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Tumblagooda Sandstone

At Red Bluff, the Tumblagooda Sandstone in this section is different to the Tumblagooda Sandstone found at Natures Window, and thus, the deposition environment is different. Red Bluff was most likely deposited in a fluvial-intertidal environment (shallow waters) as indicated by the high abundance of skolithos trace fossils, skolithos generally reside in shallow waters (refer to notebook, page 58). The higher abundance of trace fossils in the upper section of the sandstone than the lower section suggests a change in environment. The lower sections of the Tumblagooda Sandstone represent a high energy braided fluvial environment.

Evidence to support this include trough cross-bedding, which requires a flow medium, generally medium-grained and large pebble horizons (notebook page 57). The upper section of the unit is dominated by finer grains, trace fossils (skolithos) and planar cross-bedding, which suggests a lower energy environment, perhaps in a delta, where skolithos can form in relatively shallow waters. At The Loop, the Tumblagooda found in this section most likely formed in an intertidal environment due to the high abundance of vertical and horizontal burrows (notebook page 61).

High tides have caused the skolithos to burrow, and during the low tides, the skolithos come out of the burrows. 2. 7 Comparison/contrast of the core logs Red Bluff The Tumblagooda Sandstone found at Red Bluff (see figure 1 for the log of the area), is vastly different to the redbed found at Nature’s Window. Grain sizes vary at this location, from fine to very coarse, but generally the unit is fining upward. Trough cross-bedding dominates the lower bedding of the units and indicates an environment of a fluvial-braided river system, also observed by Hocking & Mory in 2006.

This type of environment indicates a higher energy environment, the lack of trace fossils and larger grain sizes are evidence of this. The lower units also display pebble horizons, higher envergy is needed to move such large grains, so it is possible they were deposited during storm events. At around 8. 5m there’s a section of pebbly sandstone known as the Gabba Gabba Member, this most likely formed in a storm event where pebbly material washed down from the top of the unit and deposited. The likely facies association for Tumblagooda Sandstone in this area is FA3, it displays fluvial-braided river sedimentary units and fining up cycles.

Upper units of redbed are finer grained unit and are generally better sorted. Finer grains may be due to the dying down of the system, or a change in environment. The upper units consist of sandstone and siltstone, that are thinly bedded. No pebble horizons were observed in the upper sections of the sandstone. Trace fossils (burrows) from gastropods were found in a section of the siltstone and skolithos were found at the very top of the section suggesting a lower energy environment, such as intertidal fluvial. The upper section of the log represents facies association 4.

Figure 20 displays the stratigraphical log for this section and is found in the appendix below. Nature’s Window The redbed found at Nature’s Window, consists mainly of medium grained sandstones that differ to the sandstone at Red Bluff, due to disruptions to the sediment from bioturbation. Lack of trace fossils in this section cannot lead to a diagnostic of the creature that caused the bioturbation. Differences also consist in the cross-bedding of the sandstone. The sandstone found at Nature’s Window, is dominated by planar cross-bedding indicating it was deposited in a calmer environment than Red Bluff.

Toward the top of the sequence and the middle of the sequence, horizons of trough cross-bedding are displayed, which may indicate the beginnings of a change in environment. There is no siltstone was found in this outcrop, which indicates that Red Bluff and Nature’s Window represent two different depositional processes. The characterisitcs of Nature’s Window suggests that belongs to facies assiciation 2. Figure 21 displays the stratigraphical log for this section and is found in the appendix below.

2. 8  Petroleum Potential Tumblagooda Sandstone is found in the Gascoyne Platform and displays minor gas show. Literature suggests it could be a potential reservoir (Lasky & Mory 1999). The Tumblagooda has poor porosity at shallower depths, because of infilling of pore space, at greater depths the Tumblagooda has retained excellent reservoir characteristics. The Tumblagooda is around 1500m thick, has a porosity around 23% and permeability between 1280mD. The potential seal could be the Dirk Hartog Group lying underneath the Tumblagooda Sandstone, however, research suggests it is unlikely (Lasky & Mory 1999).

The Gneudna Formation could act as a potential top seal for this section. 2. 9 Discussion The relationship between the Williambury area is hard to compare to the Kalbarri area, the only defining factor is that the Gneudna Formation found on the Gascoyne Platform, is found overlying the Tumblagooda Sandstone (picture). They could also share a petroleum system, whether or not this petroleum system is successful is unknown. Tumblagooda Sandstone is also found in the Merlinleigh Sub-basin, overlain by the Dirk Hartog Formation, the Nannyarra Sandstone and the Gneudna Formation and Munabia.

May possibly share a petroleum system. Figure 13 a stratigraphic column shows the relationships between the Williambury and Kalbarri units. 3. 0 Area A Williambury, Gneudna Formation and Associated Units 3. 1 Introduction The purpose of this section is to provide a general introduction to the geology of the Williambury area, a description of the units present, along with its depositional processes and geological history are included. As well as this a map completed during this section of the trip will be included for visualisation of the geological history.

The Gneudna Formation and associated unite are found in the Merlinleigh Sub- basin of the Southern Carnarvon Bain. The formations range from Mid to Late Devonian in age and consists of limestones, siltstone, sandstone and horizons of dolomite. The Gnuedna Formation can be divided into 3 facies types, based on the varying type of fossils found in each section, this will be discussed later. 3. 2 Objectives The objectives of this section were to traverse the Gnuedna Paddock are of Williambury (Area A), taking notes on different rock type and different Formations.

A map of the associated units in Area A was made using the notes taken during the field days. A cross-section of the area was made based on the map. 3. 3 Methods Methods used included, taking GPS points at different rock units and marking them onto an aerial map to distinguish bearings and lithologies, traversing the area, and identifying rock and fossils types. 3. 4 Regional Geology The Merlinleigh Sub-basin of the Southern Carnarvon Basin formed during the Ordovician. The basin formed as an interior-sag basin or intracratonic basin, which is formed by diverging plates.

As the basin developed sediments were deposited and different formations and members formed. The Gneudna Formation formed during the Mid-Late Devonian on the eastern margin of Merlinleigh. There is a high abundance of fossils within this section, however, low abundance as the unit gets older. Further east, the Gneudna Formation transitions (Collins, 2016) into the Nannyarra Sandstone, and then into the basement rock (granite). 3. 5 Lithology/Palaeontology Gneudna Paddock is generally made up of calcareous fossiliferous rocks interbedded with siltstones, sandstones, and a granitic basement rock.

Grain size varies throughout as do sedimentary structures as the environment fluctuates. There are a high abundance of fossils and trace fossils within this area. A coarse granite, rich in quartz and feldspars dominates the western edge of Gneudna Paddock, this represents the oldest unit and is the basement rock of the system (notebook page 11). The Nannyarra Sandstone is a quartz-rich sand, that displays ripples and cross bedding, there are variations in the composition of the sandstone, some of these variations match the composition of

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