Abstract: Marine and eolian carbonate microfacies of the upper member of Lower Sundance Formation (Middle Jurassic), eastern Bighorn Basin, Wyoming

Zoran Kilibarda, David B. Loope

The distribution and petrology of discontinuous carbonate sand bodies below and above the J-3 unconformity at the crest of the Sheridan arch reveal episodes of exposure, eolian dune building, and marine reworking during transgression. The shoal grainstone facies is composed of poorly sorted, large skeletal grains, large ooids (0.7-1 mm), few intraclasts, and very rare fine quartz grains, cemented with syntaxial and blocky spar. The shallow shelf grainstone facies contains ooids (0.2-0.5 mm) and fine quartz that are mixed or segregated in thick laminae, and well-rounded, elongated skeletal grains that show good imbrication. It is dominantly cemented with micrite, but shows some spar-filled pores and late diagenetic euhedral dolomite (<0.1 mm). A diverse assemblage of highly spherica , well-abraded grains, a strongly bimodal grain-size distribution, and inversely graded thin laminations characterize the eolian grainstone facies. Ooid-rich laminae show extremely close packing due to vadose compaction. Micrite is the dominant cement; several varieties of spar are less abundant. The lagoon packstone facies contains abundant ostracodes, ooids, fine quartz, and rare large (0.5-0.7 mm), well-rounded, and spherical quartz grains. Numerous grains float in carbonate mud. Sparry cement fills pores between grains and rare fenestrae in carbonate mud. The marine reworked grainstone facies is composed of well-sorted ooids that are cemented by coarse, blocky spar that occasionally shows excellent spiky calcite.

The shoal and shallow shelf facies lie below the J-3 unconformity and were cemented in meteoric vadose and phreatic environments during lowstand. Once exposed, these facies provided a source of sediment for the eolian facies that lies above the J-3. Micrite cement and several varieties of spar cement indicate meteoric vadose diagenesis of eolian facies in a warm, dry climate. Hardened surfaces of the shoal, shallow shelf, and lagoon facies were resistant at the onset of the next transgression, but the uppermost parts of the poorly consolidated eolian facies were reworked into a thin veneer that blankets the dunes.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994