Salt Tectonic Controls On Facies And Sequence Stratigraphy Of The Triassic Chinle Formation, Gypsum Valley Salt Wall, Colorado

This study focuses on mapping the facies geometry and depositional sequences of the Chinle Formation adjacent to the Gypsum Valley Salt Wall (GVSW). Interactions between salt tectonics and fluvial systems can affect sediment distribution. The upper Triassic Chinle Formation was deposited across the western U.S. in an intra-cratonic, fluvial environment. Throughout the Paradox Basin in Utah and Colorado, deformation associated with rising salt anticlines and subsiding minibasins coincided with Chinle deposition. In most areas, only small and isolated remnant Chinle outcrops actually expose the contact between Chinle sediments and the salt diapir. In Gypsum Valley three study areas were mapped and sampled based on proximity to the GV diapir, and a fourth location, 12 km from the diapir was chosen as a standard to compare and contrast with Chinle facies documented proximal to the diapir. Fifteen correlated stratigraphic sections illustrate the stratigraphy and the distribution of the Chinle facies. Near the diapir, conglomeratic facies contain clasts predominately composed of carbonate eroded from the diapir caprock. The carbonate conglomeratic lenses disappear within 0.7 km of the diapir. Individual sandstone channels are 0.5 â?? 1.0 m thick and extend roughly 2.0 m on outcrop. Channels are laterally stacked and, in some units, vertically stacked. Red siltstones, over 16 m thick form overbank deposits, that separate channel complexes. Within the expanse of overbank deposits, paleosols 0.5 meters in thickness consist of blocky clay peds, and in some instances exhibit extensive sand-rich lateral rooted horizons. Along strike channels, overbank deposits and lacustrine deposits are traceable for ~3.5 km. Lacustrine units contain abundant septerian nodules and sandstone concretions. Debris flows are restricted to areas where the Chinle Formation is in contact with the underlying diapir caprock. By contrast, 12 km from the diapir, ~50 meters of coarse to medium grained sandstone predominate, no caprock conglomerates occur, indicating that the material either breaks down before traveling far, or the presence of a depositional barrier to the diapir derived carbonate clast/grain dispersal. Petrographic analysis indicates a complex cement history that changes with proximity to the diapir. A fourth Halokinetic Sequence is reported that records angular unconforities of 3-10 degrees

Evidence for 2.0 Ga continental microbial mats in a paleodesert setting

Early evolved microbial communities characterized the initial biological invasion of Precambrian continental landscapes. In modern arid settings, microbial mats and biological soil crusts are well-developedand stabilize sediment. The Paleoproterozoic Makgabeng Formation in South Africa is one of the oldestand best preserved, dryland systems on Earth. Six types of microbial mat-related structures are nowrecognized within these depositional systems. This paper presents three newly discovered structures that include tufted microbial mat, biological soil crusts, and gas-escape features, in addition to three previously documented structures that include roll up features, sand cracks, and wrinkled features. These discoveries demonstrate that microbial communities were well-established and inhabited diverse continental settings by 2.0 Ga, approximately 200 million years after the onset of the Great Oxidation Event

Evidence for climate shifts in the similar to 2.0 Ga upper Makgabeng Formation erg, South Africa

Eolian ergs, and associated environs are sensitive to short- and long-term climate changes. In one of the oldest, erg deposits, the ~2.0 Ga Makgabeng Formation, facies association changes reflect one of the earliest recordings of short-term climatic shifts in a continental setting. The Makgabeng Formation is separated into lower and upper erg deposits by a playa or saline pan deposit. The lower erg deposit consists of dune sets with thin lenses of dry and deflationary interdunes that transitions vertically to thicker damp to wet interdunes. A laterally persistent playa deposit in the middle of the section consists of mudstone with deep and shallow penetrating mud cracks and subordinate siltstone and sandstone interbeds. Above this lower mudstone interval, the playa strata are sandy. Overlying the playa deposit, the upper erg deposit consists of thick eolian sets with thin lenses of dry interdune deposits. Grain size change near the top of the upper erg deposit corresponds to the appearance of fluvial, sheet flood, eolian cross-beds sculpted by mass flows, and thin playa deposits. The vertical facies association demonstrates shifts in precipitation and fluctuating water tables. Fluvial and playa deposits record high water tables whereas low-water tables are reflected in the core erg. The transition from a lower to higher water table is recorded by wet interdune interspersed within the dune strata towards the top of this interval. Rapid climatic amelioration occurred near the termination of the Makgabeng erg resulting in impingement of ephemeral river systems, development of playas, and generation of massive sand flows. This 2.0 Ga erg demonstrates the impact of climate change on erg development, resulting from shifts in the monsoonal impingement through time.Kutztown University Undergraduate Research Committee, Kutztown University Research Committee, Kutztown University Foundation, Kumba-Exxaro, the National Research Foundation, and the University of Pretoria in South Africa.http://elsevier.com/locate/palaeohb201