Palaeogeographic significance of black pebbles (Lower Cretaceous, Prebetic, southern Spain)

Abstract

The presence of black pebbles in carbonate stratigraphic sections constitutes a criterion for identifying emergence episodes, which are significant to palaeographic reconstructions and to location of sedimentary cycle boundaries. Three genetic models can be inferred from the stratigraphic, petrologic and geochemical study of black pebbles that occur in sediments of the Lower Cretaceous of the Prebetic (southern Spain). In the first model, black pebbles are the product of redeposition in tidal and lacustrine environments from layers rich in organic matter (carbonaceous layers). In the second model, blackening takes place as a result of very mature organic matter, mainly generated from terrestrial plants (possibly burnt ones), selectively staining some of the intraclasts of the storm layers in intertidal environmnts. In the third model, applicable only to pedogenic and lacustrine environments, black pebbles, to a large extent, originate from a redeposition of pedogenic carbonate materials affected by natural forest fires, where blackening was caused by a sudden and rapid increase in temperature. The third hypothesis is supported by data obtained from an experiment in which samples underwent progressive heating to monitor the blackening process and accompanying change in isotopic composition. In all cases, the effects of storms and/or currents and waves caused erosion and redeposition.