Depositional and Diagenetic Models to Explain Origin of Laterally Continuous Carbonate Cemented Layers, Upper Lias Sands, Southern England

I. D. Bryant, J. D. Kantorowicz, C. F. Love

The Upper Lias (Bridport) Sands contain numerous carbonate cemented layers up to a meter thick that extend continuously for over 3 km at outcrop in Dorset. These cemented layers contain bioclast-rich, clay-poor sediments and a high-energy palynofacies assemblage, and are characterized by relatively high-energy sedimentary structures. The layers are interbedded with porous, bioclast-poor, clay-rich sediments that contain a low-energy palynofacies assemblage and are characterized by extensive bioturbation. These clay-rich and bioclastic sediments were deposited by continuous fair weather and episodic storm processes, respectively, on a submerged marine shoal.

Following deposition, the clay-rich fair-weather sediments are weakly cemented and compacted. Fringing cements stabilized the bioclastic storm deposits, inhibiting mechanical compaction. The resulting porous framework was subsequently cemented by calcite and dolomite supplied during dissolution of unstable aragonite and high-Mg calcite. Carbonate remobilization at elevated temperatures during burial progressively shifted oxygen isotopic compositions from Jurassic marine bioclast compositions to cement compositions of ^dgr¹⁸O -6 to -7 ^pmil PDB.

In these sediments, depositional texture and mineralogy controlled cementation. The cemented beds are laterally continuous because of the sheetlike geometry of the original storm deposits. Despite diagenetic inversion, these sediment's reservoir properties remain facies controlled (storm deposits: less than 10% porosity, negligible permeability; fair-weather deposits: 30% porosity, 300 md permeability). Similar areally extensive cements may present fluid flow barriers within hydrocarbon reservoirs. This study should provide an analogue to aid prediction of cement distributions in reservoirs within comparable storm-dominated shelf sediments.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.