Middle Jurassic Great Oolite of Southern England, Facies Development and Regional Significance of "Dedolomitization" During Basin Evolution

B. W. Sellwood, R. Evans

The oolitic and skeletal grainstones and packstones of the Great Oolite accumulated as a southward-prograding ramp sequence adjacent to both the emergent London-Brabant Massif and the rift-related swamps of the North Sea. Both at outcrop and in the subsurface, complete transitions are seen from lignitic terrestrial facies, through muddy restricted shelf to open-water grainy associations. The latter facies provide major reservoir targets in south England. Southward, below south England, grainstones give way to oncolitic packstones and eventually to basinal mudstones. Minor coral-rich thickets occur sporadically in back-shoal settings. The Great Oolite shelf prograded as two major regressive limestone units, each separated by a minor but regionally extensive transgressive u it. Temporary, and some extensive, phases of exposure led to the generation of early meteoric diagenetic features.

In contrast to its outcrop character, the Great Oolite in the subsurface is often extensively dolomitized. Dolomite occurs in two forms: (1) as finely crystalline rhombs replacing original micrite and (2) as weakly ferroan saddle (or "baroque") dolomite.

Saddle dolomite occurs within primary pores, secondary intergranular and intragranular pores, as intergranular displacive crystals, within fractures, and locally replacing ooids and ferroan calcite cements. Microrhombic dolomite represents an early phase of mixing zone dolomitization. Saddle dolomite was emplaced at a late stage during deeper burial and at temperatures well in excess of 55°C. Saddle dolomite precipitated from ferroan waters derived from compacting shales within deeper parts of the basins.

Saddle dolomite is frequently replaced (both partially and wholly) by mildly ferroan calcite ("dedolomitization"). Relict saddle dolomite is concentrated along stylolites, protected by films of bitumen from calcitization. Calcitization of dolomite could have been triggered by meteoric recharge in the Great Oolite leading to a deep-seated water drive.

After deposition and early diagenesis (as discussed in a previous paper), the Great Oolite was onlapped by a later Jurassic sequence dominated by shales. The Great Oolite is not likely to have acted as an aquifer until the Early Cretaceous when it was reexposed along the southern flanks of the London-Brabant Massif and in the Cotswolds. It was subsequently onlapped during the later Cretaceous, only being reexhumed along the Cotswold belt during the mid-Cenozoic.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.