William James Kennedy

Chemostratigraphy versus biostratigraphy: data from around the Cenomanian–Turonian boundary

A detailed isotopic profile is presented for a stratigraphically expanded Cenomanian–Turonian boundary section in Chalk facies exposed at Eastbourne, Sussex and compared with data from Pueblo, Colorado. In both sections macro- and micropalaeontological markers (first appearances and disappearances) are well-constrained, and their relative positions and relationship to the structure of the carbon-isotope curve are identical. This consistent relationship between two independent phenomena, one geochemical, the other biostratigraphical, is taken as evidence for the likely synchroneity of both the biostratigraphical markers and the carbon-isotope excursion in these two areas. This interpretation contrasts with suggestions made recently by other authors whose data have been taken to imply a lack of correlation between the carbon-isotope excursion in Europe and North America.

ENVIRONMENTAL AND BIOLOGICAL CONTROLS ON BIVALVE SHELL MINERALOGY

Bivalves lay down two forms of calcium carbonate in their shells, aragonite and calcite. Shells may be wholly aragonitic, or may contain both aragonite and calcite, in separate monomineralic layers. Shells are built up of several layers of distinct aggregations of calcium carbonate crystals. These aggregations are referred to as shell structures and their general features are described. Aragonite occurs as prismatic, nacreous, crossed‐lamellar, complex crossed‐lamellar and homogeneous structures. Calcite occurs as prismatic or foliated structures. Myostracal layers (calcium carbonate laid down below sites of muscle attachment) are always aragonitic. The ligament and byssus when calcined are also invariably aragonitic. A summary of the occurrence of calcite and aragonite and the associated shell structures is given. Calcite is found only in the outer layer of superfamilies belonging to the subclass Pterio‐morphia with the exception of two species only from the Heterodont superfamily Chamacea. Generally within a superfamily shell structure and mineralogy are very constant. In all superfamilies these combinations have existed for many millions of years.

Strontium isotope stratigraphy for Late Cretaceous time: Direct numerical calibration of the Sr isotope curve based on the US Western Interior

The 87Sr/86Sr of Sr in macrofossil carbonate from Upper Cretaceous strata deposited in the Western Interior Seaway, USA, provides a Sr-isotope curve for Late Cretaceous time (Cenomanian-Early Maastrichtian). The curve is calibrated biostratigraphically against the most refined ammonite zonation known for the interval, and calibrated numerically with 39Ar/40Ar dates for 20 bentonites within the sequence. Marine 87Sr/86Sr decreased from 0.70743 in the late Middle Cenomanian to a Late Cretaceous minimum of 0.70730 in the Late Turonian (89 Ma). From the minimum, 87Sr/86Sr increased through a Middle Campanian inflexion (minimum at 77 Ma, maximum at 78–80 Ma) to reach 0.70772 at the Campanian/Maastrichtian boundary. Thereafter 87Sr/86Sr increased through a very short inflexion in the latest Early Maastrichtian to the limit of our sampling in the earliest Late Maastrichtian. The inflexions provide global event markers for correlation. For most of the period covered by our curve resolution in dating and correlation of ⩽0.8 Ma should be achievable with our 2 s.d. precision in measurement of ±18 × 10−6. Our 87Sr/86Sr data show no evidence of having been affected by influxes of freshwater into the US Western Interior Seaway from rivers, including those that drained the Sevier Orogenic Belt.

The Correlation of the Lower Chalk of South-East England

The Lower Chalk of south-eastern England is a richly fossiliferous sequence 54-104 m. thick, spanning most of the Cenomanian Stage. It rests with a slight break or without interruption on Upper Albian dispar-perinflatum Sub-zone sediments of Gault or Upper Greensand facies. The upper limit as here adopted is marked by a slight break at the base of the Cenomanian-Turonian plenus Marls (Metoicoceras gourdoni Zone), the sub-plenus erosion surface of Jefferies (1962, 1963). Of the many groups of fossils present, the ammonites provide the most useful means of subdivision and correlation. The distribution of these, and of other originally aragonitic fossils, is largely controlled by preservation, and there are levels at which there is evidence of seafloor dissolution of aragonite. Nevertheless, three ammonite zones can be recognised: 3. Zone of Calycoceras naviculare 2. Zone of Acanthoceras rhotomagense 1. Zone of Mantelliceras mantelli Within the M. mantelli Zone three assemblages are identified, characterised by: (a) Hypoturrilites carcitanensis, (b) Mantelliceras saxbii, (c) Mantelliceras gr. dixoni. There are also three assemblages in the Zone of A. rhotomagense, characterised by: (a) Turrilites costatus, (b) Turrilites acutus, (c) Acanthoceras jukes-brownei. The C. naviculare Zone is not subdivided. These zones are comparable to those proposed by Hancock (1959) for the type Cenomanian, and are equivalent to the Lower, Middle and Upper parts of the Stage. Bivalves, corals and brachiopods also have a use in correlation, and even fragments of the commonest fossils—Inoceramus and Holaster—are of local use.

Eustatic sea-level record for the Cenomanian (Late Cretaceous)—Extension to the Western Interior Basin, USA

A combination of biostratigraphic markers (ammonites, inoceramid bivalves) and carbon isotope excursions is employed to establish a high-resolution correlation between the middle to late Cenomanian successions of the Western Interior Basin (USA) and the Anglo-Paris Basin (southern UK). Sequences identified from sedimentologic criteria in the Pueblo succession and elsewhere in the Western Interior Basin are shown to coincide precisely with globally recognized sea-level events and were therefore under eustatic control. This evidence refutes arguments that Cenomanian sequences in the Western Interior Basin were formed by local tectonic events. The interaction of longer-term tectonic movements and more rapid eustatic change may have simply enhanced the amount of erosion associated with sequence boundaries. A crossplot of radiometric ages derived from North American bentonites against an orbitally tuned time scale developed in the Anglo-Paris Basin provides support for the argument that the sequences were controlled by the 405-k.y.-long eccentricity cycle.

Trace Fossils in Carbonate Rocks

Most terrigenous clastic sequences have calcareous equivalents, and trace fossil suites well known from terrigenous clastic sediments have their counterparts in carbonates. Well-studied examples include trace fossils from situations in which carbonate fades predominate: the Bahaman-type shallow-water environment—as represented by European Mesozoic and tropical Pleistocene limestones; and pelagic ooze—as represented by shelf-sea and deep-sea chalks, the latter now available for study as a result of the Deep Sea Drilling Project. In shallow water carbonates, trace fossil associations range from beach-shoal assemblages having Ophiomorpha, to Thalassinoides-Rhizocorallium-Chondrites-dominated intertidal and sub-in deep-sea chalks, a Zoophycos-Teichichnus-Chondrites association dominates.

Aragonite in Fossils

The distribution of aragonite in the skeletal parts of living organisms is reviewed, and its distribution in fossils is described on the basis of several hundred new determinations. Aragonitic fossils are extensively preserved in Tertiary sediments, and are common in Mesozoic rocks, particularly where the enclosing lithology is argillaceous. No aragonite was found by the authors in fossils from Palaeozoic rocks. The most important requirement for the preservation of aragonite is the presence of reducing conditions. It is suggested that the preservation of aragonite is due to the protective effect of organic skeletal matrix, which in turn requires reducing conditions for its preservation. The protective action of the organic matrix is attributed to the formation of a hydrophobic monomolecular layer on the crystal surface, composed of aminoacids derived from the breakdown of skeletal matrix proteins.

Cenomanian sequence stratigraphy and sea-level fluctuations in the Tarfaya Basin (SW Morocco)

We investigated the sequence architecture of two expanded Cenomanian successions along a depth transect in the Tarfaya Basin (SW Morocco) and correlated these successions to published records from northwest Europe and India. Changes in terrigenous material, carbonate and organic carbon content, carbonate microfacies and foraminiferal biofacies, as well as nondepositional and erosional surfaces were used to define depositional sequences and systems tracts. We identified two main transgressive cycles in the lower and middle-upper Cenomanian separated by a major regression at the early-middle Cenomanian transition (sequence boundary Ce 3). This regressive interval is characterized by lagoonal low-stand deposits indicating an overall sealevel fall of more than 30 m. Superimposed on the two main transgressive cycles, there are 11 third-order depositional sequences that correlate to globally recognized sealevel fluctuations and appear to be paced by long eccentricity variations (400 Ka period). Positive carbon isotope excursions in the middle Cenomanian (96.0 Ma) and latest Cenomanian (94.0 Ma) following sealevel lowstands together with planktonic foraminiferal and ammonite datums provide a robust framework for stratigraphic correlation. We suggest that the onset of these excursions was triggered by eccentricity minima during periods of low variability in obliquity (nodes), which probably coincided with glacioeustatic lowstands.

Origin of solution seams and flaser structure in upper cretaceous chalks of southern England

Abstract Flaser chalks consist of small ellipsoidal bodies or lenses of relatively pure chalk surrounded by clay-rich solution seams. The latter may be simple, individual clay partings or composite aggregations of clay partings. These flaser structures formed during late burial diagenesis in response to mechanical compaction and pressure dissolution of calcium carbonate. Dissolution preferentially affected the coccolith-rich, fine-grained matrix of the flaser chalks, leaving coarser skeletal particles largely unscathed; in addition, solution was most intense in those parts of the chalks which were originally most argillaceous. Variations in flaser chalks resulted when the above mentioned processes acted on and modified the products of early diagenetic lithification such as nodular chalks, intraformational conglomerates and incipient hardgrounds. True stylolites produced by pressure solution are present only in these early lithified chalks. The most probable range of burial depths at which flaser structure formed in chalks was approximately 300–2000 m.

Strontium isotope stratigraphy for the Late Cretaceous: a new curve, based on the English Chalk

Abstract Marine 87Sr/86Sr decreases from 0.70775 in the Cenomanian to 0.70730 in the middle Turonian before increasing in a near-linear manner to >0.70775 in the early Maastrichtian. This variation has been defined using samples from the English Chalk that are closely integrated with the macrofossil and microfossil biostratigraphy of northwestern Europe. With this new isotope curve a stratigraphic resolution is attainable in correlation that is typically ±0.8 Ma for the Santonian and Campanian stages. Isotopic and biostratigraphic correlations between Dorset and Norfolk, in the UK, agree within the limit of analytical error in 87Sr/86Sr.