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The geochemical studies of Sites 534 and 391 and their comparison allow us to improve the chemical characterization of different geological formations dating from the early Callovian to the Maestrichtian along the continental margin of eastern North America. Three of the formations are favorable for the preservation of organic matter: (1) the unnamed formation (middle Callovian to Oxfordian), (2) the Blake-Bahama Formation (Berriasian to Barremian), and (3) the Hatteras Formation (Aptian to Cenomanian). The organic matter is mainly detrital, except for a few organic-rich layers where a contribution of aquatic material occurs. In these organic-rich layers, the petroleum potential is medium to good. Maturation has not quite reached the beginning of the oil window even for the deepest organic material. INTRODUCTION Hole 391C of Leg 44 and nearby Hole 534A of Leg 76 (Fig. 1), penetrate the same geological formations. We studied 33 samples of the latter. They came from the following formations (Fig. 2): Great Abaco Member, Bermuda Rise Formation, Plantagenet Formation, Hatteras Formation, Blake-Bahama Formation, Cat Gap Formation, and a new unit—the unnamed formation— beneath the Cat Gap Formation. This new unit makes it possible to study the sediments deposited during the proto-oceanic stage of seafloor spreading in the Atlantic. Since the publication of data in the Leg 44 Initial Reports (Deroo et al., 1978), a large number of additional analyses of Hole 391C samples have been made in our laboratory. In order to define the geochemical characteristics of the different formations, the new data available for 92 samples from Hole 391C will also be considered here. All samples were studied with a LECO apparatus in order to investigate the amount of total organic carbon, and with a Rock Eval apparatus (Espitalié et al., 1977) to determine the petroleum generating potential, the nature of the organic material, and its stage of maturation. Furthermore, in some samples rich in organic carbon we studied the extractable compounds. The first part of this report is devoted to the results of the carbon analyses and the nature of the organic matter, the second part to the petroleum potential and maturation of the organic matter, and the third part deals with the composition of the chloroform extracts. In each part the geological order is followed for both the formations (from older to younger) and the lithological subunits (from bottom to top) within each formation. MINERAL CARBON, ORGANIC CARBON, AND TYPE OF ORGANIC MATTER IN THE DIFFERENT FORMATIONS OF HOLES 391C AND 534A Both pyrolyses and elemental analyses of kerogens enabled us to characterize the samples in relation to the different reference types of organic matter described in Tissot et al. (1974) and Espitalié et al. (1977). The samples can be assigned either to aquatic organic matter (Type II), to continental organic matter (Type III), or to residual organic matter (when they have a very low hydrogen index [H.I.] or H/C ratio) (Tissot et al., 1979). Residual organic matter has undergone strong alteration or oxidation before its deposition. Frequently, samples are located between the different reference paths and can be assigned either to Type II + residual, or to Type II + III, or to Type III + residual (see Fig. 3). Moreover, when the H.I. is very low (< 50) and the O.I. (oxygen index) very high (> 200), the organic matter is called undetermined; usually the related samples have a very low organic carbon content (0.2% and less). The origin of such an undetermined organic matter could be aquatic or continental. Unnamed Formation This Callovian to Oxfordian unit corresponds to Unit 1 (unsampled facies) of Leg 44 (Sheridan et al., 1978). We studied seven samples of this formation from Hole 534A. Three subunits were distinguished, and they are designated from bottom to top as 7c, 7b, and 7a (Fig. 2)2. Subunit 7c (Samples 534A-127,CC [10 cm] to 534A125-4, 14 cm) is dated Callovian and is composed of greenish black to olive black nannofossil claystone to carbonaceous claystone. The basal contact is located at Sheridan, R. E., Gradstein, F. M., et al., Init. Repts. DSDP, 76: Washington (U.S. Govt. Printing Office). ' Editor's note: See Site 534 report for a new subdivision of Unit 7.

Organic Geochemistry in the Mesozoic and Cenozoic Formations of Site 534, Leg 76, Blake-Bahama Basin, and Comparison with Site 391, Leg 44