Quantifying clinothem geometry in a forced-regressive river-dominated delta, Panther Tongue Member, Utah, USA

Abstract

This study presents a methodology for the quantitative description of small-scale delta clinothems. The quantitative bed-data analysis is based on three-dimensional virtual outcrop models generated by ground-based laser scanning (Light Detection and Ranging). A large number of clinothem bed measurements have been collected from the ancient forced regressive delta system of the Panther Tongue that crops out in Utah, USA. In river-dominated marginal marine environments, clinothems separated by clinoform surfaces represent the former position of the delta front as it prograded. Systematic collection of data from virtual outcrop models has allowed for accurate, spatially constrained measurement of individual bed thicknesses and the compilation of a detailed database on clinothems and associated clinoform geometries. Measurement locations were selected so that each measurement was 10 m down depositional dip from the previous one. The study area covers 5 km2 within which 2376 measurements were made from 50 separate clinothems in 320 different positions within the virtual outcrop. A bed taper parameter permitted the thinning of the clinothems to be described as a single number and thus allowed relative comparison between beds. Combined measurements were also used to calculate the average dip angle of the clinothems. Analysis of the vertical and lateral stacking of the clinothems has revealed a series of stratigraphic cycles which are termed bedsets (stream-mouth bars). The surfaces that bound the bedsets are unremarkable and it is unlikely that their significance would be recognized without this style of detailed analysis. A cyclic depositional pattern, interpreted as related to autocyclic processes and compensational stacking of mouth bars, is proposed as the origin of these packages. Mapped length/thickness trends constrain the spread of these variables, and can be used to constrain subsurface models of analogous hydrocarbon reservoirs.