Diagenetic studies of the Asbian and Brigantian limestones of Southeast Derbyshire
Asbian and Brigantian limestones of southeast Derbyshire were deposited in several environments including shallow water platform, platform margin and off platform/slope settings. Their diagenesis occurred within marine to deep burial environments. Marine cements are restricted to Brigantian platform margin mud-mounds and associated 'foreflank' limestones and include microspar, radiaxial and cryptocrystalline calcite cements of both aragonite and h.m. calcite original composition. The marine cements have undergone fabric-destructive and fabric-retentive recrystallisation. Penecontemporaneous subaerial exposure of the platform limestomes resulted in the formation of calcrete profiles. Four morphological types of calcrete were recognised; mottled, massive, laminar and rhizocretionary which are mainly composed of cryptocrystalline calcite. Rhizocretions within the calcrete profiles indicate the presence of plants growing on the exposure surface. Crypto-crystalline calcite faecal pellets indicate the presence of a soil microfauna. Pre-Zone 1 cements (high length/width ratio columnar and fibrous crystals) were precipitated in the meteoric vadose environment. Crystal growth habits depended on the precipitating water film thickness, the air/water interface and carbonate supersaturation levels. The concentration of manganese in the pore waters and pore water redox potential probably determined cement luminescence characteristics. Volumetrically, the most important cement within the Asbian and Brigantian limestones is inclusion-free sparry calcite. In cathodoluminescence this can be divided into Zone 1 (non luminescent), Zone 2 (bright orange), Zone 3 (dull orange/brown) and Zone 4 (bright orange/yellow) cements. Zone 1 to Zone 4 cements were precipitated in; (a) shallow meteoric phreatic water bodies (b) meteoric-marine mixing zones, (c) shallow to deep burial environments and (d) deep burial late diagenetic environments, respectively. Platform margin microdolomite-rich cements probably represent meteoric-marine mixing zone cements precipitated during penecontemporaneous exposure of the Asbian and Brigantian limestones. Based on Asbian D. septosa brachiopod isotope analyses Carboniferous sea water is thought to have had a δ180 value similar to that of the present day. Cement oxygen and carbon isotope data confirms and refines cement precipitation models based on petrography. 180 values were dependent on meteoric water δ180 values, temperatures of precipitation and water/rock ratios. δ13C values were dependent on marine-derived carbonate δ13C values and the influence of organic 12C-enriched CO2. Cherts formed by silica recrystallisation of the limestones and silica cement precipitation, commencing soon after deposition until sometime during Zone 3 cementation. Oxygen isotope data suggest the opal CT/quartz transition occurred in isotopically homogeneous waters (?Zone 3 type water). Asbian platform limestones do not contain chert because intraformational biogenic silica was dissolved during subaerial exposure.