To help improve our understanding of the Pricaspian reservoirs of Kazakhstan, outcrops of the margins of the Mississippian Derbyshire carbonate platform have been examined in the UK. These outcrops allow detailed comparison with problematic facies known from discontinuous short cores in Kazakhstan.
The exposed margins are of Asbian and lower Brigantian age. The Asbian margin of the platform is characterised by the rapid passage laterally of platform top cyclic peloidal/bioclastic packstones and grainstones into highly fossiliferous wackestones at the break of slope and upper slope. The matrix of these are largely composed of clotted peloidal micrites with a high proportion of radial fibrous calcite cements. The Asbian margin was steep (>30 degrees in places), around 200m in height and primarily aggradational.
A third order sequence boundary at the base of the Brigantian was followed by the development of bioclastic sandbodies along the northern margin. The high angle slope at this stage was largely bypassed with a base-of-slope apron forming. The sediments platformwards of the sandbodies are highly heterogenous. Dark, bioclastic packstones are interbedded with mud mounds, coarse stormbeds and banks of bioclastic debris. Washover systems also developed in places. In contrast, the southern margin was a lower angle and generally muddier system. Ooid shoals formed at lowstands. Carbonate sand bodies isolated within muddy sediments are interpreted as material razored off platform during sea level fall then drowned and preserved in situ by the subsequent transgression. Comparison of northern and southern margins suggest a leeward setting to the south. These margins represent a type distinct from those documented from the Pennsylvanian, Permian and Triassic.