44. ON THE JURASSIC ATLANTIC OCEAN AND A SYNTHESIS OF RESULTS OF DEEP SEA DRILLING PROJECT LEG 761 Felix M. Gradstein, Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada, B2Y 4A2 and Robert E. Sheridan, Department of Geology, University of Delaware, Newark, Delaware INTRODUCTION SITES 533 AND 534—SUMMARY OF In this Initial Report of the Deep Sea Drilling Proj- ACCOMPLISHMENTS ect, detailed studies of Sites 533 (gas hydrates) on the Hydraulic piston coring, rotary coring, and deploy- Blake Outer Ridge and 534 (oldest ocean history) in the ment of the heat flow-pore water sample and pressure Blake-Bahama Basin have provided answers to many core (PCB) barrel to a depth of 399.5 m at Site 533 geological and geophysical questions posed over the de- (Figs. 1, 2) on the Blake Outer Ridge has given us: cade that deep drilling has been undertaken in this part (1) A stratigraphically continuous record of middle of the western North Atlantic. The history of drilling Pliocene-Holocene gray green mudstones used by Moul- and a historical review of key scientific accomplish- lade (this volume) and Blanc-Vernet (this volume) to ments have been presented in the Introduction (Grad- evaluate sedimentation rate, climatic fluctuations, and stein and Sheridan, this volume). In this final chapter deep circulation. we review highlights of new geological, geophysical and (2) A temperature gradient of 5.1°C/100 m near the paleoceanographic interpretations presented in this vol- seafloor to 3.6°C/100 m at the bottom of the hole, ume, and offer a critical review of this information. We which agrees with the prediction that the strong bottom conclude with a listing of some outstanding problems simulating reflector at 0.60 s is the result of gas hydrate and recommendations for future research, including inversion. data collection. (3) Direct observation of a few thin beds (3.5 cm) of The following subjects are addressed: (1) summary of gas hydrate at 240 m sub-bottom, confirming that gas Sites 533 and 534—litho-, bio-, magneto-, and seis- hydrates exist in the Blake Outer Ridge sediments, but mostratigraphic interpretations (Figs. 1-3); (2) the quest for the oldest oceanic sediments (Fig. 4); (3) the age of basement at Site 534; (4) the Jurassic time scale and a stratigraphic synthesis chart for the western North At- lantic (Figs. 5, 6); (5) implications for early spreading rates and the age of the Blake Spur Marine Magnetic Anomaly (BSMA) (Fig. 7). (6) implication of the age of the Blake Spur Marine Magnetic Anomaly (BSMA) to the North American margin; (7) pulsation tectonics; (8) 160 m.y. of subsidence and sedimentation and the con- troversy around the Cenomanian-Miocene gap in the ocean record (Figs. 9, 10); (8) sediment accumulation at Site 391 and at Site 534, as well as Late Cretaceous to Paleogene deposition and erosion, or nondeposition in the Blake-Bahama Basin; (9) Jurassic paleogeography and paleocirculation (Fig. lla-c); (10) black shales de- positional models and hydrocarbon source potential (Fig. 12); (11) seismic stratigraphy of the Blake-Bahama Basin; (12) origin of the mass flow deposits of the Mio- cene Great Abaco Member and the Eleuthera Fan com- plex; (13) gas hydrates on the Blake Outer Ridge; (14) heat flow in the Blake-Bahama Basin; (15) contourite formation of the Blake Outer Ridge; and (16) recom- mendations for future drilling and research. Figure 1. Location of Site 533, on the Blake Ridge, and Sites 534 and 391, in the Jurassic Marine Magnetic Quiet Zone relative to the marine magnetic Anomalies M-25 and the Blake Spur. (BSMA Sheridan, R. E., Gradstein, F. M., et al., Init. Repts. DSDP, 76: Washington (U.S. = Blake Spur Magnetic Anomaly and ECMA = East Coast Mag- Govt. Printing Office). netic Anomaly.) 913 F. M. GRADSTEIN, R. E. SHERIDAN possibly only in small lenses. There is at present no satis- Kimmeridgian, and Tithonian, as recognized in a pelag- factory chemical model with which to relate the low ic biofacies. It also has significantly increased our un- methane generation in Site 533 to the gas hydrate pres- derstanding of biota distribution and oceanic paleoenvi- ence, which requires higher CH4 concentrations than ronment in the Jurassic. were observed (Claypool and Threlkeld, this volume). (6) Continuous coring of Jurassic strata that has led There is uncertainty about how the amounts of gas hy- to recovery of basal sediments, classified here as of pre- drates present in the hydrated sediments produce the Cat Gap Formation character and age. It should be acoustic properties of slightly higher seismic velocities pointed out here that Ogg et al., (this volume) have pro- and apparent transparency. posed a change in the definition of the Upper Jurassic (4) Indirect evidence of the presence of gas hydrate Cat Gap Formation as observed at Site 534. The new that comes from PCB retrievals in which the pressure de- definition (Fig. 3) now includes the dark variegated clay- cline survey shows the sawtooth pattern characteristic stones and limestones of Subunits 7a and b (Cores of hydrate decomposition (Kvenvolden and Barnard, this 119-111) below the micritic-bioclastic limestone and volume; Brooks et al., this volume). greenish gray clays in Subunit 6b (Cores 110-104) and (5) Molecular and isotopic ratios of gas samples from the variegated (reddish) claystones of Subunit 6a in the the hydrates and hydrated sediments that indicate a lo- Cat Gap Formation. This expanded section, which has cal biogenic source for the methane gas, the predomi- only been observed at Site 534, now places all of the Ju- nant gas constituent. There appears no migration of gas rassic sediments above seismic Horizon D in the Cat from deeper horizons, derived from thermal maturation. Gap Formation. The silty radiolarian-rich claystones (6) A lack of obvious current-derived visual struc- and pelletal limestones below Core 119 and older than tures in the calcareous clay and mud makes it difficult to seismic Horizon D make up the basal, unnamed oceanic identify the Blake Outer Ridge sediments as contourites lithostratigraphic unit. This new definition makes the (Site 533 report, this volume). turbiditic limestones forming Horizon D an integral Figure 2 (also in the Site 533 report) shows the compo- part of the Cat Gap Formation. We are somewhat hesi- site core, biostratigraphic, and geochemical and physical tant to accept redefinition of oceanic formations based properties record at Site 533. on single site criteria, without proof of mappability, but The composite stratigraphy of Site 534 is shown in we do show the new subdivision in Figure 3. Future Figure 3. This site, in the Blake-Bahama Basin, was con- drilling in the western North Atlantic will have to sub- tinuously cored from 545 to 1666 m sub-bottom (56% stantiate this lithologic record on a regional scale. recovery). The sediment record spans 160 m.y. of ocean (7) Better dating of the Jurassic-Neocomian geomag- history and cored the oldest (middle Callovian) oceanic netic record (J. Ogg, this volume) between M-24 and sediments yet recovered. The Site 534 report and a large M-26. Seismic Horizon D has been shown to be early number of specialty chapters in this volume deal with Oxfordian (approximately equivalent in age to Anomaly this excellent geological and geophysical record and in- M-27), and Horizon C has been confirmed to occur at clude, among other data: the Jurassic/Cretaceous boundary (M-18-M-19) (Fig. 3). (1) A Miocene record of extensive debris flow, grain (8) Recovery of Callovian shales on ocean crust-type flow, and turbidite deposition that can be traced to part basalt (Logothetis, this volume). These shales represent of a deep-sea fan complex, funneling out of the Bahama the oldest recovered sediments used to document ocean- Canyons. ic spreading. (2) Evidence for hiatuses and nondeposition in the (9) Recovery of the first samples of Jurassic Magnetic Late Cretaceous as well as in the Paleogene. Slow depo- Quiet Zone oceanic basaltic basement. Steiner (this vol- sition coupled with nondeposition, rather than rapid de- ume) finds this Quiet Zone crust to be of uniform mag- position followed by drastic erosion in a single Oligo- netic susceptibility and constant polarity, which leads to cene event, may have caused the 60-m.y. gap in the rec- the assumption that the Jurassic Magnetic Quiet Zone is ord. truly quiet. (3) A complete section of Cretaceous black shale that has been more closely sampled and better dated than in THE QUEST FOR THE OLDEST any previous studies. OCEANIC SEDIMENTS (4) Documentation of a more shaly upper section of The principal objective of Site 534, Hole 534A was to the Neocomian Blake-Bahama Formation revealing a fa- extend significantly our knowledge of the Jurassic At- des change with respect to nearby Site 391. This facies lantic Ocean beyond what was known from earlier drill- change, related to increased influx of terrigenous com- ing during Legs 1,11, and 44 in the western North At- ponents toward land, occurs all along the North Ameri- lantic and Legs 41 and 50 in the eastern half, off north- can margin. west Africa. Prior to 1980 no ocean site had penetrated (5) Dating of an excellent Middle and Upper Jurassic rocks much below seismic Horizon C, (i.e., older than pelagic record of variegated shales, radiolarian silts, tur- the Late Jurassic) or closer to the continental margin biditic limestones, and reddish clays. The multiple bio- than M-25. In a nutshell this history is depicted in Fig- stratigraphic and paleoecologic studies (see specialty ure 4. Sites 100 and 105 (Leg 11, Hollister, Ewing, et al., chapters in this volume and Site 534 report) greatly in- 1972) and Site 367 (Leg 41, Lancelot, Seibold, et al., crease our insight into zonal correlation problems con- 1978) were drilled within the area of Anomaly M-25, or cerning stage boundaries of the Callovian, Oxfordian, landward of the edge of the so-called Jurassic Magnetic 914 SYNTHESIS OF RESULTS Quiet Zone (JQZ), which is devoid of high-amplitude not have been drilled.