Depositional environments in an extensive tide‐influenced delta plain, Middle Devonian Gauja Formation, Devonian Baltic Basin

Abstract

The Middle Devonian Gauja Formation in the Devonian Baltic Basin preserves tide-influenced delta plain and delta front deposits associated with a large southward prograding delta complex. The outcrops extend over 250 km from southern Estonia to southern Lithuania. The succession can be divided into 10 facies associations recording distributary channel belts that became progressively more tide influenced when traced southwards towards the palaeo-shoreline, separated by muddy intra-channel areas where deposition was characterized by crevasse splays, delta plain lakes, abandoned channel deposits and tidal gullies. Tidal currents influenced deposition over the entire delta plain, extending up to 250 km from the contemporary shoreline. Tidal facies on the upper delta plain differ from those on the lower delta plain and delta front. In the former case, deposition from river currents was only occasionally interrupted by tidal currents, e.g. during spring tides, resulting in mica and mudstone drapes, and distinctive graded cross-stratification. The lower delta plain was dominated by tidal facies and tidal currents regularly influenced deposition. There was a change from progradation to aggradation from the lower to the upper part of the Gauja Formation coupled with a vertical decrease in tidal influence and a decrease in coarse-grained sediment input. The Gauja Formation contrasts with established models for tide-influenced deltas as the active delta plain was not restricted by topography. The shape of the delta plain, the predominant southward (basinward)-directed palaeocurrents, and the thick sandstone succession, show that although tidal currents strongly influenced deposition at bed scale, rivers still controlled the overall morphology of the delta and the larger-scale bedforms. In addition, there are no signs of wave influence, indicating very low wave energy in the basin. The widespread tidal influence in the Devonian Baltic Basin is explained by changes in the wider basin geometry and by local bathymetrical differences in the basin during progradation and aggradation of the delta plain, with changes in tidal efficiency accompanying the change in basin geometry produced by shoreline progradation.

Depositional Environment of the Devonian Quartz Sandstones in the Bāle Sand Pits

“…Presence of mica drapes suggests the influence of tidal processes on sedimentation [3]. Small-scale deformations most likely have been formed due to vertical shear from escaping pore water.…”

“…Trough cross-stratification, rather small thickness of individual cross-beds and presence of quartz pebbles suggest deposition of this sandstone by migration of 3D dunes in a fluvial channel [3]. Decrease of thickness of cross-beds upwards indicates that channels gradually became shallower.…”

“…Medium size deformations in the lower part of the section suggest the possible influence of small scale slumps occurring shortly after the deposition of sediments. Small scale deformations which have disturbed the primary structure of deposits, found in the package 1a most likely have been formed due to vertical shear from escaping pore water [3]. Package 2 is composed of fine to coarse-grained sand material.…”

“…They suggest [3] that in the Gauja time sediments were deposited in tide-influenced delta environment and the lower part of the Sietiņi Fm consists of sub-tidal bars overlain by tide influenced channels, and fine to coarsegrained quartz sands of the Sietiņi Fm were deposited on the delta plain.…”

“…According to the interpretations of P. Plink-Björklund and A. Pontén [3] the lower, sandy, rather coarse-grained part of the Sietiņi Fm corresponds to maximum progradation of the Middle Devonian delta system with a large area of tidally reworked facies. They suggest [3] that in the Gauja time sediments were deposited in tide-influenced delta environment and the lower part of the Sietiņi Fm consists of sub-tidal bars overlain by tide influenced channels, and fine to coarsegrained quartz sands of the Sietiņi Fm were deposited on the delta plain.…”