Organic Geochemistry of Cretaceous Black Shales and Adjacent Strata from the Galicia Margin, North Atlantic Ocean

Abstract

Organic-carbon-rich "black shales" and adjacent organic-carbon-poor rocks from three different Cretaceous settings encountered during ODP Leg 103 have been studied by organic geochemical methods. Rock-Eval analysis, carbon isotope data, and lipid biomarkers show organic matter to contain varying proportions of marine and continental materials. In Hauterivian-Barremian organic-carbon-rich marlstone turbidites, large amounts of land-derived organic matter are found. Aptian-Albian black-colored shales are interspersed within green claystones, from which they differ by containing more marine organic matter. An abbreviated layer of black shale from the Cenomanian/Turonian boundary is dominated by well-preserved marine organic matter. Downslope transport and rapid reburial within a predominantly oxygenated deep-water setting created most of these examples of black shales, except for the Cenomanian-Turonian deposits in which deep-water anoxia may have been involved.

The Late Hauterivian Faraoni ‘Oceanic Anoxic Event’: an update

“…Although the drilling recovery was not complete, organic-rich layers were recorded in the upper Hauterivian marly-limestone deposits off Morocco (DSDP site 370) and along the Iberian margin (DSDP site 398 and ODP site 638 [Dunham et al, 1988]). These black shales are related to a minor positive carbon-isotope excursion [Arthur et al, 1979;Renard et al, 1979].…”

Geochemical comparisons of organic matter in Cretaceous black shales from Site 897, Iberia Abyssal Plain, Sites 638 and 641, Galicia Margin, and Site 398, Vigo Seamount

“…and Stein et al, (1988) note that the vitrinite contents of the Lower Cretaceous black shales have a broad range of reflectance values, indicating that the organic matter is indeed a mixture of primary and recycled material. Some black shales, particularly the Cenomanian samples from Site 641, have relatively high HI values (Dunham et al, 1988). These samples show that deposition of black shales rich in marine organic matter occurred on this margin during the Cenomanian-Turonian Boundary Event (CTBE) (Thurow et al, 1988).…”

“…Organic carbon isotopic ratios are useful to distinguish between marine and continental plant sources of sedimentary organic matter. Most photosynthetic plants incorporate carbon into organic matter using the C 3 Calvin pathway which biochemically discriminates Erdman and Schorno (1979) and Dunham et al (1988).…”

“…Carbon isotope ratios can be affected, however, by photosynthetic dynamics and by postdepositional diagenesis (Dean et al, 1986;McArthur et al, 1992) and consequently must be interpreted cautiously. Dunham et al (1988) and Erdman and Schorno (1979) provide the results of organic δ 13 C determinations of black shales from the Galicia Margin and Vigo Seamount, respectively. The values range widely between -21.5‰ to -28.5‰ (Fig.…”

The drowning of a siliciclastic shelf: insights into oceanographic reconstructions of the northern Arabian Platform during the Early Cretaceous

“…Both of these mechanisms would be more effective earlier during the transgression than during later phases, as both shore distance increases through time and the initial erodible material will be hence consumed. Interestingly, a similar concurrent increase in MTDs was reported from offshore Galicia (ODP core 641C) (Dunham et al ., ; Haggerty & Germann, ). In this case, phases of thick MTDs were identified as punctuating a finely laminated, fine grained organic‐rich pelagic sediments.…”