Sequence Stratigraphy and Systems Tracts of a Mixed Carbonate and Siliciclastic Platform-Basin Setting: The Albian of Lunada and Soba, Northern Spain

Abstract

In the Lunada-Soba carbonate platform-basin setting (Albian of Basque-Cantabrian region, northern Spain), nine type 1 depositional sequences have been identified. Their boundaries represent times of complete exposure of the platform with accompanying karstification and fluvial erosion. Relative sea level falls of 15-50 m have been estimated from depths of erosion or from diagenetic effects on platform materials and facies analysis. During lowstands, deposits of the corresponding systems tract onlapped the platform foreslope or filled incised valleys on the platform top. These deposits comprise a variety of facies: limestone megabreccias derived from the platform margin, allodapic grainstones, hemipelagic marls, fine-grained turbiditic sandstones, basinal lutites, fine-gra ned estuarine sandstones, bank bafflestones and grainstones, fluvio-deltaic pebbly conglomerates and sandstones, and shallow-water orbitolinid grainstones. Relative sea level rises were characterized by deposition of monotonous marls in the basin, and by the growth of coral-rudist mud mounds and accumulations of skeletal grainstones and marls, sometimes with back-stepping stacking, on the platform. During highstands, hemipelagic marls and prodeltaic and talus delta lutites and sandstones dominated in the basin, whereas coral wackestones and orbitolinid grainstones, sometimes with prograding clinoforms, dominated on the platform. Transgressive surfaces have erosion or simply omission features, such as pebble lags or numerous burrows, respectively. No well-developed condensed sections exis , and they are represented by thin beds of marls rich in orbitolinids. Transgressive and highstand systems tracts are normally difficult to differentiate, both in the basin and on the platform, but in sequences with an accused wedge shape, the presence of back-stepping and prograding stacking patterns makes it easier. Four main types of sequences are documented, based on geometrical characteristics. Three of them are continuous from the platform to the basin: tabular, wedge shaped with tilted base, and wedge shaped with incised base. The fourth type (scalloped) is discontinuous from the platform to the basin. It results from the collapse of the platform margin, whose debris contributed to form the following sequence, and the unveiling of the underlying sequence at the site of collapse. Most of the sequence boundaries suggest a tectonic influence, and three of them show angular unconformities in the basin margin and/or the inner platform. These angular unconformities show major erosional vacuities at the cores of anticlines on the inner platform. They are attributed to phases of accentuated transpression in a major strike-slip tectonosedimentary environment, related to the opening of Bay of Biscay. A local curve of Albian relative sea level changes correlates, in part, with the eustatic curve of the Vail charts.