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September Meeting: The Four Corners World of the Jurassic Morrison Formation: Flashy Streams and Ash Clouds and Animas Waters. Oh My!
September 22, 2023 @ 5:30 pm - 8:30 pmFree – $20.00
Come one, come all, for the first talk of the new FCGS season! Details below:
Speakers: Christine Turner and Neil Fishman, USGS (ret.)
Title: The Four Corners World of the Jurassic Morrison Formation: Flashy Streams and Ash Clouds and Animas Waters. Oh My!
Location: Room 710, Sitter Family Hall (Geology Building), Fort Lewis College
5:30 – 6:30 pm: Social Hour and Dinner – Barbeque!
6:30 pm – 7:30 pm: Society Business / Presentation
7:30 – 7:45 pm: Raffle to raise money for students
Abstract of Talk:
Recognized world-wide for its giant herbivorous dinosaurs, the Upper Jurassic Morrison Formation of the Western Interior has intrigued geologists for
centuries. Interest in the formation was also stoked in the last century by several uranium booms. More recent studies of the formation have led to a
greater understanding of its stratigraphy and sedimentology, as well as the delineation of a previously unrecognized, areally extensive, alkaline,
saline wetland (“Lake” T’oo’dichi’) that characterizes the Brushy Basin Member from Gallup, New Mexico to Moab, Utah. Stratigraphic understanding of the Morrison Formation has evolved through time and is essential to accurate paleoenvironmental interpretations. Recognizing the lower and upper bounding unconformities that mark the lower and upper contacts of the Morrison Formation was a crucial step to establishing the time equivalency of units that comprise the formation. In the Four Corners area, the J-5 unconformity underlies the eolian Bluff Sandstone Member of the Morrison Formation and separates it from the underlying Middle Jurassic Wanakah Formation. In addition, interfingering between the Junction Creek Sandstone Member (equivalent to the Bluff Sandstone Member in Utah) and the adjacent and overlying Salt Wash Member and lower Recapture Members has been documented, as well as onlap of the Salt Wash Member onto the Junction Creek Sandstone Member. In the ancestral conjoined Paradox-San Juan (P-SJ)
basin that extended from Gallup, New Mexico to Moab, Utah, an ancient alkaline, saline wetland (“Lake” T’oo’dichi’) developed during deposition of
the Brushy Basin Member. Groundwater that entered the ancestral P-SJ basin from source areas to the west was ponded downstream by the
ancestral Uncompahgre Uplift that lay to the east. With the addition of significant volumes of silicic volcanic ash that originated in calderas in the
western Cordillera and were carried eastward by paleowinds, a unique and extensive wetland system developed. This wetland was characterized by a
combination of extreme evaporative concentration of pore waters in response to hot and dry climatic conditions, and hydrolysis of the great volumes of
silicic volcanic ash that increased the alkalinity and salinity of the pore waters. Through varying degrees of alkalinity and salinity over time, a concentric
pattern emerged across the depositional basin that can be mapped by the dominant authigenic mineral(s) that formed in the tuffs. From the margins of the wetland to the center, the transition from less concentrated to more concentrated pore waters led to a pattern from the margins to the center: Smectite formed from pore waters at the margins, which gave way basinward to the zeolite clinoptilolite, followed by analcime+/- k-spar, and thence albite in the center of the basin. Outcrops in the Four Corners area exemplify the nature of the extreme evaporative conditions required for the formation of “Lake” T’oo’dichi’. Together with recognition of the eolian Bluff/Junction Creek Sandstones as a part of the Morrison Formation, we infer a drier paleoclimate
throughout deposition of the Morrison Formation than was previously thought.
More info including author bios in the September Newsletter!