Abstract: Burial Cements, Mineralization and Petroleum Emplacement in the Lower Carboniferous of Northern Britain

Cathy Hollis, Gordon Walkden

The diagenesis of the Dinantian carbonates of the Derbyshire Platform consists of four calcite cement zones, identified using cathodoluminescence, trace element and stable isotope geochemistry. Early, near-surface meteoric phreatic and marine cements (zones 1 and 2) are followed by a shallow burial phase (zone 3) and finally a deep burial phase (zone 4). This study concentrates upon these late diagenetic zone 4 calcite cements.

Zone 4 calcite is ubiquitous across the Derbyshire Platform. It occurs primarily as a vein and fracture fill and also appears as syntaxial overgrowths on earlier cements. Cross-cutting relationships between veins and variations in cement type illustrate several phases of zone 4 cementation. Field and petrographic evidence indicates that these late-stage calcites are contemporaneous with both hydrocarbon emplacement and the precipitation of Mississippi Valley Type (MVT) ore deposits. This suggests that zone 4 calcite can be used to study late fluid flow patterns on the Derbyshire Platform during burial diagenesis.

The Derbyshire Platform is surrounded to the north, west and south by shale basins which offer a source of fluids, metal ions and hydrocarbons, expelled during compaction and deformation. Appropriate fluids were probably available in the late Westphalian, coinciding with compressional Variscan (Hercynian) deformation and fracturing. Stable isotope, trace element and fluid-inclusion studies are defining more clearly a fluid flow model. Full understanding of zone 4 calcite precipitation will constrain hydrocarbon emplacement and MVT mineralization and ultimately relate these to Variscan tectonic events in Northern Britain.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994