CHEMOSTRATIGRAPHIC (δ13Ccarb) TRENDS AND ABNORMALITIES IN UPPER ORDOVICIAN (KATIAN) DEEP SUBTIDAL FACIES: “DIAGENETIC NOISE” OR RAPID LATERAL CHANGES IN TIME-ROCK FABRIC?

Small variations (0.2-1.0 ‰) between δ¹³C_carb profiles are often written-off as signatures of “diagenetic noise”, but alternative scenarios of chronostratigraphic patchiness are seldom rigorously tested as they require substantial comparative analysis of closely spaced sections. A cluster of cores penetrating the deep subtidal, mixed limestone-marl successions of the (lower Katian) Lexington-Point Pleasant interval in the subsurface of southern Ohio provide an opportunity to investigate the origin of such rapid lateral δ¹³C_carb variability. Differentiating global time-stratigraphic from local diagenetic signals is essential, not only in advancing understanding of this pivotal interval in the evolution of the Ordovician world, but also for clarifying interpretations of similar signatures throughout the geologic record.

The lower Katian Lexington-Point Pleasant interval of the Cincinnati region is exquisitely preserved having escaped both deep burial and dolomitization. Three major features of this lower Katian δ¹³C_carb profile in the deep subtidal facies of southern Ohio appear to be shared with the carbonate-dominated platform successions to the south and west: 1) Logana positive excursion (i.e., GICE), 2) Macedonia positive excursion and 3) the “Stamping Ground” negative excursion. Between these features, δ¹³C_carb profiles show relatively similar trends, but variable morphologies at the scale of 0.2 to 1.0 per mil. Scrutiny of closely spaced sections suggests these variations in profile morphology are spatially gradational. Cross plots of rock texture, grain size and bulk composition vs δ¹³C_carb show little to no correlation. Thin section and polished sections show abundant evidence for differential compaction related to early emplacement of concretionary cements. The distribution of these cements also shows little correlation with δ¹³C_carb.

The results of this study show that southern Ohio sections are not perfect lithologic replicates of one another, just as the δ¹³C_carb profiles are not. In the absence of a clear and repeated diagenetic signal, this variability may reflect a higher degree of patchiness in the chronostratigraphic fabric of deeper subtidal sections, relative to more shallow water successions where lithologic homogeneity may reflect a greater degree of time averaging.