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The Application of Chemostratigraphy to Fluvial-Deltaic Sequences: from Example from the Ferron Sandstone Member, South-Central Utah

Wright, Amelia 1; Ratcliffe, Ken 1; Bhattacharya, Janok 2; Zhu, Yijie 2; Wray, David 3
1 Chemostrat Inc, Houston, TX.
2 Department of Geosciences, University of Houston, Houston, TX.
3 School of Science, University of Greenwich, Chatham Maritime, United Kingdom.

The Cretaceous-aged Ferron Sandstone Member (Mancos Shale Formation) comprises a series of fluvial and deltaic sequences deposited as part of the Notom Delta complex, south central Utah. Although a significant proportion of the deltaic successions are mudstone-dominated, these fine grained lithologies have received little attention compared to the coarser grained lithologies. In this study, a better understanding of the mudstone lithologies is attempted using whole-rock geochemical data. Specifically, to determine the efficacy of chemostratigraphy to these predominantly pro-delta to shoreface/delta front sequences, and secondly to better understand their relationship to overlying fluvial mudstones within Ferron Sandstone Member.

Chemostratigraphic analysis was carried out on approximately 210 silty-mudstone field samples collected from eight sections and sampled at approximately 6m intervals. Whole-rock inorganic geochemical data were acquired for all samples using inductively coupled plasma - Optical emission spectrometry (ICP-OES) and inductively coupled plasma - mass spectrometry (ICP-MS). By combining these two analytical methods, a total of 50 elements are determined for each sample.

In this study it is demonstrated that within pro-delta to shoreface/delta front sequences of the Ferron Sandstone Member a number of “chemostratigraphic units” can be recognized and correlated between the analysed sections, based on changes in K/Rb, Mg/Al and Ti/Nb ratio values. The geological factors controlling the geochemical variations within each chemostratigraphic unit are thought to be a complex mixture of changing provenance, changing clay mineral species and periodic volcanogenic influences. Additionally, the inorganic geochemical signature of mudstone samples from fluvial sequences shows a significant difference to that seen from mudstone samples analysed from shoreface/delta front sequences, implying that mudstones in the two settings were potentially derived from different provenances.

Comparisons between the resulting chemostratigraphic correlation and detailed sequence stratigraphic correlations show that mudstone-based chemostratigraphy is a potent correlation tool in shoreface/deltaic settings. The methods applied here within the Mancos Shale Formation can be readily applied to fluvial-deltaic sequences of any age, in any basin, including petroleum basins, where outcrop studies cannot be applied.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009