Sedimentologic Observation and Stratigraphic Interpretation of the Lower Devonian (Lochkovian) Manlius Formation along the Mohawk River Valley in Upstate New York

Abstract

Observations from nine measured sections through the lower Helderberg Group across upstate New York suggest that component rock units within the Manlius Formation extend laterally over 150 km across upstate New York, the most notable being a meter‐thick zone of microbial bioherms at the base of the formation. Overlying this zone of microbial bioherms is a 5–10‐m‐thick zone of wavy‐ and lenticular‐bedded ribbon rocks that grade up into microbial‐laminated mudstones that are, in turn, overlain by disrupted dolomitic mudstones deposited in playalike supratidal flats. The upper 10–15 m of the Manlius Formation comprises meter‐thick cycles of nodular‐bedded, stromatoporoid‐bearing thin sets that grade up into microbial‐laminated mudstones. Much of this succession of rock types is traceable across nearly the entire basin and strongly suggests that lower beds of the Manlius Formation do not pass laterally westward into, nor are they coeval with, the underlying Rondout Formation. This, in turn, necessitates that the time‐transgressive model of Manlius and Coeymans limestone deposition proposed by Rickard is incorrect and that the Helderberg succession of Manlius–Coeymans–Kalkberg–New Scotland formations instead comprises one third‐order stratigraphic sequence. The lower sequence boundary may comprise the disrupted dolomitic mudstones overlying the microbial‐laminated mudstones in the middle of the Manlius Formation, an interval that appears to correlate with a disconformity between the top of the Manlius Formation and the Coeymans Formation in the section exposed at Schoharie, New York. This interpretation of stratigraphic relations within the Manlius Formation necessitates reevaluation of possible correlations among meter‐thick shallowing‐upward successions in the formation.