High-resolution environmental change in the late Jurassic Kimmeridge clay formation

Abstract

Several discrete intervals of the Late Jurassic Kimmeridge Clay Formation were analyzed to assess the high-resolution environmental change, which occurs within laminated lithologies. The material was from new boreholes drilled in Dorset, southern England, for the Rapid Global Geological Events (RGGE) project. Complementary techniques of back scattered electron microscopy, palynology, total organic carbon and atomic H/C ratios were used to determine the fabric structure and composition and the character of the organic matter component. The sampling resolution was higher than for previous studies, based on two scales, a lower resolution 5cm point sample scale and a high-resolution lamina scale. Modern day water column, benthic and sedimentary processes were applied to explain fabric composition and structure. This shows the KCF to have been a complex marine system. All lithologies were dominated by three major components of organic matter, carbonate and clays/silts, the flux of which was a primary control on both the lithology and microfabric, together with water column oxygenation levels and terrestrial input. Sedimentation rates, based on a yearly assumption for organic-rich and coccolith-rich laminae couplets, ranged from 4.5cm per 1000 years for the oil shales to 30-118cm per 1000 years for the coccolith limestone. TOC % ranged from 3-51%, while an average atomic H/C ratio of 1.5 indicated type I to type II kerogens. Optical microscopy found the organic matter to be dominated by amorphous forms (AOM). Palynological analysis of structured organic particles revealed a close correlation between the marine and terrestrial environments and significant variability between lithologies. Oil shales were charcterized by very low particle abundances, while coccolith-rich lithologies showed high frequency and high-amplitude pulses of organic particles. These pulses are interpreted as the result of storms causing re-suspension and transportation of organic material, clays and nutrients from the proximal shelf into the distal basin. These storm events occur at the beginning and end of the coccolith limestones and thus, are suggested to have been linked to the initiation and cessation of coccolith limestone deposition. An intensification of the palaeo-Atlantic storm track due to an increase in climate humidity and instability is postulated to have resulted in these high frequency and high intensity storms. The frequency of these storm episodes was calculated to represent periods of 60 to 100 years, and possibly up to 200-400 years. Environmental and climatic reconstructions were made for the intervals by combining the fabric, geochemical and palynological results and interpretations. High-resolution change below the Milankovitch frequency was found to be present within the intervals on a number of scales. These changes ranged from yearly 'varve couplet' alternations of organic-rich and coccolith-rich fabric, to 60-100 year and perhaps 200-400 year storm events and larger scale events of the order of several 1000 years. The later represent changes in climatic humidity over the limestone lithologies.

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