Abstract

Clinothems are seaward-dipping beds which are deposited during the progradation of delta-front mouth bars. Measurements taken from clinothems in the field and from geospatially constrained, photo-realistic virtual outcrops were used to record clinothem dip, maximum bed thickness, and bed thickness every 10 m along a series of depositional-dip-oriented profiles from two ancient river-dominated delta successions which crop out in central Utah, USA. These data show systematic changes in bed dip and bed thinning which highlight discrete packaging of the beds into bedsets, interpreted to represent the progradation of stream mouth bars in the delta front. Individual bedsets are characterized by a progressive steepening of beds coupled with a more rapid thinning.

Systematic collection of 2800 locations along 73 separate clinothems from the Panther Tongue Sandstone Member and parasequence 1f from the Ferron Sandstone were used to study the relationship between bed thickness, dip, and decay gradient parameter (τ), which describes the down-dip bed thinning. Clinothems in the Panther Tongue are much longer and more gently dipping than the clinothems in the Ferron Sandstone. In both systems, beds can be clustered into groups (bedsets) which show a systematic decrease in τ-values and an increase in dip angle. Boundaries between the groups are defined by a sudden increase in τ-values and a concurrent decrease in dip angle. These bedsets are interpreted to represent individual stream mouth bars which amalgamate to form mouth bar complexes within the delta-front sand bodies. The use of virtual outcrops was essential for the collection of the large volumes of thickness data and the recognition of very subtle (< 1°) changes in dip angle which have made these interpretations possible.