Combined sea-level and climate controls on limestone formation, hiatuses and ammonite preservation in the Blue Lias Formation, South Britain (uppermost Triassic – Lower Jurassic)

Abstract

Lithostratigraphic and magnetic-susceptibility logs for four sections in the Blue Lias Formation are combined with a re-assessment of the ammonite biostratigraphy. A Shaw plot correlating the West Somerset coast with the Devon/Dorset coast at Lyme Regis, based on 63 common biohorizon picks, together with field evidence, demonstrate that intra-formational hiatuses are common. Compared to laminated shale deposition, the climate associated with light marl is interpreted as both drier and stormier. Storm-related non-deposition favoured initiation of limestone formation near the sediment–water interface. Areas and time intervals with reduced water depths had lower net accumulation rates and developed a greater proportion of limestone. Many homogeneous limestone beds have no ammonites preserved, whereas others contain abundant fossils. Non-deposition encouraged shallow sub-sea-floor cementation which, if occurring after aragonite dissolution, generated limestones lacking ammonites. Abundant ammonite preservation in limestones required both rapid burial by light marl during storms as well as later storm-related non-deposition and near-surface carbonate cementation that occurred prior to aragonite dissolution. The limestones are dominated by a mixture of early framework-supporting cement that minimized compaction of fossils, plus a later micrograde cement infill. At Lyme Regis, the relatively low net accumulation rate ensured that final cementation of the limestones took place at relatively shallow burial depths. On the West Somerset coast, however, much higher accumulation rates led to deeper burial before final limestone cementation. Consequently, the oxygen-isotope ratios of the limestones on the West Somerset coast, recording precipitation of the later diagenetic calcite at higher temperatures, are lower than those at Lyme Regis.

From Oppel to Callomon (and beyond): building a high-resolution ammonite-based biochronology for the Jurassic System

“…Weedon et al . (), however, demonstrate that sedimentation rates were not uniform throughout the West Somerset sequence, hence implying that the Hettangian might actually be slightly longer. Nevertheless, preliminary figures calculated for an average biohorizon‐plus‐interval duration still indicate minimum durations of around 60,000 years or less.…”

“…; Page ; Weedon et al . ). As with the base of a stage established through a ratified GSSP, a ‘standard zone’, or chronozone correlated primarily with ammonites, can be considered to mark an isochronous surface, or time plane. This is possible using fossils such as ammonites, as the rapidity of the geographical spread of the correlating nektonic‐planktonic fauna is potentially much more rapid than the processes that facilitate the use of most other correlative tools (see Fig.…”

“…Such cycles are particularly obviously in mudrock‐dominated sequences with a significant planktogenic carbonate input such as the famous ‘Blue Lias’ mudrock–limestone alternations in the Hettangian to Lower Sinemurian of SW England (Weedon ; Weedon et al . ). These cyclical changes in ecological and diagenetic conditions could have had strong controls on ammonite presence and also, crucially, preservation (cf.…”

“…And for the Hettangian, work in progress, indicates that around 55 biohorizons can recognized within standard eight subchronozones and chronozones (see Weedon et al . ) – a nearly seven‐fold increase in available resolution.…”

“…For instance for the Sinemurian, from an original 17 subzones (following Dean et al 1961), around 77 biohorizons can currently by recognizedan 94.5 increase in available stratigraphical resolution. And for the Hettangian, work in progress, indicates that around 55 biohorizons can recognized within standard eight subchronozones and chronozones (see Weedon et al 2017)a nearly seven-fold increase in available resolution. Surprisingly, however, it has taken a long time for any of these available high-resolution schemesboth French and UKto permeate UK Jurassicfocussed stratigraphical literature, which has often continued to use Arkellian-style 'standard zonations', some dating from the early 1960s (e.g.…”