TIDAL RHYTHMITES: A STUDY OF AN ANCIENT OHIO TIDAL ENVIRONMENT
Two drill cores of rhythmically laminated shales and sandstones from the Conemaugh Group and Allegheny Group in northeast Ohio were obtained from the East Fairfield Coal Company, courtesy of Mr. Tim Miller. High-resolution digital images were taken and lamina thickness was measured using imaging software. A down-core sinusoidal variation in lamina thickness is present. This cycle repeats about every 14 laminae in the Conemaugh Group sample and about every 26 laminae in the Allegheny Group sample. Additionally, within each individual lamina there are gradational changes in grain size from fine sand to very fine sand in the Conemaugh Group sample and very fine sand to silt in the Allegheny Group sample.
The repeated pattern in thickness variation suggests tidal influence on the depositional environment. A 14 laminae cycle in the Conemaugh Group sample suggests one deposit per day (a diurnal tidal pattern) and the 26 laminae cycle in the Allegheny Group suggests two deposits per day (a semidiurnal tidal pattern). The position of depositional the site in relation to the tidal range can also influence the number of lamina deposits per day. If the site of deposition is within the full tidal range, there may be up to four deposits per day. A location farther upland may only deposit one lamina per day. The gradational change within an individual lamina may be due to change in flow velocity during the tidal cycle. The thickness of the deposits will vary depending on the range of the tide, with thicker deposits during spring tide and thinner deposits during neap tide.