Thorntonia Region

Sequence 1, represented by the Mount Hendry Formation, is characterized initially by sandstone, conglomerate, arkose, siltstone, and shale, representative of coastal plain, fluvial, and valley-fill environments in the lowstand systems tract. These sediments are extremely sparsely fossiliferous, with only occasional ichnofossils occurring. Phosphorite, phosphatic limestone, and limestone of the Border Waterhole Formation and Thorntonia Limestone formed in the transgressive systems tract, and platform car-

Figure 6.7 Trilobite diversity in the Middle Cambrian sequences of the eastern Georgina Basin. Numbers represent estimated taxa present.

bonate and peritidal carbonate of the upper Thorntonia Limestone represent the highstand systems tract. An oncolitic unit marks the top of sequence 1. Biostratigraphi-cally, sequence 1 is distinguished by the co-occurrence of the trilobite genera Redlichia and Xystridura and, in certain other basins, Onaraspis. Redlichia predominates in the Border Waterhole Formation, the Thorntonia Limestone, and the Yelvertoft bed, with 10 species described from the Yelvertoft bed and associated lithofacies by Opik (1970b). In earlier publications, this fauna has been used to diagnose the Ordian stage (Opik 1967b) and the Redlichia chinensis Zone (biofacies).

The condensed section of sequence 2, containing xystridurid trilobite coquinite, stromatolite, encrinite, and phosphatic hardgrounds of the transgressive Bronco Stro-matolith Bed, is dominated by an undetermined species of the Xystridura (Xystridura) templetonensis Zone (biofacies).

On the Barkly Tableland, to the west of the Thorntonia region, the probably contemporaneous Burton beds, containing limestone, shale and chert overlying coquinite, contribute a further 15 species to the "Pentagnostus" Zone in figure 6.7.

Upper levels of the transgressive and highstand systems tracts in Middle Cambrian sequence 2 contain the following: Inca Formation, comprising organic-rich black shale belonging to the late Templetonian-Floran Stage, Triplagnostus gibbus-Acidusus atavus zones; Gowers Formation, composed of peritidal phosphorites and phosphatic limestones of late Floran, Euagnostus opimus age; ramp, platform margin, and platform carbonates of the Currant Bush, Age Creek, Mail Change, and V Creek limestones belonging to the Undillan, Ptychagnostus punctuosus and Goniagnostus nathorsti zones. Estimated numbers of trilobite taxa recorded from these zones are plotted on figure 6.7.

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