STRATIGRAPHIC TIMING CONSTRAINTS ON POST-GLACIAL EMPLACEMENT AND EROSIONAL EVENTS PRESERVED IN THE IBEX FORMATION OF DEATH VALLEY CALIFORNIA

The Neoproterozoic succession of Death Valley California contains a lithologically distinctive carbonate deposit, the Noonday Dolomite, commonly viewed as a cap carbonate because it overlies glaciogenic sediment, contains peculiar sedimentary features, and records a regionally persistent negative carbon isotopic excursion (<-4‰ d¹³C).  Lateral studies at the stratigraphic position of the Noonday in southern Death Valley reveal a range of possibly time equivalent lithofacies.   This includes, to the south, arkosic turbidite beds and siltstones (the lower member of the Ibex Formation) commonly interpreted to be the basinal time equivalent unit to the lower member of the Noonday.  The co-occurrence of a carbonate system with arkosic sands delivered to a proximal slope is uncommon in Paleozoic successions and raises the possibility that the Ibex Formation comprises a younger unit resting unconformably (or correlatively conformable) on the lowermost member of the Noonday Dolomite.  To determine the relationship of these units, mapping along well exposed terrains was conducted to identify the lateral expression of possible erosion surfaces.

Exposures of the Ibex in the southern (basinal) sections reveal an erosion surface between the middle member, composed of interbedded limestone, dolomite, and sandstone, and the lower arkosic member.  The middle member is in turn erosionally truncated by a surface beneath the uppermost member of the Ibex consisting of a mixed quartz-dolomite sandstone commonly referred to as the upper Noonday.  Up-dip (to the north) these units show the coalescence of erosional surfaces (unconformities) near an inferred platform-slope break.  In the northern sections, sandstone of the upper Noonday unconformably overlie the lower Noonday and the lower and middle members of the Ibex are absent.  In contrast, to the south, the arkosic member appears conformable with the lower Noonday as indicated by interfingering of these facies suggesting their co-occurrence.  This evidence shows that multiple changes in base level are preserved in the Neoproterozoic Ibex Formation which may represent local tectonic/isostatic uplift and/or glacioeustasy.