Jost Wiedmann

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

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.

Mesozoic-Cenozoic Development of the Eastern North American and Northwest African Continental Margins: A Comparison

During the last decade much information has been accumulated on the evolution of the Mesozoic-Cenozoic basins bordering the North Atlantic. This paper ties together the available seismic and stratigraphic data from the conjugate margins of eastern North America and Northwest Africa and contributes to a better understanding of the history of the North Atlantic Ocean with its passive continental margins. Due to the differing actual bathymetric position of the basins at the western and eastern oceanic margins, the available stratigraphic data from these margins are not entirely concordant. These data are mainly based on drilling activity in the west, and both drilling and on- shore-based studies in the east.

THE HETEROMORPHS AND AMMONOID EXTINCTION

The heteromorph ammonoids are quoted as a favourite example of degeneration and the decline of a Bauplan‘condemned’ to extinction. With astonishing tenacity this view of the heteromorphs as ‘phylogenetic end‐forms’ has embedded itself in the palaeontological literature and is still current. This is contradicted by the most recent investigations, directed especially at the Cretaceous heteromorphs, which necessitate correction of the typolysis concept as well as modification of the most uncontested of the phylogenetic ‘laws’, Dollos ‘law of irreversibility’. Contrary to the usual textbook hypothesis, the heteromorphs return in several evolutionary lineages to normal coiling of the shell and, in general, to a phylogenetically older type of suture line. At the same time these results encourage fresh reflexion on possible exogenous causes of phylogenetic extinction of the ammonoids. A clear causal connexion exists between this extinction and the far‐reaching epirogenic changes in sea level in the late Cretaceous; cosmic explanations are unnecessary.

EVOLUTION OR REVOLUTION OF AMMONOIDS AT MESOZOIC SYSTEM BOUNDARIES

1. Biological revolutions at major stratigraphical boundaries have been given numerous explanations involving endogenous biological, exogenous ecological, physical, and cosmic, as well as sedimentary or chemical factors. In an attempt to elucidate the true nature of these faunal revolutions and to assess the possible influence of biological and/or physical factors, the evolution of ammonites at the boundaries of Mesozoic stratigraphical Systems is reviewed. It is believed that the more detailed data now available can give a clearer impression of evolutionary events at these boundaries.

Cretaceous Stratigraphy, Environment, and Subsidence History at the Moroccan Continental Margin

In contrast to other regions around the North Atlantic, good exposures in the Moroccan coastal basins offer an excellent opportunity to study the Mesozoic development of a passive continental margin including the relationship between oceanic and coastal sediments and datum levels of the pelagic fossils. From south to north, the Cretaceous sediments of the coastal basins of Tarfaya, Agadir, Essaouira, and at the margin of the Meseta are described and compared regarding macro- and microfauna, sedimentology, and paleoenvironment. For the mainly marine 2500 m or 1700 m thick Cretaceous sequences of Agadir and Essaouira, respectively, we propose a correlation of the ammonite and foraminiferal zones. Probably both sections formed in one basin (the “Atlas Basin”, see front cover), but certain facies differences were caused by different water depths since Middle Cretaceous times.

Maastrichtian molluscan biostratigraphy and extinction patterns in a Cretaceous/Tertiary boundary section exposed at Zumaya, Spain

Ammonites and inoceramid bivalves were stratigraphically collected from lower and upper Maastrichtian units in continuous exposure along the seacoast near Zumaya, Spain. Three ammonite teilzones can be recognized: (1) a lower zone correlative with parts of the Globotruncana ganserri planktonic foram zone and characterized by numerous inoceramids among three different species as well as Pachydiscus neubergicus , a noded Baculites , Polyptychoceras sipho , and Hauericeras renbda ; (2) a middle zone that has no inoceramids but that has Pachydiscus fresvillensis and P. neubergicus and is correlative with the lower parts of the Abathomphalus mayaroensis Zone (planktonic foram); and (3) an upper zone that has P. colligatus and is correlative with the upper parts of the A. mayaroensis Zone. These three teilzones may be the basis for a Tethyan, facies-wide ammonite zonation of the Maastrichtian. The four main components of the fossil record at the Zumaya section show differing range characteristics with respect to the Cretaceous/Tertiary (K/T) boundary exposed in this section. The inoceramids disappear at the top of the lower Maastrichtian, except for the small enigmatic form Tenuipteria , which has a restricted range in the uppermost levels of the Cretaceous. Ammonites range upward to levels approximately 10 m below the boundary. Most larger planktonic forams and many nannofossil species disappear within several centimetres of the boundary. Echinoid fossils range up to, and possibly across, the K/T boundary. The Zumaya section is thus characterized by apparently nonsynchronous or graded extinctions of most of its fossil content.

Turonian Black Shales in the Moroccan Coastal Basins: First Upwelling in the Atlantic Ocean?

In comparison with those of the deep Atlantic, the Upper Cretaceous black shales, chiefly Turonian bituminous marls of the Morrocan coastal basins take an exceptional position. They were deposited in a water depth of 200 to 300 m during a period when the deep Atlantic water circulation lead to oxygenated sea-floor conditions and incomplete sediment sequences. The Turonian shallow water “black shales” are characterized by carbonaceous laminated marls with intercalated pelagic limestones and cherts. Their occurrence in marginal basins coincides with the peak of the Cretaceous marine transgression and decreasing terrigenous sediment supply from the continent. Their sedimentation rate ranges from 5 to 20 m/m.y. and their fauna contains elements of Boreal assemblages of the Temperate zones in northern Europe and North America. They are interpreted as an indicator of the onset of coastal upwelling along the NW African continental margin, although longdistance cold bottom currents probably did not yet exist. The lack of a middle and upper Cretaceous black shale facies in the conjugate North American shelf basins, however, may be explained by the already existing influence of a trade wind system on coastal upwelling in the eastern North Atlantic and the onset of oceanic circulation.

Mid-Cretaceous paleogeography and paleoceanography of the Betic Seaway (Betic Cordillera, Spain)

Abstract Sedimentary and micropaleontological features of Cretaceous formations in the Betic Seaway have been examined. In the External Zone of the Betic Cordillera (Southern Spain) planktic and benthic foraminiferal assemblages were studied to establish a biostratigraphic framework and to provide estimates of paleo-waterdepth. Further environmental interpretations are based on the analysis of clay minerals, sedimentary petrography and organic geochemistry. We suggest the existence of several subbasins with differing subsidence histories created by transform tectonics along the axis of the Betic Seaway. A comparison with mid-Cretaceous sequences of adjacent DSDP/ODP holes indicates that the general paleoceanographic conditions within the seaway were similar to the North Atlantic. Our results are integrated in a paleogeographic reconstruction for the Western Mediterranean during the late Albian.

Sequence stratigraphy in the upper cretaceous of the Basco-Cantabrian Basin (northern Spain)

The sedimentary cycles of the Cenomanian to Maastrichtian were investigated in the Basco-Cantabrian Basin (BCB) in northern Spain (Provinces of Alava, Vizcaya and Burgos). The depositional area was a distally steepened carbonate ramp which extended from Catalonia northwestwards to the Basque country. The investigated sediments range from calciturbidites and pelagic marls to marl-limestone alternations deposited on a distal carbonate ramp. Shallow marine limestones, marls and intertidal clastics and carbonates were deposited on the proximal part of the carbonate ramp. The establishment of a regional sequence analysis is based on the investigation of seismic profiles, well logs and outcrop sections. Examples of outcrop sections are interpreted in terms of sequence stratigraphy (unconformities of third- and second-order cycles, depositional geometries, systems tracts). The sequence stratigraphic interpretation of outcrop sections is based on facies analysis, interpretation of observed depositional geometries and correlation of unconformities and marine flooding surfaces through the basin. A biostratigraphic framework is established based on ammonites, inoceramids, planktonic and benthic foraminifera. As a result, a regional sequence stratigraphic cycle chart is presented and compared with published global cycle charts. The correlation of the regional cycle chart with published cycle charts is good. In the Cenomanian and Turonian, several sequence boundaries in the BCB are shifted by up to one biozone compared with the global chart. Some type 1 boundaries of the standard chart are only type 2 in the BCB. Important type 1 boundaries in the BCB are: top Geslinianum Zone with a 100 m lowstand wedge at the basis of the sequence (sequence boundary 92.2) ; base Petrocoriense Zone with a 250 m shallowing-upwards lowstand wedge at the basis (sequence boundary 89.2); and within the Syrtale Zone (sequence boundary 85.0).The Campanian-Maastrichtian sequence record is strongly disturbed by local compressive tectonics. Several sequences are recognizable and can be correlated with the global cycle chart. Correlation is hampered by the low biostratigraphic resolution in the western basin part. Subsidence analysis of several sections of the Upper Cretaceous of the BCB and its interpretation in the regional tectonic context leads to a discussion of the causes of the observed cyclicity. A regional eustatic curve is presented for the Upper Cretaceous of the BCB. Stage and substage names were used according Code-Committee (1977).

The upper cretaceousLacazina limestone in the Basco-Cantabrian and Iberian basins of northern Spain: Cold-water grain associations in warm-water environments

SummaryThe Upper Santonian to Lower CampanianLacazina Limestone consists of massive, often amalgamated beds of packstones and grainstones which were deposited in a shallow marine environment. The most abundant skeletal components are miliolid foraminifera, echinoderm, bivalve and bryozoan fragments, peloids and sparse red algae. Small, solitary corals only occur sporadically. Hermatypic corals, sponges and green algae are missing. The series which reaches thicknesses between 60 m and 160 m, was sampled at intervals of 0.5 m at five localities. The petrographic features throughout the series are mainly a product of changing depositional energy. The limestones are well cemented. Diagenesis is characterized by a transition from marine phreatic to burial cementation. Syntaxial and blocky calcite cements predominate over early acicular to bladed and microgranular cements.The faunal association within theLacazina Limestone exhibits features typical for temperate water i.e.foramol carbonates. On the other hand, oxygen (δ18O =-1.7 to −6.3 ‰ PDB) and carbon (δ13C to 3.0‰ PDB) isotope values of diagentically unaltered bivalve shells indicate warm surface waters corresponding better to the palaeogeographical situation of theLacazina Limestone. Nutrient-surplus is proposed as a limiting factor preventing the development of reefs and finally ofchlorozoan sediments.In the sense of sequence stratigraphy, theLacazina Limestone is interpreted to contain transgressive and highstand systems tracts. This interpretation fits well into theHaq-Vail-curve. The series shows no visible high-frequency cyclicity in the field. Several cycles were found by means of principle component analysis and spectral analysis. Their relationships to the Milankovitch spectrum are discussed.The ammonite fauna of the unit and of preceding sediments (late Coniacian to early Campanian) is described and some inoceramids are figured. They permit—for the first time—the exact dating of theLacazina Limestone in the Basco-Cantabrian Basin (BCB) and throw light on a prominent faunal change at the Coniacian/Santonian boundary. The Cenomanian to Coniacian ammonite faunas which were dominated by endemic Tethyan pseudoceratitic faunas are replaced by cosmopolitan species dominated by Madagascan elements. This drastic change permits speculations about the installation of a new oceanic current system in the Santonian.