The aim of this honours project was to examine the coal seam interburden at Caval Ridge Coal Mine and to determine the likely depositional environment. This project required the detailed examination of highwalls, thickness and lithology distribution maps, borehole geophysics, core image logs, and outcrop photos.

Highwall Interburden Sedimentary Study

Depositional Reconstruction of the Late Permian Moranbah Coal Measures – Caval Ridge Mine, Northern Bowen Basin, Eastern Australia

By Meggie Doran Supervisor: Professor Joan Esterle

Environmental Interpretation:

Pit 20N

Pit 40N

Point of Difference:

Uncertainties in the MCM environmental interpretation centre on a singular bed form featuring large scale dipping beds which extends from the top to the base of individual bodies. This sedimentary feature could have either been formed by deltaic style deposition such as a lacustrine delta or subaerial crevasse splay (Flood 1985, Johnson, 1984, Flood and Brady, 1985, Galloway and Hobday, 1996; Herbert 1997) or alternatively the deposits of laterally accreting point bars of a channel (Fielding et al, 1993 and Fielding and Alexander 2001 ). A crevasse splay is distinguished from a channel by a coarsening up gamma ray signature, whilst a channel will be fining up. The toes of these deposits are also different, where a splay will be finer grained in their toes (due to flow expansion and settling of muds further from the channel, whilst a channel point bar will have coarser grains in its toes closer to the channel where the flow velocity of the river is fastest

Crevasse splay vs. Point Bar

Sedimentary Study Caval Ridge

Moranbah Coal Measures: The Moranbah Coal Measures (MCM) are the oldest coal bearing interval of the non-marine Black Water Group, occurring in the north western reaches of the Bowen Basin. The MCM are intercalated with laterally continuous coal seams that split and coalesce for some 250 km along the strike of the basin. In the north, near Collinsville, the Moranbah Coal Measures are over 537 m thick; to the northeast at Kemmis Creek they are 760 m thick and to the south and southwest they thin to less than 20 m over the Comet Ridge and Capella Block. (Dickens and Malone 1973) The MCM’s lateral equivalent is the German Creek Formation, a marine unit, which represents a southerly progradation from fluvial and upper delta plain environments to parlalic or marginal marine environments. The retreat of the palaeoshore to the south has been attributed to uplift along the eastern margin of the Bowen Basin. As the basin filled the MCM’s environments would have included extensive shallow lakes and swamps developed in what may have been an internal drainage system (Jensen 1975) characterised by low velocity rivers and extensive floodplains. A process of cyclic crevasse splay accretion and periodic channel avulsion, accompanied by synchronous differential compaction of thick underlying peats to form complex coal seam splitting patterns (caused by thermal subsidence and foreland loading ) is thought to be the main mechanism of sediment accumulation in the MCM (Johnson 1990).

Significant findings have shown that the massive sandstone channels show an increase in thickness and width up-sequence. The two channels in each Pit 10N and 12N have a linear trend in this increase supporting the interpretation of a prograding environment, Recently, environments characterised by multiple channels radiating from an apex synchronously have been characterised as part of a larger system known as a distributive fluvial system. These deposits now are considered to make up a large proportion of the rock record (Weissmann et al 2010), their scale being so large, that they are usually only identified by their prograding signature. Distributive fluvial systems (or DFS’s) develop by repeated avulsions (Nichols and Fisher, 2007) and will experience an overall decrease in channel width and depth downstream and an increase in the proportion of floodplain area relative to channel area . DFS deposits have been likened to alluvial fan or ‘megafan’ deposits and are believed to form at the entry point of rivers into sedimentary basins. Their radial form although superficially comparable to a delta deposit (both coarsening upward sequences) will always have numerous main channels that are active at the same time compared to the one active channel of a delta which forms lobes which get abandoned before avulsing to new channel pathways.

Prograding massive channels up-sequence in Pit 10N and 12N

Nichols and Fisher, 2007

A DFS may have multiple types of channel forms (from meandering to braided) and depending on the main type of channels and the number of threads will determine the types of deposits it will produce and preserve. The illustration below depicts the difference to a DFS morphology comparing highest energy braided river threads to meandering rivers. Generally all distal DFS facies commonly consist of wetland and hydromorphic floodplain deposits that encase single channels although braided systems will have a well-drained floodplain and meandering will have standing water bodies and abandoned channel fills. Medial deposits show larger channel belt size and relatively well-drained soils, indicating a deeper water table. Proximal deposits of DFS display larger channel belts that are amalgamated with limited or no soil development across the apex of the DFS and can show greater channel amalgamation. Proximal deposits will extend further in braided forms of DFS. The resulting vertical sedimentary succession from progradation will display a general coarsening-upward succession of facies.

D05 Interval Dysart Lower

D47 Interval Dysart Upper

H00 Interval Harrow Creek Lower

Massive sandstone channel three

Massive sandstone channel four

Highwall Observations:

Lithology Distribution Maps - Progression Up-sequence of Interburden showing modelled facies of sandstone and mudstone that assign to a block model. Sandstone may be over-estimated.

Caval Ridge Coal Seam Schematic:

Borehole Geophysics:

Core Imagery: A total of five facies were

identified: channels, peatlands, floodplain, crevasse splay, lake deposits.

Further results are due Nov.

Modern Analog:

The Permian environment is viewed by many to have been analogous to modern day cold climate coals forming in shoestring bogs in tundra areas of north western Siberia (Ob River) (Retallak, 1999) or the lowland river basins of Canada such as the Mackenzie River. These environments are forming in high latitudes, similar to the Permian palaeoclimate which the Bowen Basin formed in, and are also characterised by extensive peatmires, marsh systems, channels, floodplain and lake environments. A very low gradient, stable floodplain is likely of the Permian Moranbah Coal Measures where it could episodically be traversed by anastomosing rivers which drained southwards into a marine basin. Recently the Ob River Siberia, Mackenzie River Canada and Yukon River Alaska, have been suggested to be more accurate analogues due to their high latitude and tectonically active setting, (Bowen Basin formed in an active foreland basin margin) but studies comparing them are limited.

b. Mackenzie River Delta, North West Canada (GoogleEarth Pro.) a. Environment envisaged: low lying channels, splays, floodplain and swamps of upper delta plain

Meggie.D, 2015

The quality and resolution of the laser scan imagery was greatly impacted by the position of sun at the time of field-work and were used in combination with photos to assist with interpretation and overall geometries within the Pits.

It is interpreted that the environmental system analysed may in fact be part of a much larger scale system such as the DFS, which is similarly characterised by coarsening up, prograding features where channels become wider and thicker up-sequence and the proportion of floodplain area relative to channel area is greater down-sequence. A larger study area would be needed to support this interpretation.

a. b.

Pit 10N

Pit 11N Pit 12N

Location:

Caval Ridge is currently operating in two pits: Horse Pit and Heyford Pit. These pits are accessed by a series of ramps named 10N to 50N. The two pits are mining stratigraphically distinct coal intervals with Horse Pit mining the older Dysart seams and Heyford Pit mining the younger Harrow Creek coal seams.

N

Geological map courtesy of BMA, 2015

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