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Abstract

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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