Mesozoic-Cenozoic clastic depositional environments revealed by DSDP Leg 93 drilling on the continental rise off the eastern United States

Summary Prior to Deep Sea Drilling Project (DSDP) Leg 93 (1983), drill data along the continental rise of the Atlantic margin of the United States were quite limited compared to those of the adjacent continental shelf and the deeper, more seaward expanses of the North American Basin. Interpretations of the geologic history and of processes that controlled sedimentation along the rise were strongly dependent on studies of seismic reflection profiles. DSDP Leg 93 drilled deep holes on both the lower and upper rise, allowing correlation with commercial wells on land and offshore (as well as with subsequent DSDP Leg 95 holes along the ‘New Jersey Transect’) and providing the first down dip suite of drill holes across a passive continental margin from the coastal plain to the abyssal plain. Site 603 on the lower rise 270 miles east of Cape Hatteras was cored nearly continuously over 1585 m to Berriasian pelagic limestones. It intersected an extensive Lower Cretaceous deep-sea fan complex which provides new information on the petroleum potential of the rise. Hauterivian to early Aptian in age, this 208 m interval of interbedded limestones, sand and blackshale turbidites calls into question the existence of any post Valanginian reefs along the Baltimore Canyon Trough. Less extensive terrigenous turbidites were encountered as far up in the section as the Cretaceous-Tertiary (K-T) boundary. The K-T boundary is marked by a current-laminated sand rich in dark, 1 mm diameter spherules which may denote an extraterrestrial impact event. DSDP Sites 604 and 605 on the upper rise, the first along the ‘New Jersey Transect’, are located some 100 miles south-east of Atlantic City, New Jersey. Hole 605, drilled 816.7 m down to mid-Maestrichtian limestones, penetrated a near complete Cretaceous-Tertiary boundary section, above which 20 m of lower Palaeocene are separated by a disconformity from an expanded 175 m Palaeocene sequence. Terrigenous silts and glauconite at the K-T boundary and immediately above suggest either significant sea-level change, increased current erosion along the adjacent shelf and slope, increased terrigenous input caused by decreased vegetation, a high energy event (tsunami?), or some combination of these possible factors. Site 604, 3 miles seaward of Site 605, was terminated by unstable hole conditions at 294.5 m within a unit of Miocene glauconitic sands and debris flows. Emplaced largely during the Tortonian (8.2–10.0 Ma; Vail cycle TM3.1), these upper Miocene sediments contain shelf-derived gravels, exotic blocks of Eocene chalk (up to 50 cm across) eroded from the adjacent slope, and clasts of middle and upper Miocene carbonates or silts derived from canyon walls or shallow water strata upslope. Study of closely spaced, high resolution seismic profiles suggests that large-scale regional erosion (canyon cutting), which is related to the debris flows, began during the late middle Miocene. On the lower rise, turbiditic silts and clays began to accumulate rapidly during the middle Miocene. Under the influence of a strengthening Western Boundary Undercurrent, these were deposited as muddy contourites in antidune-like sediment waves which, at site 603, grew rapidly with no appreciable break in sedimentation until at least early Pleistocene times to form the present Lower Continental Rise Hills of the Hatteras Outer Ridge (HOR). The somewhat elevated edge of the Lower Continental Rise Terrace formed as a natural levee behind which the coarser portions of the terrigenous turbidites were ponded to form the terrace. No coarse clastics that bypassed the pond were deposited with the clays of the HOR at this locality. Throughout the study, seismic sequence boundaries of the upper and lower continental rise were calibrated and correlated with continental margin unconformities as well as with deep sea reflection horizons.