Morphology and genesis of nodular chalks and hardgrounds in the Upper Cretaceous of southern England

Abstract

The Upper Cretaceous chalks of southern England are a thick sequence of rhythmically bedded, bioturbated coccolith micrites, deposited in an outer shelf environment in water depths which varied between 50 and 200–300 m. The products of sea floor cementation are widely represented in the sequence, and a series of stages of progressive lithification can be recognized. These began with a pause in sedimentation and the formation of an omission surface, followed by (a) growth of discrete nodules below the sediment‐water interface to form a nodular chalk, erosion of which produced intraformational conglomerates. (b) Further growth and fusion of nodules into continuous or semicontinuous layers: incipient hardgrounds. (c) Scour, which exposed the layer as a true hardground. At this stage, the exposed lithified chalk bottom was subject to boring and encrustation by a variety of organisms, whilst calcium carbonate was frequently replaced by glauconite and phosphate to produce superficial mineralized zones. In many cases, the processes of sedimentation, cementation, exposure and mineralization were repeated several times, producing composite hardgrounds built up of a series of layers of cemented and mineralized chalk, indicating a long and complex diagenetic history. Petrographic study of early cemented chalks indicates lithification was the result of the precipitation of small crystals on and between coccoliths and coccolith fragments. By analogy with known occurrences of early lithification in Recent deeper water carbonates, the cement is believed to have been either high magnesian calcite or aragonite, and more probably the former. The vast scale of operations involved in the cementation process precludes carbonate in expelled pore fluids as the source of cement, whilst quantities of aragonite incorporated in sediment are also inadequate. This, plus the observed association of horizons of early lithification with pauses in sedimentation associated with omission surfaces suggests seawater as a source of cementing materials. Stratigraphic studies indicate that processes of early lithification leading to hardground formation proceeded to completion in intervals to be measured in tens or hundreds of years. Regional studies suggest that early lithification characterized relatively shallow water phases associated with regional regression over the whole of the area, whilst in detail, the distribution of mature mineralized hardground complexes is strongly correlated with sedimentary thinning and condensation over small areas and the buried flanks of massifs. Early cementation in more basinal areas is typically in the form of nodular developments and incipient hardgrounds, whilst day contents in excess of a few percent appear to have inhibited early lithification. The striking rhythmicity of hardgrounds and nodular chalks is no more than a particular expression of the overall rhythmicity of chalk sequences. The stage of early lithification reached in any instance is dependent on sediment type, the time interval represent...

Anatomy of an unconformity on mid‐Oligocene Amuri Limestone, Canterbury, New Zealand

“…Early lithification of chalky sediment (e.g., hardground development) may require only a few hundred years at depths of 50-100 m during relatively shallow water phases associated with regional regression (cf. Kennedy & Garrison 1975); such a combination is fully compatible with an early stage in the shallowing of the Amuri Limestone. According to Kennedy & Garrison (1975), glauconitisation was consistently earlier than phosphatisation in the European chalks (also cf.…”

The origin of clay minerals in the Coniacian Chalk of London

“…The sedimentation rate can be estimated for the Coniacian Chalk because it is uncemented and uniformly compacted and omission surfaces, i.e. times of nondeposition, are poorly developed (Kennedy & Garrison, 1975). The Coniacian had a duration of 1.0 million years (Harland et al, 1982) and has a thickness of 55 m, so the sedimentation rate was 5.5 cm 1000 y-1.…”

Stratigraphy of Palaeocene phosphate pelagic stromatolites (Prebetic Zone, SE Spain)

“…The presence of these fossils suggests shallower conditions than those reigning during the Late Maas trichtian. In addition, and according to Kennedy and Garrison ( 1975), the colonization of the surface by epi zoans plus with the presence of abundant borings allows this surface to be classified as a well-developed hardground. The deposits that make up the host rock of the hard ground yielded the following association of planktonic foraminifera: A. mayaroensis, Contusotmncana contusa, Globotruncana conica, Racemiguembelina fmcticosa, Rugoglohigerina rotundata, and Rugoglohigerina scorti.…”