rganic Facies of Cretaceous and Jurassic Sediments from Deep Sea Drilling Project Site 534 in the Blake-Bahama Basin, Western North Atlantic

Abstract

The organic facies of Early and middle Cretaceous sediments drilled at DSDP Site 534 is dominated by terrestrially derived plant remains and charcoal. Marine organic matter is mixed with the terrestrial components, but through much of this period was diluted by the terrestrial material. The supply of terrestrial organic matter was high here because of the nearness of the shore and high runoff promoted by a humid temperate coastal climate. Reducing conditions favored preservation of both marine and terrestrial organic matter, the terrestrial materials having reached the site mostly in turbidity currents or in the slow-moving, near-bottom nepheloid layer. An increase in the abundance of terrestrial organic matter occurred when the sea level dropped in the Valanginian and again in the Aptian-Albian, because rivers dumped more terrigenous elastics into the Basin and marine productivity was lower at these times than when sea level was high. A model is proposed to explain the predominance of reducing conditions in the Valanginian-Aptian, of oxidizing conditions in the late Aptian, and of reducing conditions in the Albian-Cenomanian. The model involves influx of oxygen-poor subsurface waters from the Pacific at times of high or rising sea level (Valanginian-Aptian, and Albian-Cenomanian) and restriction of that influx at times of low sea level (late Aptian). In the absence of a supply of oxygenpoor deep water, the bottom waters of the North Atlantic became oxidizing in the late Aptian, probably in response to development of a Mediterranean type of circulation. The influx of nutrients from the Pacific led to an increase in productivity through time, accounting for an increase in the proportion of marine organic matter from the Valanginian into the Aptian and from the Albian to the Cenomanian. Conditions were dominantly oxidizing through the Middle Jurassic into the Berriasian, with temporary exceptions when bottom waters became reducing, as in the Callovian. Mostly terrestrial and some marine organic matter accumulated during the Callovian reducing episode. When Jurassic bottom waters were oxidizing, only terrestrial organic matter was buried in the sediments, in very small amounts.

Synthesis of Organic Geochemical Studies, Deep Sea Drilling Project Leg 93, North American Continental Margin

“…The Cenomanian black shale samples from the Hatteras Formation generally have high hydrogen indices (Dean and Arthur; Herbin et al, Katz; Meyers; Rullkötter et al; Schaefer and Leythaeuser; all this volume), representative of marine Type II kerogen. Although most previous studies have concluded that the majority of the organic matter within western North Atlantic Cretaceous black shales is from continental sources, Summerhayes and Masran (1983) suggest that samples with higher organic carbon concentrations contain proportionately larger amounts of marine organic matter. Moreover, some Cenomanian sediments from Site 105 are unusually rich in marine organic matter (Summerhayes, 1981), and Albian-Aptian sediments from Sites 417 and 418 are dominated by marine material (Deroo et al, 1980).…”

“…Highest organic carbon values are concentrated in Cenomanian claystones, and these are significantly higher than most previously reported values from the Hatteras Formation (cf. Erdman and Schorno, 1978;Deroo et al, 1980;Summerhayes and Masran, 1983), which have 10.4% as a maximum.…”

“…Possible explanations of black shale depositional conditions are exemplified by de Graciansky et al (1982), Summerhayes and Masran (1983), and Summerhayes (in press) and combine the effects of sea-level changes, changes in continental climates, nutrient availability, and basin morphology to achieve periods of mid water or bottom anoxia in the Mesozoic North Atlantic. It is probable that these are the important factors, but de Graciansky et al (1984) caution that a simple model employing one set of conditions appears inadequate to explain the geographically broad extent of Cenomanian black shales.…”

“…Although most of the organic matter appears to be terrigenous (Katz and Pheifer, 1982), the marine fraction increases with distance from North America Summerhayes and Masran, 1983). This pattern has been explained by Summerhayes and Masran (1983) to reflect the decrease in turbiditic dilution of marine sediments with continental materials as distance from shore increases. Exceptions to this generality occur where large proportions of marine organic matter are found in Cenomanian black shales at Site 105 (Summerhayes, 1981) and in Cenomanian and Valanginian black shales at Site 603.…”

“…Anoxic bottom waters have been postulated to have enabled enhanced preservation of organic matter in the Cretaceous North Atlantic Arthur and Schlanger, 1979;Jenkyns, 1980;Summerhayes and Masran, 1983;Bralower and Thierstein, 1984; as examples). The abundant presence of burrowed, oxidized sediments above, below, and sometimes within the black shales in this ocean argues against such bottom water anoxia being extensive in either volume or duration (Katz and Pheifer, 1982;Waples, 1983).…”