Arrowie Basin

The Uratanna Formation (Daily 1973) contains three informal members that indicate lowstand, abrupt upward deepening, then gradual shoaling of the succession (McDonald 1992; Mount and McDonald 1992; Mount 1993). A relative sea level curve, its component systems tracts, and a composite stratigraphic column (from Mount 1993) are illustrated in figure 6.3.

Figure 6.3 Uratanna sequence stratigraphy. Sections are drawn at different scales to illustrate their location within systems tracts.

Incised channels at the lower sequence boundary contain massive, amalgamated sandstone beds that have locally eroded to a level bearing the Ediacara fauna (Daily 1973). The beds lack fossils and are interpreted to represent the lowstand systems tract (Mount 1993) (figure 6.3). The transgressive systems tract is represented by laminated siltstone and shale with phosphorite nodules at lower levels. Rare, but upwardly increasing, interbeds of fine-grained sandstone mark the incoming highstand tract. The first recorded specimens of Sabellidites cf. cambriensis occur within the transgressive tract, and the trace fossil Phycodes coronatum occurs about 60 m above in the highstand tract. Upper parts of the highstand tract are recorded by passage into fine-grained, cross-bedded quartz sandstone deposited in upward-shallowing cycles. Within these, Mount (1993) lists 10 ichnotaxa including Treptichnus pedum (referred to as Phycodes pedum in figure 6.3), Treptichnus, and Rusophycus. Diplocraterion paralle-lum, Plagiogmus arcuatus, and the mollusk Bemella sp. occur in the overlying Parachilna Formation (Daily 1976a), which we interpret with Mount (1993) to be in the low-stand tract of the overlying sequence.

On present evidence, the first organic-walled fossils (sabelliditids) are preserved in the transgressive systems tract, the first Cambrian trace (P. coronatum) is found in the lower part of the highstand tract, and abundant traces occur in its upper part.

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