A DIAGENETIC INVESTIGATION OF LOWER TRIASSIC CARBONATES: IMPLICATIONS FOR GEOCHEMICAL STUDIES OF THE EARLY TRIASSIC BIOTIC RECOVERY IN THE WESTERN US
The Lower Triassic of the Western United States provides an invaluable source of well-exposed, mixed carbonate and siliciclastic strata for studying the aftermath of the end-Permian mass extinction. Samples from the Woodside, Dinwoody and Thaynes Formations at Hidden Pasture in southwestern Montana were analyzed for petrographic and geochemical indicators of alteration. Carbonate phases in thin sections were screened for diagenesis via transmitted light and cathodoluminescence petrography. Representative carbonate phases were then micro-sampled using a microscope-mounted drill. Major (Ca, Mg) and trace element (Mn, Sr and Fe) abundances were measured using a quadrapole ICP-MS.
Elemental abundance data provide evidence for both meteoric and burial diagenetic events altering the geochemical signatures of these rocks. High Fe levels, as well as petrographic studies, indicate fabrics representative of burial realm alteration. Trace element concentrations towards the top of the section suggest that this zone might also have experienced meteoric diagenesis. Best-preserved phases exhibited a minimum Mn/Sr ratio of 1.5, while the most abundant phases had an average value of 14.6. An Mn/Sr ratio of less than 0.5 is accepted as representing good preservation of marine seawater signatures. Thus, these ratios suggest that there has been alteration of the original material, indicating poor preservation. As such, caution should be used when interpreting isotopic data from this locality.