Facies Patterns that Define Orbitally Forced Third-, Fourth-, and Fifth-Order Sequences of Sixth-Order Cycles and their Relationship to Ostracod Faunicycles: The Purbeckian (Berriasian) of Dorset, England
Published:January 01, 2004
Edwin J. Anderson, 2004. "Facies Patterns that Define Orbitally Forced Third-, Fourth-, and Fifth-Order Sequences of Sixth-Order Cycles and their Relationship to Ostracod Faunicycles: The Purbeckian (Berriasian) of Dorset, England", Cyclostratigraphy: Approaches and Case Histories, Bruno D’Argenio, Alfred G. Fischer, Isabella Premoli Silva, Helmut Weissert, Vittoria Ferreri
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The Purbeck Group (Berriasian) in Dorset, England, is divisible into meter-scale rock cycles that are in turn bundled into a multi-tiered hierarchy of cycle sets (sequences). These cycles and sequences are the product of “Croll-Milankovitch” orbital forcing, by which the smallest-scale cycles (sixth order) are the product of precession and three orders of cycle bundles (sequences) are produced by modulation of precession by variation in the degree of eccentricity of the Earth’s orbit. The Purbeckian at the thickest section in Durlston Bay (over 100 m of marginal marine, brackish, and freshwater facies and paleosols) is divisible into four third-order (2 My) sequences that in turn comprise fourth-order (400 ky) and fifth-order (100 ky) sequences. All levels of this hierarchy are recognized by asymmetry in their patterns of facies distribution. Larger facies changes occur at the bases of cycles lower in sequences whereas smaller facies changes occur at cycle boundaries higher in sequences. In the Purbeck Group coarse-grained skeletal limestone is the dominant facies at the base of cycles and low in cyclic sequences whereas fine carbonates, shale, and paleosols characterize the upper parts of cycles and sequences.
Clements (1969, 1993) described the biofacies and lithofacies of 245 beds that constitute the Purbeck Group at Durlston Bay. Morter (1984) described nine molluscan salinity (~ depth) zone assemblages and used them to interpret patterns of sea-level rise and fall through the Middle Purbeck stratigraphic interval. Anderson (1973) divided the Purbeck of southern England into four ostracod assemblage zones and later (1985) into 40 ostracod faunicycles. Each faunicycle consists of a lower more-marine S-phase and an upper more-fresh-water C-phase assemblage of ostracods. These faunicycles are traceable laterally, and Anderson (1985) suggested that they might be the products of salinity fluctuations. Using Clements’ (1993) beds as markers in association with the ostracod assemblage zones it is possible to integrate Anderson’s ostracod faunicycles and Morter’s salinity-based molluscan assemblages with the lithologically determined four-tiered hierarchy of allocycles developed in this paper. Anderson’s faunicycles (1985) appear to coincide closely with fifth-order (100 ky) sequences, and the transgressive events interpreted by Morter (1984) occur at either the first or second fifth-order boundaries in fourth-order sequences. This independent paleobiological evidence corroborates the interpretation of a cyclic hierarchy determined by lithofacies analysis on the basis of assumptions of orbital forcing. In turn, recognition of the orbitally forced hierarchy of rock cycles provides an explanation for the published patterns of repeated (cyclic) faunal change.
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Cyclostratigraphy: Approaches and Case Histories
This volume is derived from an SEPM international workshop entitled Multidisciplinary Approach to Cyclostratigraphy, organized by the editors in May 2001 and held in Sorrento (Naples, Italy). In the Introduction we offer a brief history of how concepts of orbital cyclicity and its effects on the Earth evolved, an appraisal of the present state of research, and an overview of the papers in this volume. The main body of the volume consists of the contributed studies. These include a paper on conceptual and pragmatic approaches to stratification cycles by one of the pioneers of cyclostratigraphy, Walther Schwarzacher, who, in the 1940s, discovered the hierarchical expression of orbital cycles in rocks. The other contributions are specific studies of cyclic sequences, extending from the Quaternary back to the Triassic, covering the range from continental deposits to the deep sea, and employing a wide variety of techniques for extracting and processing the information.