A PETROGRAPHIC ANALYSIS AND COMPARISON OF MODERN BAHAMIAN AND MIDDLE CAMBRIAN OOIDS FROM THE CONESTOGA/ELBROOK FORMATIONS OF SOUTH CENTRAL PENNSYLVANIA AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE

Classic studies of modern ooids have identified conditions under which layered carbonate grains develop. In most cases, they form in warm, shallow waters and are deposited on shoals and are transported onto neighboring beaches. Such is the case of the ooid-rich environments of North Andros Island, Bahamas. Rounded to sub-rounded grains form as minerals precipitate on the surface of a pre-existing cortex forming a singular or multilayered coating. Ooids range from a mm to a cm or more and growth may be biofilm-mediated. The environmental geochemistry of ooids, the significance of layering, and grain shape as a consequence of environmental energies experienced during grain formation is receiving renewed attention. Studies by Sipos et al (2018) show that non-layered ooids result from sphere growth in a low-energy environ that prevents grain contact and abrasion, or conditions when wave energy suspends particles so collisions are limited.Complex multi-layered and irregularly shaped ooids appear to be influenced by grain-to-grain contact whether during bed transport or post-depositional settling.

Despite the progress, there are still questions related to the presence/absence of ooids during certain time periods. That is to say, ooids are not present in nearshore carbonates for some time intervals. The concept of time-restricted facies suggests there are facies with distinctive sedimentologic, or diagenetic characteristics that persist for a limited window of time when conditions are just right (Brett et al, 2012). At other times, conditions do not allow the facies to develop. Cornell (2008) suggested ooid-rich intervals fit the definition of time-restricted faces (i.e. forming during transgressive periods) and are not ubiquitous throughout carbonate deposition during the Late Ordovician. This study is a comparative study of modern Bahamian ooids (and studies thereof) and ooids deposited within the Middle Cambrian Conestoga/Ledger formations of West York, PA. Results of petrographic analyses of ooid-bearing samples are reported. Elements of cortical structure, layer morphology, intergranular pore space characteristics, as well as grain composition are investigated to better understand the depositional and diagenetic conditions of deposition and post-depositional processes.