Organic-rich sedimentation at the Cenomanian-Turonian boundary in oceanic and coastal basins in the North Atlantic and Tethys

Abstract

Summary One of the most striking results of the Deep Sea Drilling Project is the proof that organic-rich sediments have a widespread geographical distribution during the period from Upper Cenomanian to Middle Turonian. Such sediments were drilled at North Atlantic DSDP Sites: 105, 135, 137, 138, 367, 398, 551, 603. They are also present (from outcrops or oil wells) on the shelf of the African continental margin (Senegal, Tarfaya and Agadir Basins), in the Tethys (former Alboran Block, Southern Spain, Algeria, Tunisia and Umbrian Apennines/Italy), and in the North Sea. Although these sediments have different lithologies and depositional environments (from shelf areas to the deep sea), their study, mainly based on organic geochemistry with additional data on sedimentology, biostratigraphy and palaeo-bathymetry, suggests that a unique ‘pulse’ of organogenic accumulation characterizes the Cenomanian/Turonian Boundary Event (CTBE). The content and type of organic matter are related to the depositional environment and organic preservation. The organogenic accumulation is distributed according to various trends. Off the African continental margin the organic content increases from onshore areas to the shelf (Casamance area), and, moreover, increases also in deep sea areas, with a gradual transition from terrestrial type III to marine type II (the best preservation of the organic matter being in the deepest areas, i.e. Site 367). Off the American continental margin Site 603 shows the same TOC and type of organic matter as at Site 105. The CTBE is also well recorded in the northern part of the Atlantic (Celtic margin, North Sea) by a drastic lithological change (black shales within chalks), but the type of organic matter is mainly terrestrial. In the Tethyan area the organic matter is of marine origin and well preserved. Results are compared with those of Pratt (1984) from the Western Interior Basin of the USA. Different hypotheses to explain this synchronous widespread accumulation of organic matter are discussed.

The Cenomanian/Turonian Boundary Event (CTBE) at Hole 641A, ODP Leg 103 (Compared with the CTBE Interval at Site 398)

“…9) (E. G. Kauffman, pers, comm., 1987). Similar sequences are commonly recorded from other sites in the deeper North Atlantic, as well as from onshore outcrops in Europe and northwest Africa (compilation in Herbin et al, 1986b). Examples include the southern part of the Galicia mar gin at Deep Sea Drilling Project (DSDP) Site 398 (Fig.…”

“…SEM studies revealed the typical fissile character of the CTBE black shales with scattered pyrite crystals or framboids of pyrite. Samples from Section 398D-56-2 have the typical organic geochemistry characteristics: total or ganic carbon reaches 10% and the organic matter is of domi nantly marine origin (amorphous kerogen; Herbin et al, 1986b). Habib (1979b) described the interval as belonging to his xeno morphic palynofacies type (amorphous palynodebris composed mostly of optically translucent yellow, brown, and orange parti cles), which supports the organic geochemistry results.…”

“…Such sediments occur within a wide range of paleobathymetric and paleogeo graphic settings (e.g., Schlanger and Jenkyns, 1976;Wiedmann et al, 1978Wiedmann et al, , 1982Arthur and Schlanger, 1979;Jenkyns, 1980;Schlanger and Cita, 1982;Thurow et al 1982;Herbin et al 1986b;Arthur et al, 1987;Schlanger et al, 1987). This bound ary event, originally termed the Cenomanian/Turonian Oceanic Anoxic Event (OAE) (Schlanger and Jenkyns 1976), and subse quently referred to as the Bonarelli Event, the Oceanic Anoxic Event 2 (OAE 2), the Evenement 2 (E 2 = OAE 2), and the Ce nomanian/Turonian Boundary Event (CTBE), has become one of the most studied Mesozoic oceanic events.…”

“…This layer is described as a laminated black claystone rich in marine organic matter and with a very low sed imentation rate, overlain by a multicolored claystone interval, and dated as Cenomanian and possibly basal Turonian. Herbin et al (1986b) and Thurow and Kuhnt (1986) introduced the term CTBE for this layer in the North Atlantic and similar se-quences in the western Tethys. This special type of Cretaceous sedimentation can also be traced into the Western Interior Sea way of the United States (Pratt and Threlkeld, 1984;Herbin et al, 1986b).…”

Secular variation in Late Cretaceous carbon isotopes: a new δ13C carbonate reference curve for the Cenomanian–Campanian (99.6–70.6 Ma)

“…3) is one of the largest and best documented carbon-isotope events in the geological record (Jenkyns, 1980(Jenkyns, , 1985Scholle & Arthur, 1980;Schlanger et al 1987). This event is conventionally interpreted as being related to accelerated burial of marine organic matter, particularly in the Atlantic, but also in other ocean basins (Herbin et al 1986;Arthur et al 1990;Kuypers et al 2002Kuypers et al , 2004. The phenomenon is widely referred to as the Cenomanian/Turonian Boundary Event (CTBE), or Oceanic Anoxic Event 2 (OAE2; Arthur et al 1990).…”