E. J. Anderson

Punctuated Aggradational Cycles: A General Hypothesis of Episodic Stratigraphic Accumulation

According to the hypothesis of punctuated aggradational cycles presented here, the stratigraphic record consists of small-scale (1-5 m thick) shallowing-upward cycles (PACs) separated by surfaces marked by abrupt change to deeper facies. This motif, pervasive in time and environment, is produced by relatively long periods (tens of thousands of years) of base-level stability punctuated by geologically instantaneous relative base-level rises of at least basin-wide extent. As basin-wide lithologic time-stratigraphic units, PACs are fundamental to all aspects of stratigraphic analysis including correlation, paleoenvironmental interpretation, and paleogeographic reconstruction. Of several possible allogenic mechanisms for a pervasively cyclic stratigraphic record, glacial eustasy driven by orbital perturbations is preferred. The PAC hypothesis is presented as a comprehensive model to be tested and modified by examination of the stratigraphic record from an episodic perspective.

Associated Physical and Biogenic Structures in Environmental Subdivision of a Cambrian Tidal Sand Body

The Cambrian Chickies Quartzite was produced by a migrating mosaic of subtidal channels, intertidal flats and tidal flat ponds. These tidal zone subenvironments are interpreted from assemblages of physical and biogenic structures, each of which has unique hydrodynamic implications. The highest flow regimes and most persistent currents occur in channels represented by non-burrowed tangentially cross-stratified dune deposits. The range of flow conditions on the intertidal sand flat is represented by burrowed sandstones containing a sequence of physical structures from tangential to avalanche cross-stratification to lower flat bed. The high-energy low flat deposits contain closely spaced Scolithus burrow forms. Lower-energy high flat deposits contain Monocraterion burrow tops in conjunction with widely spaced Scolithus dwelling tubes. Quiet water ponds in abandoned channels on the intertidal flat are represented by lenses of non-burrowed gray clay immediately overlying channel deposits.

Episodic accumulation and the origin of formation boundaries in the Helderberg Group of New York State

At any single locality in the Helderberg Group of New York State, most formation and member boundaries coincide with boundaries of punctuated aggradational cycles. In each case of coincidence, the boundary is a discontinuity that was produced by a rapid rise of relative base level, rather than a facies contact representing gradual migration of contiguous paleoenvironments. Apparently diachronous formation boundaries in the sequence are not single continuous surfaces; instead, each diachronous boundary is actually a stratigraphic series of isochronous surfaces that are separate punctuated aggradational cycles (PAC) boundaries produced by distinct geologically instantaneous base-level rises. These conclusions about the origin of formation boundaries suggest that the concept of the formation as a fundamental unit of stratigraphic description and interpretation may need revision if the stratigraphic record accumulated episodically on a small scale.

Role of stratigraphic discontinuities in episodic development of paleogeography, Helderberg Group, Central Appalachians

The Lower Devonian Helderberg Group is segregated into paleogeographically significant packages by stratigraphic discontinuities at two scales. All stratigraphic sections are completely divisible into punctuated aggradational cycles (PACS) whose boundaries are synchronous stratigraphic discontinuities produced by rapid sea level rises that recurred at intervals of thousands or tens of thousands of years. Environmentally disjunct facies were superimposed basin-wide at each PAC boundary. Facies within PACs represent a continuous spectrum of paleoenvironments that coexisted and evolved through aggradation. At a larger scale, correlation of PACs reveals cryptic unconformities with a recurrence interval of hundreds of thousands of years. At these stratigraphic discontinuities, PACs are locally or regionally missing as a result of nondeposition or erosion associated with major sea level falls (and subsequent rises). Large basin-wide facies changes across these discontinuities indicate major reorganizations of paleogeography. Therefore, Helderbergian paleogeography developed episodically in response to allogenic stratigraphic events at two scales and two frequencies. Reorganization of the paleographic significance of these allogenic events emphasizes the need to distinguish between stratigraphic and sedimentologic processes in stratigraphic analysis. Facies architecture and paleogeographic patterns were determined by stratigraphic processes, not sedimentologic processes, and therefore require stratigraphic models for interpretation.