Giant ooids Neoproterozoic Johnnie Formation Incised valley Carbon isotopes Glaciation Was provided by the Agouron Institute and the NASA Astrobiology Institute of Technology under the direction of J. Petterson for directions, aerial images, andĪdvice for finding field localities. MacLeod at the University of Missouri for Bergmann for her assistance in theįield and with sample preparation M. Received 4 December 2009 revised 2 March 2010 accepted 9 March 2010. The most plausible explanation for this bed is that it is a raft derived from a so far unobserved pre-incision Rainstorm Member giant ooid bed, which would also have been the source of many of the giant ooid clasts in the incision breccia. Carbon isotope values from the ooid bed indicate that it was probably deposited after the Shuram δ^(13)C excursion, making this the youngest known Precambrian giant ooid unit. All lines of evidence suggest that the Rainstorm giant ooids form a unit distinctly different from those of the Beck Spring Dolomite. Many have cerebroid outlines, are broken and recoated, and are poorly sorted. This study uses mapping, petrography, and isotope geochemistry to compare giant ooids from the Rainstorm Member and the Beck Spring Dolomite, with the goal of determining the provenance of the Rainstorm giant ooids. Distinctive oolite units on the Late Jurassic (Tithonian) Adriatic Platform, Croatia (Tiljar, 1985), are characterized by oversized ooids with radial primary calcite fabrics. However, since the giant ooid bed is associated with the incised valley fill, it could simply be a large block of oolite derived from the stratigraphically lower Beck Spring Dolomite, known to contain giant ooids. If these giant ooids were deposited as part of the Rainstorm Member, they would represent the youngest known occurrence of Neoproterozoic giant ooids and the only such unit deposited after the last Precambrian glaciation (the Gaskiers). Although the Johnnie Oolite and the incised valley occur in most Johnnie Formation outcrops scattered throughout the Death Valley region, the giant ooids have only been observed in one area, restricted to the southern Nopah Range. Fracture-associated dolomitization may have occurred within a large-scale post-orogenic groundwater system, with Mg 2+ being supplied through the release of deeply buried diagenetic brines.The Rainstorm Member of the late Neoproterozoic Johnnie Formation contains several distinctive features of possibly regional to global significance-the Johnnie Oolite, an incised valley with a carbonate breccia fill, and an enigmatic bed of giant ooids (the Rainstorm giant ooids). Some of the veins clearly relate to Triassic exhumation of the Carboniferous Limestone, but others may be related to post-Mesozoic uplift and erosion of South Wales. Fracturing of the Gully Oolite during the Hercynian Orogeny and subsequent post-orogenic uplift led to localized dolomitization, several generations of calcite veins, and the restricted occurrence of 18O depleted cements in inter- and intragranular microporosity. The extensive allochem overpacking appears to have occurred during the first few tens of metres of burial and intergranular macroporosity was eliminated prior to deep burial. Ooid isotopic composition (δ 18O=-7♸0° to -3♱0° and δ 13C = -2♳8° to +3♲8°) is similar to that of associated phreatic cements and the data suggest that the bulk of ooid stabilization and cementation occurred within meteoric groundwaters. 1), forming the initial division of the Tobosa Basin into the western Delaware Basin and the eastern Midland Basin (Galley, 1958). Late Mississippian tectonism raised the Central Basin Platform (Fig. More usually, however, phreatic cements postdate extensive overpacking of allochems. Texas Pennisula was subject to erosion from the late Ordovician through the Devonian it was submerged beneath Mississippian seas (Holmquest, 1955). Subsequent nonferroan phreatic cements are non- to dully luminescent and in restricted vertical intervals predate significant compaction. Early diagenetic phases include isopachous, fibrous submarine cements, nonluminescent vadose cements, and mixing zone dolomite. Deposition of the Gully Oolite was locally interrupted by emergence and a regionally extensive palaeosol is present at the top of the unit.

Oolite subjected to diagenesis