TITLE:
Siltstone Geochemical Compositions: Applications for Event Size and Correlation
AUTHORS:
Kannipa Motanated, Michael M. Tice
KEYWORDS:
Windblown Dust, Laminated Siltstones, Geochemical Mapping, Upper Permian Brushy Canyon Formation
JOURNAL NAME:
International Journal of Geosciences,
Vol.7 No.2,
February
29,
2016
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.