Year

1998

Degree Name

Doctor of Philosophy

Department

School of Geosciences

Abstract

The Capertee High in the Mudgee-Capertee region was a Late Silurian to earliest Middle Devonian shallow marine to subaerial palaeogeographic unit to the east of the Hill End Trough, north-east Lachlan Fold Belt, southeastern Australia. Rocks of the Capertee High occur in structurally-complex sequences along with Ordovician basement, thick Late Devonian siliciclastic sequences, and Middle Carboniferous granitic intrusions. Flat-lying Permian-Mesozoic sequences overlap the High in the east and north.

Early Devonian sequences associated with the Capertee High can be broadly subdivided into two categories: (a) Platform sequences, which were deposited on the shallow marine to subaerial platform areas of the Capertee High and have been collectively defined as the approximately 4000 m thick Kandos Group; and (b) Western Platform Margin Sequences, which were deposited in slope and base-of-slope settings along the western margin of the Capertee High platform and have been subdivided into the Queens Pinch and Limekilns Groups. The latter sequences are transitional westwards with strata of the eastern Hill End Trough.

This thesis focuses on: (a) elucidating the stratigraphy, depositional processes and environments, and palaeogeography of the Kandos Group (particularly in the Rylstone-Cudgegong area); (b) establishing a biochronological framework for the Early Devonian succession by obtaining conodont faunas from the carbonates; (c) attempting correlation of the Kandos Group with platform margin units and Hill End Trough sequences; and (d) producing a synthesis of Early Devonian sedimentation and volcanism on the Capertee High and eastern Hill End Trough. Results show that the Kandos Group spans almost the entire Early Devonian and is separated from Late Silurian sequences by a widespread unconformity. The Kandos Group can be accurately correlated with the western platform margin sequences which are also dated by conodonts; however, correlation with the Hill End Trough succession is imprecise and relies on a small number of SHRIMP U/Pb zircon dates (which are inaccurately aligned with conodont zones) and rare macrofaunas.

The Lochkovian portion of the Kandos Group is a transgressive to regressive shallow marine sequence deposited on a west- to southwest-sloping shelf flanked to the east by eroding sedimentary basement and active silicic volcanoes, and passing to the west into deepwater facies along the western margin of the High (basal Mullamuddy Formation) and in the eastern Hill End Trough (upper Crudine Group). Sedimentation in the Kandos Group began in the early Lochkovian woschmidti Zone and comprised (in ascending order): a transgressive sequence of wave-dominated, fan-deltaic elastics (Warrah Conglomerate); a unit of transgressive biostromal to biohermal carbonates (Clandulla Limestone; woschmidti to eurekaensis Zones); a transgressive to regressive muddy, storm-influenced shelf deposit (Yellowmans Creek Formation; eurekaensis to delta Zones); and a thick regressive unit of storm-dominated siliciclastics deposited in middle shelf to nearshore settings (Roxburgh Formation; delta to pesavis or early sulcatus Zones). Facies prograded west during the late Lochkovian regression, culminating in shelf exposure and some erosion in the east. The early Lochkovian transgression may reflect a eustatic sea-level rise, whereas the late Lochkovian regression more likely indicates uplift of nearshore areas ahead of increasingly proximal volcanism.

Thick sequences of dacitic to rhyolitic pyroclastics and volcaniclastics (Riversdale Volcanics and Huntingdale Volcanics) demonstrate that the Capertee High was dominated by subaerial silicic pyroclastic volcanism during the early to mid Pragian sulcatus and kindlei Zones. Medium-scale calderas, centred southeast of Cudgegong and in the eastern Capertee Valley, were flanked by extensive, shallowly-dipping sheets o f silicic ignimbrites and epiclastics which probably overlapped, forming a continuous sheet. Short-lived transgressions on the platform resulted in minor carbonate deposition during volcanically quiescent periods. Huge volumes of volcanic detritus were transported to the west and deposited as lowstand fans and aprons along the margin of the Capertee High (Mullamuddy and Kingsford Formations) and in the Hill End Trough (Merrions Formation).

The Carwell Creek Formation overlies the Riversdale Volcanics and represents a possibly eustatic transgression in the latest Pragian to earliest Emsian (pireneae to dehiscens Zones) which initially reworked then drowned the existing volcanic topography, creating a broad southwest-sloping shelf with local eroding volcanic islands. Sedimentation in this setting was dominated by a laterally and temporally extensive regime of storm- and tidally-influenced nearshore to shelf siliciclastic sequences deposited in barrier island settings, along with local biostromal and biohermal carbonate buildups deposited in lagoonal and open-marine environments (Carwell Creek Formation, Myrtle Grove Formation and basal Mt Frome Limestone). Transgression cut off the supply of clastic detritus from slope and basin areas, and condensed mudstone sequences were deposited along the margin of the Capertee High (Warratra Mudstone and 'Rosedale Shale') and in the Hill End Trough (basal Cunningham Formation). A transgressive pulse in the perbonus Zone was followed by regression, local platform exposure and erosion of platform areas in the middle Emsian (late perbonus to inversus Zones), and a number of carbonate aprons developed along the western margin of the High (basal Sutchers Creek Formation and Inglebum Formation).

A transgression (possibly eustatic) resulted in further deposition of carbonates on the Capertee High during the late Emsian serotinus Zone (middle to upper Mt Frome Limestone and unpreserved equivalents). Regression later in the serotinus Zone resulted in erosion of some of these platform carbonates and redeposition in fans along the western margin of the Capertee High (Jesse Limestone and upper Sutchers Creek Formation). This regression probably continued into the earliest Eifelian partitus Zone, when carbonates of the Mt Frome Limestone were replaced by wave-dominated shoreline and shelf elastics of the Boogledie Formation. Deposition of this clastic sequence continued for an unknown length o f time into the early Eifelian before it was terminated by uplift and regression related to the mildly manifested Middle Devonian Tabberabberan deformation which effectively ended the depositional history of the Capertee High.

Modem analogues for the Early Devonian Capertee High-Hill End Trough-Molong High exist in the Sumisu Rift (off the southeast coast of Japan), the southern Havre Trough (north of New Zealand), and the Sulu Sea (between Borneo and the Philippines). The Tertiary Waitemata interarc basin (north of Auckland, New Zealand) displays gross similarities in facies, palaeogeography and scale compared to the Early Devonian Hill End Trough and its flanking highs.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.