The Ecology of the Cambrian Radiation - Andrey Zhuravlev - Chapter 6

06-C1099 8/10/00 2:06 PM Page 107 CHAPTER SIX David I. Gravestock and John H. Shergold Australian Early and Middle Cambrian Sequence Biostratigraphy with Implications for Species Diversity and Correlation This description of Lower and Middle Cambrian strata from the Stansbury, Arrowie, Amadeus, and Georgina basins combines elements of biostratigraphy and sequence stratigraphy. The record of some South Australian Lower Cambrian sequences is missing, or has not been recognized, in central Australia. Deposition in the Middle Cambrian of the central Australian basins and the Stansbury Basin reflects subsi-dence-induced transgression, but these sequences cannot be differentiated in the al-most unfossiliferous clastic deposits of the Arrowie Basin. Trace fossil assemblages in basal siliciclastic rocks are most diverse in lowstand half-cycles of relative sea level. Archaeocyath species diversity is highest in transgressive tracts, whereas lowstands are accompanied by extinction on shallow to emergent carbonate shelves. Trilobite species diversity is likewise highest in transgressive tracts but is seemingly unaffected by lowstand conditions. Duration of the Early and Middle Cambrian is 25–35 m.y. and 10–15 m.y., respectively, indicating very high rates of trilobite speciation in successive transgressive systems tracts. AUSTRALIAN LOWER AND Middle Cambrian sedimentary rocks contain rich assem-blages of fossil marine invertebrates, calcified and organic-walled microbial fossils, and traces of organic activity. Knowledge of the taxonomy and affinities of Australian Cambrian invertebrate fossils has increased significantly in the past decade, but at present only the archaeocyaths and trilobites have been studied in detailed strati-graphic successions. Progress is being made in the further study of mollusks and other small skeletal fossils, superbly described by Bengtson et al. (1990). InthischapterwedocumentthespeciesdistributionofarchaeocyathsintheLower Cambrian and trilobites in the Middle Cambrian of the Stansbury and Arrowie basins inSouthAustraliaandtheAmadeusandGeorginabasinsintheNorthernTerritoryand western Queensland (figure 6.1). Upper Cambrian trilobite faunas are well preserved 06-C1099 8/10/00 2:06 PM Page 108 108 David I. Gravestock and John H. Shergold Figure 6.1 Cambrian and undifferentiated Cambrian-Ordovician sedimentary basins of central and eastern Australia. Source: Modified after Cook 1988. in the Georgina and Warburton basins, but are beyond the scope of this study be-cause correlative strata in the Stansbury, Arrowie, and Amadeus basins have yielded few fossils. Trace fossils occur in basal Cambrian siliciclastic rocks beneath archaeocyath-bearing carbonates in all of these basins (Daily 1972). For completeness the occur- 06-C1099 8/10/00 2:06 PM Page 109 AUSTRALIAN EARLY AND MIDDLE CAMBRIAN SEQUENCE BIOSTRATIGRAPHY 109 rences of trace fossils are investigated, together with archaeocyaths and trilobites, in a sequence stratigraphic context (sensu Vail et al. 1977; van Wagoner et al. 1988). On the basis of our analysis, we discuss three key attributes of the Cambrian radiation in Australia: species diversity and relative sea level change; correlation of sequences be-tween basins; and rates of speciation, assisted by the increasing number and accuracy of radiometric ages of Cambrian successions. SEQUENCE BIOSTRATIGRAPHY AnumberofsequencestratigraphicframeworkshavebeenproposedfortheEarlyand Middle Cambrian of Australia (Amadeus Basin: Lindsay 1987; Kennard and Lindsay 1991; Lindsay et al. 1993; Arrowie Basin: Gravestock and Hibburt 1991; Mount and McDonald1992;StansburyBasin:Gravestocketal.1990;Jagoetal.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 sedi-ment supply and subsidence. The depositional components of a sequence are systems tracts (Brown and Fisher 1977), which describe the associations of shelf-to-basin fa-cies 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 frame-works,whichoftenrepresentawidesampleofpaleoenvironments.Withoutadetailed faunal succession, it is difficult to determine whether all sequences have been pre-served. In this work, archaeocyath and trilobite biostratigraphic schemes correlate se-quences and determine which are missing. Within a sequence, facies analysis of sys-tems tracts helps explain why a particular species assemblage occurs at a given place and time relative to a cycle of sea level change. SequencenomenclatureintheStansburyandArrowiebasinsisshowninfigure6.2. Fourthird-ordersequences(Uratannasequence, – 1.1,– 1.2,– 1.3)spanmuchofthe Early Cambrian. The late Early to Middle Cambrian sequences – 2.1–– 3.2 rely prin-cipally 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- 06-C1099 8/10/00 2:06 PM Page 110 110 David I. Gravestock and John H. Shergold 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. URATANNA SEQUENCE BIOSTRATIGRAPHY The Uratanna sequence (Mount and McDonald 1992) is represented by the Uratanna Formation in the Arrowie Basin and the Mount Terrible Formation in the Stansbury Basin. Mount (1993) has reported a new occurrence of Sabelliditescf. cambriensisfrom the Uratanna Formation interpreted here to be at or just beneath the level of Daily’s (1976a) Mount Terrible skeletal fauna, and well below his first reported occurrence of Saarina. Arrowie Basin The Uratanna Formation (Daily 1973) contains three informal members that indi-cate 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. 06-C1099 8/10/00 2:06 PM Page 111 AUSTRALIAN EARLY AND MIDDLE CAMBRIAN SEQUENCE BIOSTRATIGRAPHY 111 Figure 6.3 Uratanna sequence stratigraphy. Sections are drawn at different scales to illustrate their location within systems tracts. ... - tailieumienphi.vn