The Winton Formation is a Early to Late Cretaceousgeological formation, mainly in central-western Queensland, Australia. It is late Albian to early Turonian in age, between ~103 and 93.9 million years ago, and is the latest formation of the Rolling Downs Group. The formation blankets large areas of central-western Queensland, as well as northwestern South Australia, the southwestern corner of the Northern Territory, and parts of northern New South Wales.
Geology
Map showing the extent of the Winton Formation within Queensland, and in the vicinity of the town of Winton specifically
The Winton Formation, a subject of interest for both basic research and economic geology, is stratigraphically widely established to be the uppermost unit of the Manuka Subgroup, conformably overlying the marineMackunda Formation, also forming the uppermost strata of the Rolling Downs Group. It also underlies Paleogene or younger Cenozoic strata. Significant erosion of the Winton Formation has oxidisedpalynomorphs in layers close to the surface, once limiting the capacity to estimate the youngest age of deposition. However, U-Pb isotope dating with laser ablation of detrital zircons later enabled more precise dating, suggesting that sedimentation of the Winton Formation concluded around or after the Cenomanian–Turonian boundary, about ~93.9 million years ago. The same results also suggest that deposition began in the latest Albian, between ~103 to 100.5 million years ago. The strata of the Winton Formation consists primarily of sedimentary rocks such as volcanolithicsandstone, siltstone, mudstone, arenite and claystone, within complex facies. Pockets of coal deposits and intraformational conglomerates also known.
Localities in the greater Winton region which host fossils, such as those in the Belmont, Elderslie and Lovelle Downs Stations, Lark Quarry, and the Bladensburg National Park, all come from a layer of the Winton Formation which was settled during the late Cenomanian to early Turonian epochs, about ~93.9 million years ago. These possess a number of distinct facies and horizons associations that are rather continuous across the area. Other localities, such as in the Isisford area, were deposited about 10 million years earlier during the late Albian to early Cenomanian epochs, between ~103–100.5 million years ago. With this knowledge, the age of the upper Mackunda Formation was constrained to the latest Albian, about ~104 to 102 million years ago, which also constrains the age of the early Cretaceous layers of the Winton Formation to be no older than this.
Hydrology
The sediment localities in the wider Winton region indicate that the upper layers of the Winton Formation were derived from freshwater and terrestrial depositional environments. This is further supported by the occurrence of freshwater gastropods like Melanoides and freshwater bivalves such as Hyridella and Megalovirgus. Previous studies have proposed that lakes may have been present at sites including near Lovelle Downs Station, and the Lark Quarry Conservation Park. However more recent findings have revealed that the lacustrine sedimentation required for this proposal did not occur during or even before the creation of the track-bearing surface at Lark Quarry, and lakes were seemingly only a small portion of a landscape that consisted primarily of channels and floodplains. While older sediments are suggested to have been deposited in brackish waters, perhaps in a fluvial-deltaic setting influenced by tides along the Eromanga Sea, the younger strata occur in sediments which indicate a more terrestrial and freshwater alluvial plain.
The sheer thickness of the Winton Formation in some areas (about 1,000 metres (3,300ft)) suggest that there was once a large river system, which over a span of at least 10 million years in a variety of forms, filled the Eromanga Sea, with periods in which this river flowed southwest as part of the continental Ceduna River system. The primarily siltstone as well as mudstone and dolostone sediments of the formation host plant fossils are associated with trough and tabular cross-stratification, current ripple cross-lamination, and possible ox-bow lakes, giving further evidence that the upper layers of the Winton Formation represents a meandering river system, and have indeed been confirmed by detailed sedimentological studies.
Palaeoclimate
The Winton Formation was once treated as representing a single palaeoenvironment in older palaeoclimatic studies, regarding it as a generally warm temperateecosystem. Although the presence of coal suggested humidity, the composition of the floral biota was interpreted to indicate dry conditions. More recently, higher resolution dating of the local biota communities and additional fossils described in the last 25 years allows for an improved investigation of the local palaeoclimate.
Various methods have been utilized to study the palaeoclimate of the late Cretaceous layers of the Winton Formation in better detail. Research has found that the palaeoclimate was quite warm (~16°C (61°F)mean annual temperature) and wet (1,300 millimetres (51in) mean annual precipitation), estimated by the foliar physiognomcic methods of Climate Leaf Analysis Multivariate Program, Leaf Area Analysis and Leaf Margin Analysis, as well as mutual climatic range methods hailed from the floral assemblage. However high and seasonal precipitation appears to have been, it was lower than in modern highland forests today, neither were there any indications of the usual extremes of dry and wet as known from modern monsoons. For instance, in modern Darwin, often a victim of monsoonal weather patterns, the cumulative three driest months receive about 8.1 millimetres (0.32in), while the wettest months receive an average of 1,120 millimetres (44in) in rainfall. Compared with the late Cretaceous strata of the Winton Formation, estimated to have received about 268±137 millimetres (10.6±5.4in) during the driest three months, and 630±206 millimetres (24.8±8.1in) in the three wettest months. Additionally, growth indices on fossil wood indicate that climate variables differed between years, to the point that it had an observable influence on ring width in podocarp-like wood. This variability seems to be oscillatory and systemic, reflecting a similar phenomena to the Pacific Decadal Oscillation were driving patterns of climate in the mid Cretaceous.
Alongside botanical indicators, parts of the fauna are also climatically informative. For instance, crocodyliform fossils are known from the late Cretaceous layers of the Winton Formation on Elderslie Station, about 60 kilometres (37mi) northwest of Winton. With the assumption that these remains are referable to Isisfordia duncani or a related species, and if eusuchians are accepted palaeothermometers, their occurrence supports a mean average temp of >16°C (61°F). Additionally, modern lungfish, serving as analogues for Metaceratodus ellioti, also from the upper strata of the Winton Formation, have a distribution that is limited to the tropics and subtropics. Undescribed freshwater turtles also from these same rocks also suggest a degree of warmth, as even cold-adapted modern turtles require warmest mean monthly temperatures over 17.5°C (63.5°F).
Australian Age of Dinosaurs, which operates the Australian Age of Dinosaurs Museum of Natural History, and holds annual dinosaur digs in the Winton Formation
12Kemp, A (1997). "Four Species of Metaceratodus (Osteichthyes: Dipnoi, Family Ceratodontidae) from Australian Mesozoic and Cenozoic Deposits". Journal of Vertebrate Paleontology. 17 (1): 26–33. Bibcode:1997JVPal..17...26K. doi:10.1080/02724634.1997.10010949.
