecanoe I sequence). Each of these sea-level cycles can be correlated from well to well and, because of their regional extent, are possibly the effect of an eustatic cause. Other relative shallowing and deepening events are apparent in some of the wells, and at various stratigraphic positions. These local events are interpreted as tectonic uplifts and downwarps. Many of these localized sea-level events are more long-lived than the regional 5-my sea-level signal, but have a shorter duration than the Creek holostrome.