Pennsylvanian high-relief microbial boundstone-dominated carbonate platform margins (Sierra del Cuera, Asturias, Spain)

Abstract

The facies character, architecture and geometry of carbonate depositional systems provide fundamental information for the understanding of the evolution of carbonate factories through time and how carbonate platforms respond to changes in carbonate production, accommodation space and environmental fluctuations. These data sets are also strategic for the investigation of rock types and reservoir properties in the subsurface. The Pennsylvanian Sierra del Cuera (SDC) carbonate platform outcrops of the Cantabrian Mts. in Asturias (North Spain) offer detailed seismic-scale examples of flat-topped and high-relief, microbial boundstone-dominated, platform margin to slope transects, formed during an ice-house period. The SDC platform margin lithofacies and geometry differ from Neogene to present-day carbonate platform margins dominated by coral-algal reefs, providing a depositional model for Carboniferous reservoirs in the Pricaspian Basin (Kazakhstan) subsurface. The SDC outcrops allow the full stratal anatomy and morphology of the platform top to basin to be examined as continuous transects. This field guide presents a two-day itinerary to the key outcrops of the SDC carbonate platform that offer an overview of carbonate lithofacies and stratal geometry from platform top to the toe-of-slope of a Pennsylvanian high-relief carbonate platform with light-independent microbial boundstone-dominated margins and slopes. The field stops will focus on: 1) the geometry of the carbonate system characterised by a horizontal platform interior gradually turning into the steep (30 to 40°) and high-relief (600 to 800 m) depositional slope without a raised reef rim, 2) the texture, composition and stratal architecture of the inner and outer platform, slope and basin lithofacies, 3) the characteristics of the microbial boundstone factory extending in situ on the slope from platform break to nearly 300 m palaeo-water depth, 4) the detrital lower slope sourced largely by the upper slope microbial boundstone; and 5) the variation in lithofacies types and geometry during platform margin aggradation and progradation. The SDC is an example of the non-actualistic type of carbonate platform margin where the microbial carbonate factory on the upper slope controlled the rates of progradation rather than the reworked sediment input sourced from the platform top (i.e. slope shedding model vs. Bahamian platform-top derived highstand shedding model).