Victor A. Gostin

Shrinkage Cracks: Desiccation or Synaeresis?

ABSTRACT Casts of shrinkage cracks found within sedimentary sequence are frequently cited as being diagnostic of depositional environments periodically subjected to subaerial exposure. The term shrinkage cracks, however, encompasses a broad suite of sedimentary structures having various origins. Shrinkage cracks can form not only at the sediment-air interface by desiccation processes but also at the sediment-water interface or substratally by synaeresis processes. Shrinkage cracks are induced in muddy sediments by variations in the salinity of the depositing medium, sediment compaction, and temperature in the case of some desiccation mudcracks. Crack morphology is influenced by the interplay of numerous factors, such as sediment composition, bed thickness, and bed surface configuration. Factors controlling desiccation mudcracks also include the rate of initial drying, total exposure time, depth of the groundwater table, and direction of surface drainage. Unfortunately, because of the complex interplay of these factors, no single feature of any shrinkage crack is necessarily useful in differentiating between a desiccation or synaeresis origin. Confidence in interpreting subaerial exposure from shrinkage cracks, therefore, depends on finding othe associated sedimentary structures diagnostic of exposure.

A chemostratigraphic overview of the late Cryogenian interglacial sequence in the Adelaide Fold-Thrust Belt, South Australia

Within the Neoproterozoic sequence of the Adelaide Fold-Thrust Belt, South Australia, two of the most severe ice ages in Earth history (Sturtian and Marinoan) are recorded in the glacigenic rocks which mark the base and top of the 4.5 km-thick Umberatana Group, the focus of the present chemostratigraphic study. Rock units from three drillholes and ten measured outcrop sections, located in the southern and central Flinders Ranges and on the Stuart Shelf, were sampled for isotopic analysis of their co-existing calcite (δ13Ccal), dolomite (δ13Cdol) and kerogen (δ13Corg). Strontium and sulphur isotopic analyses were also run on selected samples. The sample sites are in tectonic domains where regional metamorphism has been insufficient to significantly alter the isotopic composition of the kerogen. The resulting data, when integrated with the lithostratigraphy of the Umberatana Group, yield one of the most complete records of secular variation in the C-isotope signature of the late Cryogenian ocean between ∼750 and 680 Ma. Its composite δ13Ccarb curve begins with a sharp positive excursion (−3 to +1.5‰ in the dolomitic Tindelpina Shale which caps Sturtian glacial diamictites (Wilyerpa Formation). Then follows a steady climb through the shoaling upward Tapley Hill Formation reaching +4‰ in the Brighton Limestone and culminating in a plateau of 13C enrichment within the platformal carbonates of the Etina Formation (+8 to +10‰). The upper part of the temporal trend shows a decline to +6 to +7‰ in the stromatolite reef facies of the Enorama Shale; and a dramatic drop to −8 to −9.5‰, followed by a recovery to –3‰, in the youngest carbonate unit (Trezona Formation) beneath the Marinoan glacigenic Elatina Formation. The latter is capped by a thin dolostone, the Nuccaleena Formation, which records a consistent up-section decrease in δ13C (−1 to −3‰). Superimposed on this overall pattern of C-isotopic variation are second and third-order fluctuations attributable to eustatic sea-level change. Pyritic sulphur in the lower Tapley Hill Formation (δ34S=+9 to +40‰) is distinctly heavier than that in the underlying Wilyerpa Formation (+4 to +20‰). Minimum 87Sr/86Sr values obtained from the Brighton Limestone (0.7071) and Etina Formation (0.7076) are consistent with their late Cryogenian age. This Sturtian to Marinoan interglacial succession is more complete than equivalent sequences elsewhere, and thus provides a useful reference for global chemostratigraphic correlation. Notable features of its C-isotope curve are the strong post-Wilyerpa positive excursion, also seen in key sections from Svalbard and western Mongolia; the ‘Etina plateau’ of 13C enrichment which is remarkably similar to maxima in the terminal Proterozoic C-isotope records of western Mongolia, north-western Canada and Brazil; and the spectacular depletion of 13C in the lower Trezona relative to similar carbonate facies in the underlying Etina Formation and Enorama Shale. A preglacial offset in δ13Ccarb values of such proportions (∼14‰) is almost unprecedented, except perhaps for that recently documented in the upper Ombaatjie Formation beneath tillites of the Ghaub Formation, Otavi Group, Namibia. It represents a key piece of evidence in the case presented by the proponents of a Neoproterozoic ‘snowball’ Earth.

Impact Ejecta Horizon Within Late Precambrian Shales, Adelaide Geosyncline, South Australia

A solitary layer of shattered crustal rock fragments has been traced over a distance of 260 kilometers within folded 600-million-year-old Precambrian marine shales of the Adelaide Geosyncline, South Australia. The fragments consist entirely of acid to intermediate volcanics (approximately 1575 million years old) displaying shattered mineral grains, shock lamellae in quartz, and small shatter cones. Fragments reach 30 centimeters in diameter and show evidence of vertical fall emplacement. Available evidence points to derivation of the rock fragments from a distant hypervelocity impact into the Gawler Range Volcanics at Lake Acraman, approximately 300 kilometers west of the Adelaide Geosyncline.

Development of the Circum-Antarctic Current

Deep-sea drilling in the Southern Ocean south of Australia and New Zealand shows that the Circum-Antarctic Current developed about 30 million years ago in the middle to late Oligocene when final separation occurred between Antarctica and the continental South Tasman Rise. Australia had commenced drifting northward from Antarctica 20 million years before this.

Acraman impact ejecta and host shales: Evidence for low-temperature mobilization of iridium and other platinoids

Geochemical investigations of a widely dispersed Late Proterozoic impact ejecta horizon and its host shales (Bunyeroo Formation, Australia) have provided strong evidence for low-temperature mobilization of the platinum group elements (PGE), including Ir. PGE anomalies are associated with both the ejecta horizon (Ir up to 2.0 ppb, Pt up to 270 ppb) and the green shales that envelop it. Several PGE anomalies (0.073-0.45 ppb Ir, 3.11-314 ppb Pt) also occur in thin green shale beds at other stratigraphic levels within a predominantly red shale sequence (average red shale background of 0.019 ppb Ir and 0.89 ppb Pt). All green shale horizons analyzed, regardless of their stratigraphic position, have relatively high levels of Ir and other PGE. The nonextraterrestrial, postdepositional origin of the PGE at other stratigraphic levels away from the impact ejecta horizon is indicated by their occurrence in thin, permeable, green beds in a predominantly red shale sequence, their association with enrichments in Cu-V-Zn-Ni, and their nonchondritic PGE interelement ratios. The impact ejecta horizon has a more chondritic PGE geochemistry consistent with a meteoritic origin. A redox precipitation model similar to that invoked for redbed Cu-U-V deposits is proposed to explain the PGE anomalies in the green shales.

The Holocene non-tropical coastal and shelf carbonate province of southern Australia

Abstract Carbonate-dominant sediments are currently forming and accumulating over the extensive marine shelf of the passive margin of southern Australia. A dearth of continental detritus results from both a very low relief and a predominantly arid climate. The wide continental shelf is bathed by cold upwelling ocean waters that support luxuriant growths of bryozoans and coralline algae, together with sponges, molluscs, asteroids, benthic and some planktonic foraminifera. The open ocean coast is battered by a persistent southwest swell, resulting in erosion of calcrete-encrusted Pleistocene eolianites. Much sediment is reworked and overall shelf sedimentation rates are low. High-energy microtidal beach/dune systems occur between headlands and along the very long ocean beach in the Coorong region. The northern, more arid coastal areas also contain saline lakes that precipitate gypsum from infiltrated sea water, and display marginal facies of aragonite boxwork to fenestral carbonate crusts, with stromatolites and tepee structures. In contrast, the southern, seasonally humid Coorong region, has a predominantly continental groundwater regime where sulphate is rare, and the high summer evaporation precipitates dolomite, magnesite and aragonite muds. Fenestral crusts, breccias, tepees and some stromatolites are also present. St. Vincent and Spencer gulfs both afford some protection from ocean swell, but tidal amplitude and currents increase, and a depth and inundation-related zonation of plants and animals is established. Muddy carbonate sand accumulates on the sea floor below 30 m, where filter-feeding bryozoans, bivalves and sponges dominate. In shallower regions, seagrass meadows contain a rich fauna that results in rapid accumulation of an unsorted muddy bioclastic sand. Mangrove woodlands backed by saline marsh with cyanobacterial mats are common, and accumulate mud-rich and gastropod-bearing sediment. As tidal amplitude and desiccation increase northward into both gulfs, a supratidal zone bare of vegetation (sabkha) becomes the site for deposition of gypsum-rich and fenestral calcitic mud.

Sea-level history, 45,000 to 30,000 yr B.P., inferred from Benthic foraminifera, Gulf St. Vincent, South Australia

Abstract Surficial sediments of Gulf St. Vincent, South Australia, are predominantly bioclastic, cool-temperate carbonates. Benthic foraminifera are abundant and distribution of species is closely related to water depth. For example, Massilina milletti is most common at depths ca. 40 m, while Discorbis dimidiatus is characteristics of shallow, subtidal environments. Elphidium crispum, a shallow-water species, and E. macelliforme, favoring deeper water, provide a useful numerical ratio. Their logarithmic relative abundance, in the sediment size fraction 0.50–0.25 mm, correlates strongly with water depth. Vibrocores SV 4 and SV 5 recovered undisturbed sections of Quaternary strata from the deepest part (ca. 40 m) of Gulf St. Vincent. Amino acid racemization and radiocarbon age determinations show that late Pleistocene sections of the cores were deposited over the time ca. 45,000 to 30,000 yr B.P. Species of fossil foraminifera, recovered from these sections, are mostly extant in modern Gulf St. Vincent, thus allowing paleoecological inferences of late Pleistocene sea levels. These inferred sea-level maxima can be correlated with those determined from study of Huon Peninsula coral reef terraces. Initial estimates of tectonically corrected sea levels for transgressions in Gulf St. Vincent at 40,000 and 31,000 yr B.P. are −22.5 m and −22 m, respectively. The intervening regression lowered sea level to −28 m.

The stratigraphy of coastal carbonate banks and Holocene sea levels of northern Spencer Gulf, South Australia

Abstract Stratigraphic and chronologic studies of the coastal sediments of northern Spencer Gulf in the vicinity of Redcliff are described. Two Pleistocene marine units are recognised beneath an extensive Holocene peritidal sequence. The Holocene sequence is dominated by sediments of the Posidonia australis seagrass facies which form a distinctive carbonate bank fringing the coastline. Peritidal sedimentation commenced prior to 6600 radiocarbon yrs B.P. and a + 2.5 m higher relative sea level existed from about 6000 until around 1700 yrs B.P. The subsequent relative fall to present level probably resulted from local tectonic uplift. After the regression, mangrove and samphire colonization, and beach ridge progradation, occurred over the former seagrass bank.

The sedimentary framework of northern Spencer Gulf, South Australia

Abstract Northern Spencer Gulf is the landward extremity of a shallow marine embayment which occupies a structural depression that overlies a major Precambrian lineament. Situated in a warm temperate climate, the low rainfall and high evaporation have created high salinities resulting in an hydrological inverse estuary. Modern sedimentation is dominantly biogenic carbonate to mixed terrigenous-carbonate. It is controlled by a mesotidal regime with occasional storm surges, minor wave activity, and a prolific growth of seagrass in shallow water. The skeletal detritus consists of bivalves, gastropods, forams, echinoids, coralline algae and bryozoa. The subtidal zone between 10–25 m is divided morphologically into two provinces. The wide southern part has a relatively smooth floor, but the northern part is narrower, and the seafloor is either scoured free of loose sediment, or covered with wide belts of megaripples. The subtidal zone between 0–10 m is everywhere dominated by seagrass meadows. The seagrasses are largely Posidonia australis and P. sinuosa and occupy broad depositional platforms, and discrete offshore banks. The seagrass meadows produce and trap mollusc/foram detritus, resulting in the accumulation of very poorly sorted, organically bound structureless carbonate muddy sand. Intertidal and supratidal zone sediments are very extensive. The intertidal zone includes bare sand flats or those covered by the seagrass Zostera . Dense mangroves ( Avicennia marina ) from mean sea level to spring high-tide level are followed progressively by a halophytic (samphire) association and an Atriplex (saltbush) association. Extensive algal mats occur with the halophytes and extend into the mangrove forests. The sediments are muddy and only moderately calcareous. The supratidal zone consists mainly of bare carbonate flats, some stranded beach ridges, and coastal dunes. The carbonate flats contain discoidal gypsum crystals in weakly layered, fenestral, calcitic mud. Coastal changes during historic time are limited, and the dominant sedimentary regime in northern Spencer Gulf is the vertical growth of seagrass areas to form intertidal sand flats.

Mantle plume uplift in the sedimentary record: origin of kilometre-deep canyons within late Neoproterozoic successions, South Australia

In South Australia, canyons 200–250 km long are incised to depths of c. 1.5 km in late Neoproterozoic (after c. 590 Ma) marine deposits of the Adelaide fold belt and canyons 90 km long and 700 m deep occur in correlative marine strata of the Officer Basin 1000 km to the NW. In the Adelaide fold belt, the isotopic signature of limestone veneering canyon walls, canyon sinuosity and sedimentary structures in canyon fill suggest that the canyons were cut subaerially and filled by shallow-water marine deposits during coastal onlap. Eustacy cannot account for the kilometre-scale change in sea level required for subaerial canyon erosion and chemostratigraphy and the lack of evaporites at that stratigraphic level in southern Australia argue against a Messinian-style drawdown. In each region, uplift and crustal extension preceding or coeval with canyon incision are indicated by a regional unconformity, turbidite or mass-flow deposition, and extensional faults. These features and canyon incision may be explained by regional uplift above a rising mantle plume over a distance of >1000 km in southern Australia. Flood basalt volcanism in the Officer Basin tentatively dated at 563±40 Ma may have followed this uplift. Mantle plume uplift is expressed in the sedimentary record by emergent trends in facies, regional thinning, regional unconformities, turbidites, gravity slides, normal faults and incised canyons and palaeovalleys.