Regressive to transgressive transits reflected in tidal bars, Middle Devonian Baltic Basin

Abstract

A comparison between tidal bars from a deltaic to estuarine complex in the Middle Devonian Baltic Basin shows contrasts in sedimentary structures and bar preservation, related to changes from a regressive to transgressive setting. Detailed facies analysis of the 20–30 m thick tide-dominated estuarine Amata Formation is combined with previous analysis of the underlying deltaic Gauja Formation in order to develop specific criteria for differentiating between estuarine tidal bars and deltaic tidal bars. The estuarine tidal bars are characterized by: 1) preserved bar topsets; 2) dominantly landward palaeocurrent directions; 3) well to very-well-sorted, very fine- or fine-grained sandstones; and 4) overall retrogressive stacking with overlying marine mudstones. These bars, were deposited in the outer estuary, as elongated sand bodies. Tidal currents dominated the depositional regime, seen from the abundant tidal facies, and high energy is suggested by low abundance of mudstone drapes and mica drapes. The deltaic tidal bars are characterized by: 1) eroded bar topsets; 2) dominantly basinward palaeocurrent directions; 3) poorly sorted sandstone with grain-size ranging from very fine sands to granules; and 4) overall progradational stacking patterns. These tidal bars were situated seaward of the delta plain. Strong fluvial influence is reflected by coarse-grained, poorly sorted sandstone, mudstone and pebble clasts and the predominately basinward palaeocurrent directions. The criteria listed above can all be useful for separating between the two tidal-bar types in basins with similar settings as the Devonian Baltic Basin, however, to be used as a general facies model, the criteria must be filtered for local variations. We suggest that characteristics which occur due to the regressive to transgressive stacking patterns of tidal bars; the different palaeocurrent directions, and the preservation versus truncation of tidal bars, are broad diagnostic criteria that could be applied to other basins.