PALEOENVIRONMENTAL ANALYSIS OF THROMBOLITES IN THE BASAL PURBECK FORMATION (UPPER JURASSIC OF SOUTHERN ENGLAND)

The paleoenvironmental conditions and growth patterns of ancient thrombolites are not well constrained. In this study thrombolites from the basal Purbeck Formation (Upper Jurassic) on the Isle of Portland, southern England, are described and interpreted. The algal and microbial material forming the thrombolites grew around the bases of tree trunks. When the trunks fell, the algal material on its underside was compressed. This strain is evident in Fry analysis. Decay of the trunk left a cylindrical hole, which was later filled with geopetal sediments. Four main thrombolite zones were assigned in the field, and petrographic analysis confirmed zonal differences. Peloids are abundant throughout the three main zones of the thrombolite, while ostracods are rare. The growth of microbes obscures the form of constituent grains. The amount of silica and early calcite cementation increases toward the outside of the thrombolite. Worm tubes are also present in outer zones. Similar thrombolitic microbialites are found today in Storr's Lake on San Salvador Island in the Bahamas. This is a coastal lake with outlets to the sea, so its salinity varies according to the tides. The thrombolitic microbialites in Storr's Lake grow at the lake margins and are completely exposed at least part of the year. Storr's Lake represents a possible modern analog to the environment in which the Purbeck thrombolites grew.