C. Wylie Poag

A record of Appalachian denudation in postrift Mesozoic and Cenozoic sedimentary deposits of the U.S. Middle Atlantic continental margin

Abstract The complex interplay between source-terrain uplift, basin subsidence, paleoclimatic shifts, and sea-level change, left an extensive sedimentary record in the contiguous offshore basins of the U.S. middle Atlantic margin (Salisbury Embayment, Baltimore Canyon Trough, and Hatteras Basin). Isopach maps of 23 postrift (Lower Jurassic to Quaternary) a allostratigraphic units, coupled with a revised stratigraphic framework, reveal that tectonism, by regulating sediment supply (accumulation rate), dominated the interplay of forcing mechanisms. Tectonic pulses are evidenced by abruptly accelerated sediment accumulation, marked latitudinal shifts in the location of depocenters, and regional changes in lithofacies. Relatively rapid tectonic subsidence during the Early and Middle Jurassic history of the basins may have enhanced sediment accumulation rates. Beginning in the Late Jurassic, however, subsidence rates decreased significantly, though occasional short pulses of subsidence may have effected relative sea-level rises. Sea-level change heavily influenced the distribution and redistribution of sediments one they reached the basins, and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins. We conclude that source terrains of the central Appalachian Highlands were tectonically uplifted, intensely weathered, and rapidly eroded three times since the Late Triassic: (1) Early to Middle Jurassic (Aalenian to Callovian); (2) mid-Early Cretaceous (Barremian); and (3) Late Cenozoic (Middle Miocene). Intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces, geomorphic features commonly reported to characterize the central Appalachian Highlands. This series of three, irregularly spaced, tectonic/quiescent cycles does not, however, match the traditional four-cycle concept of post-Triassic Appalachian “peneplanation”.

U.S. Geological Survey Core Drilling on the Atlantic Shelf

The first broad program of scientific shallow drilling on the U.S. Atlantic continental shelf has delineated rocks of Pleistocene to Late Cretaceous age, including phosphoritic Miocene strata, widespread Eocene carbonate deposits that serve as reflective seismic markers, and several regional unconformities. Two sites, off Maryland and New Jersey, showed light hydrocarbon gases having affinity to mature petroleum. Pore fluid studies showed that relatively fresh to brackish water occurs beneath much of the Atlantic continental shelf, whereas increases in salinity off Georgla and beneath the Florida-Hatteras slope suggest buried evaporitic strata. The sediment cores showed engineering properties that range from good foundation strength to a potential for severe loss of strength through interaction between sediments and man-made structures.

Meteoroid mayhem in Ole Virginny: Source of the North American tektite strewn field

New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, peak-ring impact crater, carved through upper Eocene to Lower Cretaceous sedimentary strata and into underlying pre-Mesozoic crystalline basement rocks. A polymictic, late Eocene impact breccia, composed mainly of locally derived sedimentary debris (determined from four continuous cores), surrounds and partly fills the crater. Structural and sedimentary characteristics of the Chesapeake Bay crater closely resemble those of the Miocene Ries peakring crater in southern Germany. We speculate that the Chesapeake Bay crater is the source of the North American tektite strewn field.

Geologic evolution of Atlantic continental rises

Part I: Prerift and synrift evolution: Galicia continential margin: Constraints on formation of non-volcanic passive margins Southwest Africa plate margin: Thermal history and geodynamical implications Part II: Early postrift evolution: Antarctic continental margin: Geologic image of the Bransfield Trough, an incipient oceanic basin Southwestern Africa continental rise: Structural and sedimentary evolution Angola basin: Geohistory and construction of the continental rise U.S. Middle Atlantic continental rise: Provenance, dispersal, and deposition of Jurassic to Quaternary sediments Norweigan Continental margin: Structural and stratigraphical styles Part III: Late postrift evolution: Southern Brazil basin: Sedimentary processses and features and implications for continental rise evolution Southeastern New England continental rise: origin and history of slide complexes Western Nova Scotia continental rise: Relative importance of mass wasting and deep boundary-current activity Guinea and Ivory Coast-Ghana transform margins: Combined effects of synrift structure and postrift bottom currents on evolution and morphology Northwest African continental rise: Effects of near-bottom processes inferred from high-resolution seismic data Saharan continental rise: Facies distribution and sediment slides Part IV: Synthesis Index.

The Chesapeake Bay bolide impact: A convulsive event in Atlantic Coastal Plain evolution

Abstract Until recently, Cenozoic evolution of the Atlantic Coastal Plain has been viewed as a subcyclical continuum of deposition and erosion. Marine transgressions alternated with regressions on a slowly subsiding passive continental margin, their orderly succession modified mainly by isostatic adjustments, occasional Appalachian tectonism, and paleoclimatic change. This passive scenario was dramatically transformed in the late Eocene, however, by a bolide impact on the inner continental shelf. The resultant crater is now buried 400–500 m beneath lower Chesapeake Bay, its surrounding peninsulas, and the continental shelf east of Delmarva Peninsula. This convulsive event, and the giant tsunami it engendered, fundamentally changed the regional geological framework and depositional regime of the Virginia Coastal Plain, and produced the following principal consequences. (1) The impact excavated a roughly circular crater, twice the size of Rhode Island (∼6400 km2) and nearly as deep as the Grand Canyon (∼1.3 km deep). (2) The excavation truncated all existing ground-water aquifers in the target area by gouging ∼4300 km3 of rock from the upper lithosphere, including Proterozoic and Paleozoic crystalline basement rocks and Middle Jurassic to upper Eocene sedimentary rocks. (3) Synimpact depositional processes, including ejecta fallback, massive crater-wall failure, water-column collapse, and tsunami backwash, filled the crater with a porous breccia lens, 600–1200 m thick, at a phenomenal rate of ∼1200 m/hr. The breccia lens replaced the truncated ground-water aquifers with a single 4300 km3 reservoir, characterized by ground water ∼1.5 times saltier than normal sea water (chlorinities as high as 25,700 mg/l). (4) A structural and topographic low, created by differential subsidence of the compacting breccia, persisted over the crater at least through the Pleistocene. In the depression are preserved postimpact marine lithofacies and biofacies (upper Eocene, lower Oligocene, lower Miocene) not known elsewhere in the Virginia Coastal Plain. (5) Long-term differential compaction and subsidence of the breccia lens spawned extensive fault systems in the postimpact strata. Many of these faults appear to reach the bay floor, and may be potential hazards for motion-sensitive structures in population centers around Chesapeake Bay. Near-surface fracturing and faulting generated by the impact shock may extend as far as 90 km from the crater rim. (6) Having never completely filled with postimpact sediments, the sea-floor depression over the crater appears to have predetermined the location of Chesapeake Bay. (7) As large impact craters are principal sources for some of the worlds precious metals, it is reasonable to expect that metal-enriched sills, dikes, and melt sheets are present in the inner basin of the crater. In addition to these specific consequences, the crater and the convulsive event that produced it, have widespread implications for traditional interpretations of certain structural and depositional features of the Atlantic Coastal Plain, particularly in southeastern Virginia.

Deep Sea Drilling Project Site 612 bolide event: New evidence of a late Eocene impact-wave deposit and a possible impact site, U.S. east coast

A remarkable >60-m-thick, upward-fining, polymictic, marine boulder bed is distributed over >15,000 km[sup 2] beneath Chesapeake Bay and the surrounding Middle Atlantic Coastal Plain and inner continental shelf. The wide varieties of clast lithologies and microfossil assemblages were derived from at least seven known Cretaceous, Paleocene, and Eocene stratigraphic units. The supporting pebbly matrix contains variably mixed assemblages of microfossils from the same seven stratigraphic units, along with trace quantities of impact ejecta (tektite glass and shocked quartz). The youngest microfossils in the boulder bed are of early-late Eocene age. On the basis of its unusual characteristics and its stratigraphic equivalence to a layer of impact ejecta at Deep Sea Drilling Project (DSDP) Site 612 (New Jersey continental slope), the authors postulate that this boulder bed was formed by a powerful bolide-generated wave train that scoured the ancient inner shelf and coastal plain of southeastern Virginia. The most promising candidate for the bolide impact site (identified on seismic reflection profiles) is 40 km north-northwest of DSDP Site 612 on the New Jersey outer continental shelf.

The Goban Spur transect: Geologic evolution of a sediment-starved passive continental margin

Leg 80 of the DSDP-IPOD program drilled a transect of four core holes (548–551) across the continent-ocean boundary at Goban Spur, a prominent southwest-trending structural high on the Irish continental slope. Multichannel seismic-reflection profiles show that, during rifting, continental basement rocks of Goban Spur were broken up by northwest-trending listric normal faults to form a series of half-graben basins. Two of these half-grabens were sampled during Leg 80 (Sites 548 and 549). Site 550 was located on the adjacent oceanic crust of Porcupine Abyssal Plain. Site 551 was located on transitional crust at the foot of Goban Spur. The objectives were lo analyze the structural, the depositional, and the paleoenvironmental development of this sediment-starved passive continental margin. At Sites 548 and 549, basement comprises continental Hercynian metasediments of Devonian age; at Sites 550 and 551, the basement is tholeiitic basalt. The oldest syn-rift sediments (Barremian age, or perhaps late Hauterivian) were penetrated at Site 549, lying uuconformably below Aptian? strata. Seismic sequence analysis reveals that Aptian? strata also overlie this unconformity farther northeastward in the basin. An unconformity above the Aptian? section marks the end of rifting and the beginning of sea-floor spreading. An Albian age for the initiation of sea-floor spreading was corroborated at Site 550 where abyssal late Albian chalks rest upon and are interbedded with oceanic basalts, indicating an initial water depth of ∼2,000 m. As sea-floor spreading progressed, Goban Spur subsided rapidly, so that by Cenomanian time, bathyal to abyssal chalks were accumulating at deeper sites. After two periods of partial stagnation in the Aptian-Albian and in the Turonian, chalk deposition in well-oxygenated environments took place at all sites, modified chiefly by shifts in deep-circulation patterns and the calcite compensation depth (CCD), by periodic influx of terrigenous detritus during low stands of sea level (especially in the Cenozoic), and by frequent displacement of older carbonates from the slope to abyssal sites. A number of major unconformities correspond to those most often reported from other widespread locations in the North Atlantic Basin and on surrounding continental shelves and coastal plains. Several unconformities are preserved undisturbed in our cores and can be correlated with sea-level fluctuations, with paleoceanographic events, and with tectonic movements. A thick Quaternary section at Site 548 records prominent fluctuations of glacial-inter-glacial paleoclimates. An even thicker Paleogene section at Site 549 provides unusually well-preserved and uninterrupted sequences suitable for detailed sedimentological and stratigraphic studies.

Episodic post-rift subsidence of the United States Atlantic continental margin

Sediment thickness, paleobathymetry, and chronostratigraphy from COST wells offshore from Georgia and New Jersey indicate periods of rapid subsidence superimposed on the slower thermal subsidence of the continental margin. Rapid subsidence occurred during the Coniacian-Santonian, the Eocene and, in the COST wells off New Jersey, since the end of early Miocene. Once the maximum effects of water and sediment loading, compaction, and thermal cooling are removed, the residual vertical movements caused by tectonic and sea-level fluctuations can be analyzed. Because no global sea-level change can account for all residual movements, we propose that tectonism, variously amplified by loading, is responsible for the observed episodes of rapid subsidence. Synchroneity of subsidence with sea-floor–spreading changes in the North Atlantic suggests a unified cause for these events. Recognition of episodic subsidence may have implications for fault timing, petroleum potential, and global sea-level effects on passive margins.

Rise and demise of the Bahama-Grand Banks gigaplatform, northern margin of the Jurassic proto-Atlantic seaway

Abstract An extinct, > 5000-km-long Jurassic carbonate platform and barrier reef system lies buried beneath the Atlantic continental shelf and slope of the United States. A revised stratigraphic framework, a series of regional isopach maps, and paleogeographic reconstructions are used to illustrate the 42-m.y. history of this Bahama-Grand Banks gigaplatform from its inception in Aalenian(?) (early Middle Jurassic) time to its demise and burial in Berriasian-Valanginian time (early Early Cretaceous). Aggradation-progradation rates for the gigaplatform are comparable to those of the familiar Capitan shelf margin (Permian) and are closely correlated with volumetric rates of siliciclastic sediment accumulation and depocenter migration. Siliciclastic encroachment behind the carbonate tracts appears to have been an important impetus for shelf-edge progradation. During the Early Cretaceous, sea-level changes combined with eutrophication (due to landward soil development and seaward upwelling) and the presence of cooler upwelled waters along the outer shelf appear to have decimated the carbonate producers from the Carolina Trough to the Grand Banks. This allowed advancing siliciclastic deltas to overrun the shelf edge despite a notable reduction in siliciclastic accumulation rates. However, upwelling did not extend southward to the Blake-Bahama megabank, so platform carbonate production proceeded there well into the Cretaceous. Subsequent stepwise carbonate abatement characterized the Blake Plateau Basin, whereas the Bahamas have maintained production to the present. The demise of carbonate production on the northern segments of the gigaplatform helped to escalate deep-water carbonate deposition in the Early Cretaceous, but the sudden augmentation of deep-water carbonate reservoirs in the Late Jurassic was triggered by other agents, such as global expansion of nannoplankton communities.

Middle to late Miocene canyon cutting on the New Jersey continental slope: Biostratigraphic and seismic stratigraphic evidence

We have identified and dated a major Miocene erosional surface (M1) on the New Jersey continental slope. This surface was penetrated at Deep Sea Drilling Project (DSDP) Site 612, which was drilled near the thalweg of a buried V-shaped canyon. Biostratigraphic data at Site 612 firmly constrain the age of strata above the buried canyon surface as Zones CN7 (=NN9) and N16 (lowermost upper Miocene); the upper Miocene surface at Site 612 lies above lowermost Oligocene strata because of coalesced unconformities. We traced the M1 erosional surface to the COST B-3 well where upper middle Miocene strata underlie it. Biostratigraphic studies of other New Jersey continental slope boreholes (ASP 14, ASP 15) suggest that elsewhere the sediments immediately below the M1 surface encompass the Globorotalia fohsi robusta Zone (= Zone N12-earliest N13; middle middle Miocene). The best age estimate is that M1 was eroded between 11.5 and 10.0 Ma. This erosional event apparently correlates with a similar event on the Irish and Florida continental margins and with oxygen-isotope evidence for a glacio-eustatic lowering.