A DIAGENETIC INVESTIGATION OF LOWER TRIASSIC CARBONATES: IMPLICATIONS FOR GEOCHEMICAL STUDIES OF THE EARLY TRIASSIC BIOTIC RECOVERY IN THE WESTERN US

The end-Permian mass extinction is characterized by a severe drop in fossil diversity, as well as a number of important ecological shifts. The biotic recovery following this event was delayed during the Early Triassic by as much as 5 million years for most taxa before community structure and diversity returned to pre-extinction levels. Establishing a better understanding of the anomalous environmental and climatic conditions that could have contributed to slow recovery rates is hindered by the lack of a high-resolution bio- and chemostratigraphic framework that can be used to correlate Lower Triassic sections within different parts of the globe. However, before such a framework can be constructed, caution must be employed to ensure that the data is sourced from samples exhibiting the least amount of post-depositional alteration.

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.