Reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within British Jurassic marine mudstones

Abstract

The Coinstone is a well known hiatus‐concretion level in the Lower Lias (Lower Jurassic, Upper Sinemurian) of Dorset, southern England. It has long been recognized as a layer of bored and encrusted, early diagenetic, clay‐hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones. Several different types of concretion of variable complexity can be distinguished, of which two, probably derived from slightly different stratigraphic levels, have been juxtaposed by condensation at the erosion surface. Diagenetic and biological processes occurring before, during and after exhumation on the Jurassic sea‐floor can be recognized. The relative timing of these events can be distinguished, suggesting that initial concretion consolidation, the first generation of septarian cracking, and the precipitation of the first generation of crack‐lining calcite preceded exhumation. These, therefore, probably took place at an early stage, at shallow burial depths within the accumulating sediment pile. The early calcite is brown, UV‐fluorescent and inclusion‐rich, and is similar to the first calcite generations seen in many other clay‐hosted septarian concretions. A generally early diagnetic origin of this material is thus inferred. Observations on crack textures and geometries and the interactions of the post‐exhumation fauna of encrusters, borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body, possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today. The burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion, even in the absence of high energy physical processes. Regional stratigraphic gaps may be formed as a consequence of sea level rises or falls, or as a response to sediment supply reduction independent of sea level change. Such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized.

The Significance of Hiatus Beds in Shallow-Water Mudstones: An Example from the Middle Jurassic of Switzerland

“…Thus, the question arises how tectonism in general affected the formation of hiatus beds and hiatus concretions. Observations from this study, as well as the hiatus beds reported by Wignall (1989) and Hesselbo and Palmer (1992) that formed on differentially subsiding swells, suggest that the number of hiatus beds that formed in this way has most likely been underestimated.…”

“…Such a shift to light oxygen-isotope values is not uncommon for mudstone-hosted concretions (e.g., Hesselbo and Palmer 1992;Burns 1998). The common explanations for this phenomenon are diagenetic alteration of volcaniclastic material, rapid precipitation of carbonate minerals, interaction with organic matter, freshwater influx, and (re)crystallization at elevated temperatures (Burns 1998).…”

“…During phases 1 and 5 cracking and precipitation of additional cement may occur (e.g., Kennedy et al 1977;Hesselbo and Palmer 1992). In terms of sequence stratigraphy, hiatus concretions and beds are genetically linked to rising or high sealevel (e.g., Van Wagoner et al 1988).…”

“…Other processes, such as differential subsidence, can produce sediment starvation or seafloor erosion (e.g., Hesselbo and Palmer 1992) and thus provide the conditions necessary for formation of hiatus concretions.…”

Organic geochemistry and stable isotope composition of New Zealand carbonate concretions and calcite fracture fills

“…Textural and stable isotope evidence (Boles et al 1985;Thyne & Boles 1989;Hudson et al 2001;Raiswell et al 2002) indicates fracturing (rupturing, cracking) takes place very soon after concretion formation, although the fracture fills may span several millions of years. Rupturing appears to occur while concretions are in a plastic state (Hesselbo & Palmer 1992), perhaps during shrinkage of a colloidal precipitate, (Pratt 2001), through excess internal pore pressure during shallow overburden removal (Hounslow 1997), or stress amplification due to competence contrast (Selles-Martinez 1996). Small amounts of mineral and/or organic matter and/or fluid inclusions are present in many septarian fracture fills, giving rise to distinctive colours and eprfluorescence (photoluminescence) properties.…”