Sequence Biostratigraphy

A number of sequence stratigraphic frameworks have been proposed for the Early and Middle Cambrian of Australia (Amadeus Basin: Lindsay 1987; Kennard and Lindsay 1991; Lindsay et al. 1993; Arrowie Basin: Gravestock and Hibburt 1991; Mount and McDonald 1992; Stansbury Basin: Gravestock et al. 1990; Jago et al. 1994; Gravestock 1995; Dyson et al. 1996).

Sequence stratigraphy relates patterns of sediment accumulation at various scales to recurring cycles of marine transgression and regression, as well as to rates of sediment supply and subsidence. The depositional components of a sequence are systems tracts (Brown and Fisher 1977), which describe the associations of shelf-to-basin facies at low relative sea level (lowstand systems tracts), rising relative sea level (trans-gressive systems tracts), and falling relative sea level (highstand, or forced regressive, systems tracts).

Systems tracts or entire sequences may be condensed or incomplete, and hiatuses occur close to basin margins in regions undergoing slow relative subsidence and in structural belts where tectonic uplift opposes regional subsidence. Sequence biostra-tigraphy permits the interpretation of depositional sequences within biozonal frameworks, which often represent a wide sample of paleoenvironments. Without a detailed faunal succession, it is difficult to determine whether all sequences have been preserved. In this work, archaeocyath and trilobite biostratigraphic schemes correlate sequences and determine which are missing. Within a sequence, facies analysis of systems tracts helps explain why a particular species assemblage occurs at a given place and time relative to a cycle of sea level change.

Sequence nomenclature in the Stansbury and Arrowie basins is shown in figure 6.2. Four third-order sequences (Uratanna sequence, €1.1,-61.2,-61.3) span much of the Early Cambrian. The late Early to Middle Cambrian sequences 62.1-63.2 rely principally on data from the Stansbury Basin, with the Middle Cambrian being placed at the base of the Coobowie Limestone on Yorke Peninsula (see the section "Stansbury Basin" below).

A relative sea level curve illustrated in figure 6.2 indicates the positions of low-stands and highstands in the stratigraphic succession. Based on the ideas of Zhuravlev (1986) and Rowland and Gangloff (1988), the dashed envelope that connects high sea level culminations corresponds to the Botoman transgression and Toyonian re-

Figure 6.2 Early and Middle Cambrian sequence stratigraphy of the Arrowie and Stansbury basins. Third-order high sea level culminations are linked by a dashed curve to depict Botoman transgression and Toyonian regression.

gression. These are considered to be global phenomena. The third-order sequences illustrated in figure 6.2 operated in all basins under review where a rock record is preserved.

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