Tide-Influenced Deltas in the Ancient Record – How to Recognize Them

Anna S. M. Pontén and Piret Plink-Björklund
Göteborg University, Gothenburg, Sweden

The three-dimensional outcrops of Gauja Fm in the Devonian Baltic basin provide a database for recognizing tide-influenced deltas in the ancient record, based on detailed investigation of spatial and temporal changes in style of deposition. The outcrop belt extends over 250 km from southern Estonia to southern Lithuania. In the past, studies of tide-influenced deltas have mainly focused on modern examples, perhaps because of the difficulties to distinguish the dominating process: fluvial currents, long-shore drift, tides, waves, climate, sea level change, and the inherent complexity of the system. Additional problems are the lack of unique criteria, except for the sediment stacking pattern, like structures, facies, or subenvironments, to distinguish tide-influenced deltas from estuaries in the ancient record. Gauja Fm provides means for distinguishing tide-influenced deltas from estuaries, and for evaluating the degree of tidal influence and fluvial influence in small and large-scale bedforms. The applicability is due to the three-dimensional exposures, the excellent preservation of sedimentary structures and the unconsolidated nature of the outcrops. The dominantly basinward palaeocurrent directions across the whole delta plain, the strongly channelized sediment succession, even in the distal reaches of the delta plain, together with a systematic proximal to distal increase in tidal influence are suggested as indicators for tide-influenced deltas. The Gauja Fm database also shows that although the tidal currents strongly influenced deposition on individual bed scale, the larger-scale bedforms were more influenced by the fluvial currents and the mud/sand ratio in the sediment load.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005