Upscaling lithology and porosity-type fractions from the micro- to the core-scale with thin-section petrography, dual-energy computed tomography, and rock typing: Creation of diagenesis and porosity-type logs

We have developed and validated a new approach to upscale lithology and porosity-type fractions from thin sections to cores using dual energy and multiscale computed tomography (CT). A new rock-typing approach (genetic rock typing [GRT]) is proposed to upscale ⇋diagenetic mineral and diagenetic pore-type fractions, from thin sections to the core domain, eventually to create a diagenesis and porosity types logs. An extensive set of short cores from Mason County (Texas) provides a representative sample set of Late Cambrian microbial buildups and their interbuildup sediments to test the GRT approach. GRTs were defined by using a dolomite log as a proxy for diagenesis and the average percentage of dolomite from each observed depositional facies (buildup interior, buildup rind, and interbuildup sediment) as a cutoff. Dolomite, diagenetic calcite, and diagenetic porosity fractions are summed to form a diagenesis log, which captures depositional facies and the diagenetic overprint at a 0.5 mm resolution. The diagenesis log was subdivided based on the number of pore-throat size classes within each GRT and provided a framework to distribute porosity-type fractions from thin sections to log form. A high correlation coefficient is observed when the predicted extent of diagenetic alteration from the log is compared with that quantified for each thin section using image processing ([Formula: see text]). Multiscale CT imaging and dual-energy-derived logs could be directly linked to well-log photoelectric factor and bulk-density logs. This approach thus has the ability to span six orders of magnitude in resolution (500–0.0005 mm). The diagenesis log can be used to extrapolate porosity-type fractions from thin sections to logs, from which qualitative geologic interpretations can be generally translated into quantitative values.

Paleoecology of an Upper Ordovician submarine cave-dwelling bryozoan fauna and its exposed equivalents in northern Kentucky, USA

A bryozoan-dominated fauna that inhabited small caves underneath a carbonate hardground is here described from the Corryville Formation (Upper Ordovician, Katian) exposed near Washington, Mason County, Kentucky, USA. The dominant bryozoan,Stigmatella personata(a trepostome), is found both growing downwards from the cave ceilings and upwards on the exposed hardground surface above. Another trepostome,Monticulipora, is a minor component of the cave fauna. There are few discernible anatomical differences between the bryozoan colonies that grew upwards in presumably well-lit waters and those that grew downwards in the gloomy caves. The pendant, cave-dwellingS.personatain some cases appears to have longer zooecial tubes than its exposed equivalent. The colonies ofS.personataare rounded mounds with multiple layers formed by self-overgrowth. The overgrowths in both downward and upward growing forms are marked by thin layers of sediment infilling the upper zooecial chambers in the older portion of the colony. We suggest that biofilms developed on patches of the colony where the zooids had died. Sediment adhered to these surfaces and the colony then overgrew the patches, trapping sediment within the skeleton. The bryozoan zoaria and the carbonate hardground are extensively bored by the cylindrical ichnogenusTrypanitesthat occasionally contain cylindrical calcite-filled tubes similar to “ghosts” of organic materials. Bioclaustrations are present in some of the bryozoan skeletons. This cave fauna is one of few submarine examples known from the Paleozoic. It supports the hypothesis that early cave-dwelling organisms were little differentiated from their exposed counterparts.