Remote Sensing Investigation of a Possible Impact Structure in North-central Niger and Paleoenvironmental Conditions and Diagenesis in the Upper Edwards Formation

Date

2020-12

Journal Title

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Abstract

Investigation reveals a circular feature in north-central Niger approximately 10 km in diameter. Previous mapping by Black (1967) indicates that the feature is defined by outcrops of Devonian fluvio-marine Idekel Sandstone and the Late Proterozoic, weakly-metamorphosed molasse of the Proche Ténéré Formation, with no mention of diapiric evaporite deposits. Approximately 100 km south is the Aïr Massif which contains a suite of igneous ring dikes. Whether the feature is related to the ring dikes, structural doming, or an impact crater is evaluated from remotely sensed image analyses. Lithologies that define the feature are confirmed by classification techniques using the ENVI software platform from Landsat 8 and Sentinel 2A data. Spectral profile comparisons between the igneous intrusive rocks of the Aïr Massif, sedimentary rock near the feature, and desert sand confirm that sedimentary rocks, as previously mapped, define the feature. The shallow subsurface structure, revealed through Spaceborne Imaging Radar-C data, and 3-D modeling using Sentinel 1A data, is not consistent with ring dike structures. These data suggest that the feature did not likely form as a magmatic intrusion close to the time of sedimentary rock deposition. It is proposed that this feature formed from either a meteorite impact after deposition of host sedimentary units or by some other, but less likely, geologic process. Rudist shells, limestones, and dolostones from Lake Georgetown Spillway were analyzed using petrographic and outcrop studies to better constrain the conditions that provided the growth of a Late Albian rudist buildup, the cessation of rudist growth, and what diagenetic changes have affected the rudist buildup and overlying strata in the Cretaceous Edwards Formation of the Fredericksburg Division. The data indicate that the cessation of rudist mound growth was likely the result of a combination of increasing temperature, shallowing seas, and increasing salinity which resulted in a change of deposition from biohermal growth, to marginal microbial mats, and ultimately subaerial exposure. Limited geochemical analyses support the evidence presented through petrographic and outcrop study. Carbonate clumped isotope analyses and stable isotope analyses support the petrographic evidence, whereas complex diagenetic overprint makes constraining the precise modes of dolomite formation undetermined.

Description

Keywords

Remote Sensing, Paleoenvironment, Rudists, Impact, Meteorite, Geochemistry, Clumped Isotopes

Citation

Portions of this document appear in: Lobpries, Trey A., and Thomas J. Lapen. "Remote sensing evidence for a possible 10 kilometer in diameter impact structure in north-central Niger." Journal of African Earth Sciences 150 (2019): 673-684.