Organic-rich sediments and palaeoenvironmental reconstructions of the Cretaceous North Atlantic

Abstract

A survey of DSDP black shales from the North Atlantic has been carried out in order to study the evolution of Cretaceous palaeoenvironments in the region. The study involved a reappraisal of the biostratigraphy of the Cretaceous formations and considered accumulation rate, mineralogical and organic geochemical data. Special attention was paid to distinguishing between redeposited and autochthonous sediments. Three main phases of deposition are recognized which are separated by two unconformities:The Blake-Bahama phase (Valanginian-early Aptian (Bedoulian)) corresponds to relatively uniform depositional conditions over the whole of the North Atlantic with periods of anoxia. The clay mineral assemblages reflect different sedimentary influxes from the continents into the eastern and western parts of the basin which were separated by the midocean ridge.The transition to the Hatteras phase (Event El) is marked by a drastic fall in sediment accumulation rate and by a rapid rise in the CCD but not by any appreciable change in clay mineral fluxes.The Hatteras phase (late Aptian-late Cenomanian) is characterized by distinctly diverse depositional conditions. In the southeastern part of the basin anoxic conditions were persistent, while in the northeast along the European margin there is no, or only very rare, evidence of anoxia. In the west, periods of oxia and anoxia alternated. Over the mid-ocean ridge conditions were oxic. A 'barrier' in the region of the present-day Bermuda Rise separated two areas of distinct clay mineral deposition.The transition to the Plantagenet phase (Event E2) corresponds to low sedimentation rates. It was immediately preceded by, or was coincident with, deposition of a black shale rich in marine organic matter.The Plantagenet phase (Turonian-Senonian) was a period of uniform sedimentation under oxic conditions.Possible causes of these changes in sedimentation are outlined.

Oxygen and evolutionary patterns in the sea: Onshore/offshore trends and recent recruitment of deep-sea faunas

“…However, these later Cretaceous a͞dbc are not evident in the deep-sea deposits. Apparently, such conditions were confined to shelf environments (26). The latest Paleocene event seems to be a temporal outlyer of short duration that, although global, only affected the deep sea.…”

“…For the purposes of this analysis we define a first-order epoch of a͞dbc influence in offshore waters extending from the Permo-Triassic through the Cen͞Tur. We place this endpoint to anoxia at the Cen͞Tur boundary on the basis of (i) Cenomanian highstand and transition from firstorder transgression to first-order regression, (ii) the transition to an oxygenated state in the deep Atlantic basin (25,26), and (iii) the judgment of Tyson and Pierson (11), who observed that the frequency and anoxic dysoxic events were high before, and substantially lower after, the Cen͞Tur. Others might wish to place a change to a more oxygenated world after the Coniacian or Santonian (22).…”

Paleoenvironment and sea-level change in the early Cretaceous Barents Sea?implications from near-shore marine sapropels

“…Several authors favor basin-wide to global ''stagnation'' of deep waters, for example, deep basins with reduced vertical circulation which inhibits a renewal of water masses (e.g., Brumsack 1980Brumsack , 1988De Graciansky et al 1987). Others explain the occurrence of these black shales by an increased oceanic ''productivity'' which may lead to the formation of oxygen minimum layers which impinge on the seafloor (e.g., Schlanger and Jenkyns 1976).…”

“…This increased the prominence of shallow marine basins along the continental shelf, thereby promoting the development of restricted depositional environments, and favored the accumulation and preservation of organic matter (e.g., Demaison and Moore 1980). As a result, dark-colored, organic carbon-rich sediments, so-called black shales, occurred widely during this period, for example, in the marginal seas of the North Atlantic (e.g., Schlanger and Jenkyns 1976;Stein et al 1986;De Graciansky et al 1987), along the Norwegian-Greenland Seaway (e.g., Dore´1991; Smelror et al 2001;Mutterlose et al 2003), and in the adjacent Barents Sea (e.g., Bugge et al 1989Bugge et al , 2002. The formation of black shales is often attributed to ''oceanic anoxic events'' (OEAs) on a global scale, for example, during the Cenomanian/Turonian which is characterized by a relatively high sea-level stand (e.g., Arthur et al 1987;Erbacher et al 2001).…”

Stable oxygen and carbon isotopes from brachiopods of southern England and northwestern Germany: estimation of Upper Turonian palaeotemperatures

“…Graciansky et al (1987) described an ocean-wide erosional unconformity for upper Cenomanian to Coniacian horizons at several sites in the North Atlantic. In Europe, upper Turonian to Coniacian sediments are strongly influenced by strikeslip fault movements forming angular discordances, sedimentary hiatuses, facies changes and submarine slumps and slides in different shelf-sea basins (Mortimore et al 1998).…”