Doctor of Philosophy
Department of Geology
Senapati, Nikhilesh, The sedimentology of the northeastern Carpentaria Basin, Doctor of Philosophy thesis, Department of Geology, University of Wollongong, 1988. http://ro.uow.edu.au/theses/1418
The northeastern Carpentaria Basin, a Mesozoic sequence, is situated on northern Cape York Peninsula, Queensland. Late Devonian to Early Carboniferous Pascoe River Beds and unnamed Permian sediments underlie the Carpentaria Basin within the study area.
The Pascoe River Beds are subdivided into three informal units. The lower two units were deposited on a mature topography by a meandering fluvial system whereas the upper unit was deposited subaqueously, probably in a lacustrine environment. Some contemporaneous volcanism accompanied sedimentation of the Pascoe River Beds. The sequence was buried to depths of about three kilometres and had a complex diagenetic history, undergoing at least one episode of folding, uplift and erosion prior to the Late Permian.
Unnamed Permian sediments deposited in a cold climate by a high sinuosity meandering river system include thin overbank coal seams. The sediments were restricted to northwesterly trending, structurally controlled valleys flanked in part by crystalline basement. The sediments underwent minimal burial prior to the Jurassic.
Sedimentation in the northeastern Carpentaria Basin began in the Middle Jurassic and continued through into the Early Cretaceous. The climate was tropical, probably with seasonal rainfall and flooding.
The pre-Mesozoic surface was a mature topography with gentle relief. Highlands were present to the east, and valleys and ridges trended in a northwesterly, direction. Moderately sinuous fluvial systems occupied the valley floors and deposited mainly quartzose sands of the Wenlock Member. The sediment was derived from the highlands and shows a unimodal transport direction. Each valley's fluvial system was similar but probably unconnected. There was considerable vegetation and the water table was high facilitating the development of discontinuous coal seams.
A shallow marine transgression from the north followed, inundating both the fluvial systems in the valleys and the adjacent basement ridges. The highlands in the east remained exposed and the marine environment probably extended no further south than the Pascoe River area. Ooliths, chamositic-rich sandstone and mudstone were deposited forming the Bromley Member. Bottom water temperatures were greater than 20°C.
The sea then regressed leaving extensive low relief plains. Sediments were carried from the eastern highlands and deposited in bed-load dominated, braided fluvial systems, forming the extensive sheet sands of the Wreath Member.
The sedimentary history from this period on was dominated by a progressive marine transgression which occurred as a number of pulses. The encroachment of the sea, due to subsidence and eustatic changes, was from the west. The transgressive sequence was heralded by the deposition of the Batavia Member in a distal meandering river environment which was transitional between a braided river system and a near shore marine system. Deposition was tidally influenced, with the dominant palaeocurrent direction oriented towards the east.
The Glennie and Briscoe Members were deposited during two transgressive episodes. The coarsening upward cycles in these members consist of bioturbated laminated mudstone overlain by marine sandstone with Skolithos burrows. The cycles were deposited in a wave and tide-dominated clastic shore-line environment.
A further marine transgression accompanied deposition of the basal Rolling Downs Group. This sequence, of glauconitic mudstone and sandstone, was deposited in a shallow marine environment below wave base in waters with temperatures greater than 20°C. Additional sediments were deposited over the area because geothermal gradient studies show burial of the basal Rolling Downs Group to a depth of at least 750 m.
Sedimentological characteristics in the study area indicate the Permian sediments, Wenlock Member and Batavia Member have a low potential for economic coal deposits. However, the sedimentary sequences have suitable source rocks, reservoirs, seals, anticlinal structures and levels of organic maturation to host accumulations of hydrocarbons. Extending and correlating the studied sequence towards the west suggests the entire Carpentaria Basin has a significant hydrocarbon potential.