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.