CONCRETION COMPOSITION AND GEOMETRY IN THE LATE CRETACEOUS WAHWEAP FORMATION, SOUTHERN UTAH: EVIDENCE FOR FLUID-FLOW CONDITIONS

Three mineralogically distinct types of concretions, each associated with different stratigraphic intervals, are found within fine- and medium-grained sandstones in the upper part of the Cretaceous Wahweap Formation, Kaiparowits Basin, Southern Utah. Petrographic, x-ray diffraction, and fluid inclusion methods were used to analyze the concretions. The upper and capping members contain evidence of syndepositional movement of local normal-faults that developed along the East Kaibab Monocline.

The upper member consists of braided and meandering-river lithic sandstones and contains barite and elongate carbonate concretions. The barite concretions are found in 2-7 meter-thick seismically distorted, massive sandstone beds. The concretions contain poikilotopic barite cement, followed sequentially by sparse calcite cement. Fluid inclusion microthermetry indicates fluid salinity of >25 wt. % NaCl equiv., suggesting the barite cement is a result of sulfate reducing brines. It is likely the high salinity fluids traveled along the local normal faults from lower evaporite units of the Jurassic Carmel Formation. The elongate carbonate concretions range from 2-12 cm in length, and are composed of ferroan calcite cement. Long axes trend parallel to the N-S axis of the East Kaibab monocline. Concretions of similar mineralogy, ‘trapped’ within crossing deformation bands, constrain carbonate-precipitating fluid transport direction from north to south, up the plunge of the monocline.

The overlying capping sandstone member is composed of braided-fluvial and eolian quartz arenites. Fe-oxide concretions are found throughout the capping sandstone member, each 1-5 cm in size, and typically found in 10 cm-size agglomerations.

A thin, fluvial, lithic sandstone unit at the top of the capping member represents a sedimentary transition associated with the stratigraphic sequence boundary between the Wahweap and Kaiparowits Formations. Throughout the unit are cm-size spherical nodules that consist of, from early to late, ferroan calcite, dolomite, and Fe-oxide cements. These concretions are most abundant where bedding is distorted by seismic activity.