Depositional Cyclicity in the Lower Devonian Helderberg Group of New York State.

Abstract

The Helderberg Group of New York State, consisting of a wide range of shallow water carbonate lithologies, contains one of the finest and most complete stratigraphic records of Lower Devonian earth history. High-frequency spatial and temporal rhythmicity of carbonate accumulation in the Helderberg Group has been evaluated by examining thickness frequency distributions of individual sedimentation units in flasered ribbon rock and of individual lithofacies elements. These distributions provide important insights into the sedimentation processes that ultimately controlled stratal thicknesses. Observed thickness frequencies are closely approximated by the exponential end member of the gamma distribution, wherein flaser and lithofacies thickness frequency over each stratigraphic interval is dependent only on net sequence length and the number of stratal elements. Exponentially distributed thickness frequencies would be expected if lithologic change were a Poisson process, that is, if horizons of lithologic change occurred more or less randomly throughout. Such Poisson variation in lithologic thickness suggests a general absence of regularly recurrent high-frequency depositional forcing of stratal thicknesses during accumulation. At the individual flaser-bedded and lithofacies scale, deposition was primarily a stochastic rather than a deterministic process. Thickness distributions of supposedly shallowing-upward lithofacies associations, and magnitudes of sea-level rise inferred from lithologic change across shallowing cycle boundaries, exhibit similar skewed distributions of thickness/magnitude recurrence in Helderberg Group sections. These also are readily interpreted as generally Poisson processes of carbonate accumulation. Skewed frequency distributions of either cycle thickness or water depth change result from the grouping of a small number of elements from an exponential population of randomly distributed sizes. Modal (albeit skewed) cycle thickness distributions are simply an artifact of cycle definition and need not bear any relation to periodic extrabasinal forcing mechanisms. We conclude that thicknesses of flasered bedding units and lithofacies in the Helderberg Group largely reflect the more or less incidental migration of Lower Devonian depositional subenvironments across New York State during sediment accumulation. Stratigraphic extents of shallowing-upward lithofacies associations and apparent punctuated deepening across shallowing "cycle" tops are little different from chance groupings of stratal units randomly drawn from an exponential population. If any sea-level control is manifest in these patterns of sediment accumulation, it must have been nearly haphazard with respect to both secular recurrence and magnitude of eustatic change.