PALEOECOLOGY AND CYCLICITY OF THE TONOLOWAY AND KEYSER FORMATIONS: A GUIDE TO UNDERSTANDING LIMESTONE COMPOSITION IN MANDATA, PA

Geochemical and mineralogical analyzes of the Tonoloway and Keyser Formations (Silurian-Lower Devonian) from the Meckely Quarry, Mandata, PA, exhibit two types of cycles, representing short and long term changes in eustatic sea level. Based on abundant dolomite, laminations and mudcracks, vugs, and low biodiversity, the Tonoloway Fm. is interpreted to be a supratidal environment. It is composed of interbedded dolostone and limestone (wackestones to peloidal grainstones) containing micrite and rare, disarticulated brachiopods and ostracods. The Keyser Fm. is composed of massively bedded calcareous limestone containing ostracods, brachiopods, cephalopds, bryozoans, crinoids, trilobites, and gastropods. It is dominated by limestone (grainstones) and is interpreted to represent a subtidal environment.

Within the Tonoloway Fm. small-scale, shallowing-upward meter scale cycles are composed of ribbony limestone that grade into laminated and mudcracked beds. Within the Keyser Fm., similar cycles are composed of massive limestone that grade into shaly, ribbony units. Large-scale cycles are represented by changes in mineralogy that are related to major changes in depositional environments. In the supratidal environment, magnesium calcite and dolomite dominate, whereas, in the subtidal environment, calcite dominates the mineralogy.

By understanding the mineralogical and geochemical characteristics of the Tonoloway and Keyser Formations and how these correspond to changes in the environment, one can predict the grade of limestone within the quarry. By understanding the cyclicity, we may better understand the large- and small-scale paleoecological changes recorded in the rock record.