Sedimentology of the Christies Beach Formation Alluvial Fan Deposits
Cristino Da Cruz, B.Sc.
Honours Degree of Bachelor of Science (Petroleum Geoscience), 2013
Australian School of Petroleum, the University of Adelaide
The sedimentology of ancient alluvial fans is commonly studied by defining the lithofacies and deposit geometry followed by interpretation of depositional processes and environment. Lithofacies descriptions are focussed on the description of grain sizes, sorting and sedimentary structures of a stratigraphic unit that can be distinguished from adjacent beds. Sellicks beach is a town that located in South Australia and it is located 53 Km away from Adelaide city. The area of Sellicks Beach especially along the beach has laterally extensive exposures of alluvial fan deposits which is very accessible and great place to study this alluvial fan deposits.
The study is focused on the characterisation of the alluvial fan deposits based on field work and lab work. The field work was set to construct the sedimentary log and collecting samples. The logs are used to describe the type of facies in the fan deposits. The lab work was aimed to process the samples through both dry and wet sieving which then the results will be proceed to grain size analysis. The Christies Beach Formation is defined by the exposure of the alluvial fan deposits at the Sellicks Beach however; the representative sections were described at Sellicks Creek. Study is done on these fan deposits by constructing sedimentary log and collecting samples of sediments for grain size analysis.
The sedimentary logs resulted in classification of five lithofacies; Matrix supported massive gravel (Gmm), Massive clay/mud (Fm), Gravelly clays (Gf), Matrix supported graded gravel (Gmg), and Parallel laminated clay/mud (Fl). Matrix supported massive gravel (Gmm) is interpreted as a viscous flow from a debris flow and deposited near fan head which is located close to the Willunga Fault. Massive clay/mud (Fm) is acquired by typical river sediments (low energy environments) and therefore, depositional environment of this facies is near fan toe which is located far from the fault. Gravelly clays (Gf) facies is caused by mixture of mud and gravels that formed cohesive debris flow which is associated with depostional environment of mid fan area. Matrix supported graded gravel (Gmg) is obtained through a viscous flow from a debris flow that is also close to fan head which is located close to the fault. Parallel laminated clay/mud (Fl) suggests that there has been a laminar flow under an upper flow regime.
The grain size analysis obtained from wet and dry sieving has resulted in three types of graphs; bimodal, and two unimodal shapes. The bimodal graphs indicate a mixture of two different types of grains (Clays/mud and Pebbles) which are formed separately by two different energies and dumped together in a new depositional environment. The first type of unimodal has some percentage sand and gravels (about 40 %) but dominated by very fine sand. This type suggest the possible caused of this mixture is from cohesive debris flow (mud flow deposit) from overbank floods. Moreover, second type of unimodal of very fine sand suggests a typical river sediments associated with overbank deposit, back swamp, suspension setting and rapid mass deposition after flooding of the unconfined flows.
The major control of the alluvial fan deposits in Sellicks beach are climatic and eustatic (base level rise). The climatic control suggests that there had been a change from glacial to arid climatic condition. Tectonic of the Willunga basin area seems to only provide the topography for the fan to deposit without affecting the deposition of the sediments themselves. Thus further study of fan geometry is required.
In terms of reservoir potential, the alluvial fan deposits of Christies Beach Formation alluvial fan deposits are classified as poor reservoir quality as they have no potential seal rock and poorly sorted grains.