Siltstone Geochemical Compositions: Applications for Event Size and Correlation

Kannipa Motanated,Michael M Tice

doi:10.4236/ijg.2016.72015

Abstract

Thinly laminated siltstone and sandy siltstone are major components of the Upper Permian Brushy Canyon Formation, west Texas and south New Mexico. These rocks have been variously interpreted as the deposits of low-density turbidity currents or as windblown sediment deposited over water. Nevertheless, all models agreed that this lithology was deposited without subsequent reworking by bottom currents or burrowing organisms. These siltstones, thus, are ideal test units for quantitatively estimating hydraulic properties of the flows that formed them. In particular, the Zr/Ti ratio was tested as a geochemical proxy for flow size and transport distance. In situ geochemical abundance and grain size of particles with contrasting susceptibility to erosion—Zr- and Ti-rich particles—were mapped and measured by X-ray fluorescence analytical microscopy, μXRF. Lamination thickness was measured from Fe fluorescence intensity, which increased sharply at the top of each layer. Within the same sample, zircon grains were systematically finer than rutilated quartz grains. Zr/Ti fluorescence ratio positively correlated with lamination thickness, not particle sizes. In other words, Zr/Ti fluorescence ratio fluctuations resulted from variations in mineral abundance. Therefore, variations of Zr/Ti fluorescence ratio in these siltstones are likely caused by fluctuations in the intensity of erosional events rather than transport distance. High Zr/Ti ratios and thick laminations reflect periods of enhanced erosion. The average wind velocity during typical events was estimated to be at least 150 km?hr?1, or the equivalent of a Category 1 hurricane. The method used here could be applied to both outcrop and subsurface strata correlation.

Highlights

The unit chosen for study is the gray fine to medium grained thinly laminated siltstones and sandy siltstones of the Brushy Canyon Formation in the Delaware Mountain Group located in the Delaware basin, west Texas and south New Mexico
In every scanning location, rutilated quartz grains were systematically larger than zircon grains (Figure 3)
Results from this research found that 1) both zircon and rutilated quartz grains were well-sorted and normally distributed, 2) rutilated quartz grain sizes were systematically correlated with zircon grain sizes, 3) neither zircon grain nor rutilated quartz grain size correlated with Zr/Ti ratio, and 4) lamination thickness and Zr/Ti fluorescence ratio had positive correlations

Summary

Introduction

The unit chosen for study is the gray fine to medium grained thinly laminated siltstones and sandy siltstones of the Brushy Canyon Formation in the Delaware Mountain Group located in the Delaware basin, west Texas and south New Mexico. This basin was tectonically created and able to contain about 1000-m-thick deposits [1]. Reference [2] interpreted that the sedimentary rocks of the Delaware Mountain Group sediments were deposited from dual density flows, where saline density currents deposited sands and less-dense currents deposited detrital silt and organic materials. High-density turbidity currents generated channel filling sandstones while laminated siltstones were deposited from dilute, low-density turbidity currents [6]

Methods

Results

Discussion

Conclusion