Transgressive–regressive sequence stratigraphy of Pennsylvanian Donezella bioherms in a foreland basin (Lena Group, Cantabrian Zone, NW Spain)

摘要

A well-preserved Pennsylvanian (early Moscovian) succession including Donezella mounds, which accumulated in a highly subsiding foreland basin (Cantabrian Zone, NW of Spain), is described and discussed. This succession has been interpreted as one 3rd-order sequence reaching a thickness up to 815 m and recording ~2.3-My duration. It consists of four 4th-order transgressive–regressive (T-R) sequences (105–350 m thick and ~0.6-My duration), each subdivided into several 5th-order (~70-ky duration) meter-scale cycles (23-m average thickness). Nearly all the Donezella mounds are present in the second and third 4th-order sequences defined (Levinco Formation). They are up to ~90 m in thickness and several hundreds of meters wide, showing lenticular to domal morphologies with steep slopes up to 35–40°. Bioherms are composed of micritic boundstones with heterogeneous microfabrics and a diverse biotic community, including the microproblematic Donezella, calcitornellid foraminifers, bryozoans, agglutinated worm tubes, crinoids, and calcareous algae (red Komia/Ungdarella, beresellids, dasycladaceans and phylloids). According to the biotic assemblage and sedimentological features, these Donezella-rich bioherms thrived in a relatively shallow and low-energy environment (below fair-weather wave-base), and resulted from the baffling and binding ability of Donezella and associated biota, and the in situ-precipitation of microbial micrite. The upward evolution of the succession mainly resulted from the interplay between high tectonic subsidence rates and high-frequency moderate-amplitude glacioeustatic sea-level changes. The mounds growth mainly occurred during the transgressive phase of the 3rd-order sequence (Vereian), whereas during the regressive phase (Kashirian), deltaic siliciclastics prograding westward gradually buried and prevented the buildup development.