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

Abstract

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.

Organic facies variations in the Valanginian-mid-Hauterivian interval of the Agrio Formation (Chos Malal Area, Neuquén, Argentina): local significance and global context

“…Summerhayes & Masran (1983, p. 475) also observe that 'sediments first became commonly laminated in the Valanginian... as sea level was rising' (also Lini et al 1992, p. 380). Wise et al (1986) find that the late Valanginian sees the first occurrence of 'black to dark grey carbonaceous claystone' interbeds at DSDP Site 603B in the NW Central Atlantic; these beds form up to 33% by thickness during the Valanginian and Hauterivian, but are interpreted as distal organic-rich turbidites, perhaps redeposited from an up-slope oxygen minimum zone (Wise et al 1986, p. 41). Waples (1983, p. 969) considers that (initially scattered) definitive evidence of true deep-basin anoxia in the North Atlantic does not occur until at least the late Hauterivian; however, Summerhayes & Masran (1983, p. 476) note that on a carbonatefree basis, maximum TOC values actually increase progressively from the Berriasian to the Hanterivian (as sea level rises), reaching values of up to 6-10% (but mostly less than 2%).…”

Mesozoic–Cenozoic Depositional Environments Revealed by Deep Sea Drilling Project Leg 93 Drilling on the Continental Rise off the Eastern United States: Cruise Summary

“…These distorted blocks could denote slumped overbank deposits, which in turn would indicate the presence of well-defined fan channels. As noted by Wise et al (1986), similar features were found to be characteristic of cores taken at overbank Sites 617 and 620 in the middle Mississippi fan, where high sedimentation rates promoted locally unstable slopes (Coleman et al, 1985).…”

“…It was to narrow this data gap that DSDP Leg 93 drilled a series of deep holes on both the lower and upper continental rise, a task which was continued by DSDP Leg 95. Our Leg 93 team results, which have been made available as expeditiously as possible (see Leg 93 Staff, 1983;Leg 93 Scientific Party/Leg 94 Scientific Party, 1984;von Rad et al, 1984;van Hinte et al, 1985a, b;Wise et al, 1986), have expanded our understanding of the geologic history and the processes which have shaped the North Atlantic continental rise. Four holes were attempted, but no penetration was achieved at Hole 603A (see Table 1).…”

“…As noted by Wise et al (1986), paleontologic dating of the sands at the top of Subunit VA initially presented special difficulties, particularly when applying previous calcareous nannofossil zonations developed for this area. First, turbidites may have displaced older microfossils into younger units; secondly, the section was possibly contaminated through caving associated with the drilling process, and third, marker species might be mimicked by previously undescribed taxa.…”

“…No other microfossil group was sufficiently well represented in these deep-water sediments to provide reliable age dates; however, data points were sufficiently numerous to allow Ogg (this volume) to recalibrate portions of the Hauterivian/Barremian magnetostratigraphic column. Wise et al (1986) considered several possible interpretations for the massive, unconsolidated sands at the top of Unit VA. One possibility, first suggested by JEVH aboard ship and promulgated by van Hinte et al (1985a), is that they may have been emplaced as the result of the predicted eustatic sea-level drop that had been dated as mid to late Aptian in age (Vail et al, 1977;Vail and Mitchum, 1979;Vail et al, 1980). These sands would then represent what Vail et al (1984) describe as a Type I unconformity event.…”