Lowstand carbonates, highstand sandstones?

Abstract

The sedimentary facies, sediment dynamics and sequence architecture of modern high-energy shelves in the mid and high latitudes are largely governed by wave abrasion processes. Cool-water carbonates may form there, if the influx and/or net accretion of siliciclastics is kept at a minimum. Little dilution of the carbonate produced in situ is generally promoted by a wide “epicontinental” shelf, subdued topography of the adjacent mainland, the predominance of limestone outcrops, and an arid climate. The aforementioned requirements are rarely met, and thus will automatically lead to the formation of mixed siliciclastic–cool-water carbonates. Such an example is found in the Early to Mid-Miocene Lagos-Portimão Formation (Algarve, S-Portugal), which formed on a narrow high-energy shelf of the Atlantic Ocean that was bounded by a mountain range. The sediments of the formation consist of fossiliferous sandstone (FS), shell beds, and rhodolith blankets. Along strike, the stratification of the formation is monotonous for tens of kilometres and well exposed in coastal cliffs, whereas no outcrops of dip sections exist. The bulk skeletal composition of the sediments is typical for the warm-temperate climatic zone: various endo- and epibenthic bivalves, bryozoans, coralline algae, echinoderms, gastropods, and large foraminifers ( Heterostegina). In some very rare beds, a few isolated, not framework-forming specimens of zooxanthellate corals ( Porites, Tarbellastrea) indicate temporally elevated surface water temperatures close to the lower threshold of the coral reef ecosystem. In sandstones, the fauna is well preserved and burrowing bivalves are commonly found in life position. In limestone beds, the state of preservation of the grains ranges from intact to disintegrated and abraded specimens. We infer an accumulation of the shell beds through winnowing of fine materials (siliciclastic sand and carbonate mud) at wave abrasion depth and concentration of calcareous skeletons associated with the subsequent attraction of new epibiota in a complex shell bed. The vertical alternation of fossiliferous sandstone and shell beds, and in-phase variations of the “Photo Index” (photic biota vs. bryozoans) and “Bryozoan Index” (bivalves vs. bryozoans) is envisaged to document variations of water depth (and sea level). Sandstone units built up when wave abrasion depth (WAD) rose above the sea floor during TST (and early HST), whereas the shell beds formed during LST when the WAD for sand intersected with the sea floor. Clastic sediments were probably brought on the outer shelf during early transgression, and by longshore currents. Sea-level signatures inferred in the mixed siliciclastic–cool-water carbonate shelf setting of S-Portugal therefore significantly deviate from conventional concepts of carbonate sequence stratigraphy, which were developed for flat-topped platforms. Successful interpretations of ancient mixed sequences must therefore take into consideration the processes of production, concentration and accretion of the carbonate sediments.