Abstract
A sedimentological study of the Thacher Limestone Member of the Manlius Formation (Lower Devonian Helderberg Group) resulted in the recognition of two facies: mudstones and calcarenites. Mudstones can be dolomitic or non-dolomitic; they are typically laminated, exhibit mud- or prism-cracks, and fenestrae. Calcarenites comprise very coarse-grained intraclasts or skeletal fragments, to peloids. These grains combine to form a diversity of sedimentary structures which in turn compose a wide range of centimetre- to decimetre-thick sedimentary deposits. The mudstone and calcarenite facies dominantly compose metre-thick asymmetrical fining-upwards sedimentary sequences. Two symmetrical sequences are also described. Comparison of the Thacher Member with modern and ancient siliciclastic and carbonate deposits suggests that it was deposited mainly by storms. Tidal deposits are subordinate. Asymmetric sequences are interpreted as shallowing upwards sequences. Symmetrical sequences presumably record gradual deepening followed by shallowing. Correlation of symmetrical sequences to certain asymmetrical sequences suggest an autogenic origin for the latter, but the entire Thacher Limestone Member nay reflect regional, or eustatic sea level fluctuations.
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