GEOCHEMICAL CHARACTERIZATION OF BOUSE CARBONATES: TOWARD AN UNDERSTANDING OF DIAGENESIS OF BOUSE CARBONATES

The Miocene - Pliocene Bouse Formation is a series of carbonates that is discontinuously exposed in the lower Colorado River (CR) corridor. The recently defined basal carbonate unit of this formation is a travertine (or ‘tufa’). In the Blythe Basin, this facies is found draping topography and consists of interfingering biohermal mounds, tubular masses interpreted as serpulid worm tubes, porous tufas, and mound-like travertines. Geochemical analyses of these carbonates may help resolve the lake versus estuary/marine debate about depositional setting. To determine the paleoenvironmental settings, we have collected samples from the basal carbonate unit for isotopic (δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr) analyses. Hand sample and thin section inspection reveals complex textures that show multiple generations of carbonates and suggest that diagenetic history needs to be resolved before clear primary geochemical signals can be isolated. Hence, all sampling was done via careful sub-sampling of different textures in each hand sample. Preliminary results of C-O stable isotope analyses on samples collected in 2017 show a covariation trend that extends from near marine values of (0,0) to more negative values of (-9, -15). To explain this apparent mixing line, we tested two hypotheses: 1) seawater mixed with CR water (-7, -16), or 2) the evaporation of CR water coincidentally shifting waters toward (0,0). Radiogenic values of ⁸⁷Sr/⁸⁶Sr for Bouse carbonates of 0.710 to 0.711 have been cited as evidence that deposition within the Blythe Basin was in a lake and not marine (marine values are ~ 0.709). However, marine influence is possible if values resulted from an estuary-type mixed setting or were altered due to diagenesis. To test this, we ran a double dissolution test on three samples to isolate primary from altered portions. In each case, values changed little (0.710514 to 0.710814, 0.71056 to 0.71074, and 0.710879 to 0.710878) suggesting that either values are primary (our preferred interpretation), or alteration has been complete. We also performed SEM and microprobe analyses to inspect textural and compositional changes that may alter isotopic analysis results. We found some samples contained more than ~30% silica as an alteration product. The effect of silica diagenesis on primary carbonate signatures is still under investigation.