Seismic stratigraphy and relict coastal sediments off the east coast of Australia

MARINE aEOLOGY INTERNATIONAL JOURNAL OF MARINE GEOLOGE GEOCHEMISTRY ANO GEOPHYSICS

ELSEVIER

Marine Geology 121 (1994)81

107

Seismic stratigraphy and relict coastal sediments off the east coast of Australia Ian Browne Cable Sands Holdings Pty. Ltd., 14 Martin Place, 18th Floor, Sydney, N S W 2000, Australia Received 8 November 1993; revision accepted 6 July 1994

Abstract

During 1989 marine geophysical surveys were conducted in three regions within the Territorial Sea 1 of New South Wales (NSW) to explore for heavy mineral sand deposits. The northern and central NSW coastline is a recognised beach placer province for economic deposits of futile and zircon. Economic placer deposits of these minerals occur as spatially discrete lenses within coastal sand bodies. Exploration for marine placers was previously attempted during the late 1960s, however with mixed results. In order to improve the recognition of potential host sites for placer minerals in marine environments a custom made acquisition system was developed which resulted in digital real-time data-acquisition linked to a high-resolution seismic system. The digital data were recorded onto mass storage media and subsequently digitally processed to reproduce geophysical records. A literature search suggested that the possibility of preservation of drowned relict coastal sediments was questionable due to reworking by subsequent marine transgressions. The improved resolution of sedimentary structures beneath the seabed afforded by processing digital data and interpreted using seismic sequence analysis shows that relict structures persist, some of which display features attributed to beach-barrier preservation. These structures are commonly draped by an apron of sediments deposited during the post glacial transgression. Relict drowned coastal deposits show various degrees of preservation in the areas surveyed. It is proposed that the degree of preservation of relict structures is not dependent on the present coastal landscape. Preservation may be dominated by conditions operating at the time of deposition and to post depositional processes which uniquely characterise each location.

1. Introduction

Approximately 1100 line kilometres of geophysical traversing were conducted during the months of June through September 1989 in three separate areas shown in Fig. 1. The p r o g r a m m e was designed with the view that heavy mineral (Hm.) sands may occur in the marine environment in 1Sea corridor defined by joint Federal and State legislation commencing from a baseline on coast (usually mean-low-water mark) to the 3 nautical mile limit. 0025-3227/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0025-3227(94)00089-1

configurations which emulate economic terrestrial deposits. Terrestrial Hm. placers occur as spatially discrete lenses in Holocene and Pleistocene beach and barrier environments although the processes are not fully understood. The resolution of the seismic data required to resolve these environments offshore was perceived to represent a critical component in the survey specifications. To achieve this, a review of the existing convention of displaying the data in analogue formats was required. Furthermore, high access costs for marine exploration for Hm.

1. Browne/Marine Geology 121 (1994) 81 107

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placers, when compared to terrestrial exploration methods demanded efficient data acquisition and the development of conceptual models for structures hosting Hm. enrichment offshore. Exploration for marine Hm. placers was conducted during the late 1960s in the Territorial Sea and was based on the view that Hm. deposits occur within drowned relict strand lines that correspond to shore-parallel elevations of seabed topography. Follow-up drilling showed this correlation did no exist. This work did however identify enriched lenses of Hm. submerged well below the seabed in configurations analogous to terrestrial deposits (Layton, 1967; MacCulloch, 1969). The primary objective of the current programme was to explain these findings in terms of the sedimentary structures beneath the seabed, and to

develop geologic models to improve target definition in unexplored regions. A representative selection of three coastal landscapes were chosen for marine surveying based on drilling results from past surveys. Information from the previous data set was then married with new geophysical data. The project was commenced on a very limited knowledge base of relict sedimentation in the inner east Australian shelf. Some earlier surveys were conducted using single channel seismic methods using analogue displays that were based on regional objectives. Interpretation of the digitally processed seismic records showing improved definition of sedimentary structures suggest that it is possible to assign seismic sequences to models associated with beach and barrier preservation and related sediments.

I. Browne~Marine Geology 121 (1994) 81-107

2. Operating conditions The east coast of Australia represents a beach barrier regime established under micro-tidal conditions as a high energy coast (Roy, 1982). On this coastline average sea states usually exceed 1 m seas with 2 m seas occurring fairly regularly. In the Sydney region persistently moderate sea swells exceed 1 2 m for 63% of the year and seas of 2 3 m occur for about 21% of the time (Short et al., 1992). The quality of seismic data recorded on analogue records during earlier exploration programmes was effected by these inclement sea conditions making it difficult to confidently interpret the data. A major factor forcing a review of a reliance on analogue data was the view that marginal sea states, seas approaching and exceeding 1 m wave heights, would prevail. Additionally, it was considered that digital acquisition would extend the operating window under which data could be acceptable as data which may have otherwise been marginal on analogue records could be enhanced by post survey data processing.

3. Background and methods The east coast of Australia has a history as a beach placer mineral province (Morley, 1981 ). The principal mineral suite of greatest economic value is rutile and zircon. The mineral suite characteristically includes minor ilmenite, monazite, and gold. For the most part these minerals do not constitute the prime exploration objective. A literature study of known Hm. occurrences in terrestrial coastal landscapes shows that placer enrichment may occur in certain configurations as a function of the coastal geometry (Wallis and Oakes, 1990). It is commonly observed that Hm. enrichment preferentially occurs on beach berm crests on beaches draped around coastal headlands. It has also been observed that relict deposits are often sited adjacent irregularities in basement topography. Other sites of Hm. enrichment appear to offer no apparent explanation in terms of preexisting geomorphologies. Recent work by Roy et al. (1992) proposes several models of occurrence

83

although this evidence does not facilitate an explanation where mineralisation is conspicuously absent. The predictive science is very new. Accumulations of Hm. are identified with both Pleistocene and Holocene highstands which are found in the present coastal barrier system (Force, 1991; Wallis and Oakes, 1990; Roy, 1980). Although field evidence suggests that in extensive prograding barrier systems successive highstands are not always easily recognisable. In crosssectional perspective of Pleistocene and Holocene coastal sediments, Hm. deposits occur on berm crests above the prevailing sea level (Force, 1991; Morley, 1981). Several superimposed depositional cycles often result in massive though spatially restricted lenses of heavy minerals which commonly show a seaward dip (Force, 1991). Recent work by Force (1991) and Hamilton (1990) suggest that the depositional environment appears to be one of temporary dynamic equilibrium. In offshore sediments it is postulated that relict beach systems may be stranded on the inner shelf, as a consequence of fluctuating Quaternary sea levels, as described by Chappell (1974) and Bloom et al. (1974). The inner shelf on the east coast has been relatively tectonically stable throughout the Quaternary, although minor subsidence may have occurred on the outer shelf(Davies, 1975). Based on both geophysical and geological evidence, proposed models for beach and coastline evolution in eastern Australia are considered to arise due to alternating and sequential phases of marine transgressions and regressions (Roy and Thom, 1981; Marshall, 1979; Davis, 1975).

3.1. Transgressions and regressions It is proposed that both transgressive and regressive events provide opportunities for the preservation of beach/barrier sediments. Of particular interest from the exploration perspective is the recognition of preserved beaches and the discovery of possible host facies for heavy minerals. Heward (1981) considered that the preservation potential of beach facies in transgressive regimes was low, whereas preservation in regressive regimes may depend on a fortuitous balance of circumstances that preclude subaerial erosion.

84

1. Browne/Marine Geology 121 (1994) 81 107

Marine transgressions have been described as occurring by a process of "shoreface" retreat (Swift, 1968). Sandars and Kumar (1975), on the other hand, proposed that on occasions when sealevel rise is rapid and the transgressive surface has a relatively gentle slope barriers may be overstepped during transgressive episodes by a process of "in place" drowning. Therefore in conditions of rapid sea-level rise Sandars and Kumar (1975) suggest that the transgressive model results in isolated or stranded and partly modified barrier sediments. On occasions transgressive sediments may fill and be preserved in topographic depressions on the underlying surface (McCubbin, 1969). However, this scenario does not propose that beach and berm sediments are preserved. Hodbay and Tankard (1978) acknowledge that under circumstances of high sediment input, beach/barrier sediments may vertically aggrade during the transgressive phase in a manner representing a set of superimposed terraces. Under these very unique conditions opportunities for beach-berm preservation may exist, although they have not been reported on the Australian east coast. In regressive regimes the opportunities for preservation of beach/barrier sediments are much enhanced (Heward, 1981), although beach deposits and coastal dunes may be subject to modification by subaerial processes (P.S. Roy, pers. commun., 1992). For backbarrier facies the preservation potential is reasonably high (Heward, 1981), with the consequences that interpretation of relict barrier stratigraphy invites recognition of lagoons, tidal inlets, washover fans and tidal deltas. Although Heward (1981) proposes a basic framework in order to permit reconstruction of relict stratigraphy, it is clear that overall preservation potential must address the possibility of exposure to repeated cycles of rising and falling sea levels. Vail (1977) on global trends originally observed that a relative rise in sea level is generally gradual and a relative fall was abrupt. Vail's conclusion derives from the abundance of prograding sediments representing delta front deposits as seen on seismic profiles that correlate with sealevel highstands. Evidence of a relative sea-level fall may be poorly represented on seismic profiles.

Vail (1977) observed that contrary to perceptions coastal regression commonly occurs during a sea-level rise or sea-level stillstand, and in these circumstances opportunities for preservation of relict deposits exists. In a scenario of a rising sealevel coastal regression or transgression appears to be governed by the relative abundance of the sediment budget which may increase with rising sea levels. Therefore in a period of rising sea levels erosion of pre-existing deposits may not necessarily occur and a rising sea may overstep existing sediments. Transgression may correspond to a rise in sea level which exceeds the rate of sediment input and deposition, with the advent that erosion occurs and marine sediments directly onlap underlying sediments (Vail, 1977). During rapid rises in sea level, which exceeds the rate of sediment input, transgression may be characterised by erosion of pre-existing surfaces (Ryler, 1977). Evidence for erosion relates to recognition of a concave erosional profile scoured into underlying sediments (Schwartz, 1967; McCubbin, 1981; Ryler, 1977). Elliot (1986) describes this surface as the shoreface erosional plane which separates existing strata from landward migrating beach facies. At the culmination of this process erosion may cease when sea level stabilises or the rate of sediment supply overtakes the capacity of the sea to advance further landward with a consequence that beach barrier sediments are deposited and progradation commences. Thin prograding lenses of the order of 1 7 m have been observed which characterise times of falling sea levels with low sediment supply (Heward, 1981). Vail (1977) suggests that a high sediment input during falling sea level results in a progradational style of downslope stepwise segments which appear to onlap previous deposits. 3.2. Relict environments

Morely (1981 ) states that heavy mineral enrichment in coastal systems during Pleistocene times in eastern Australia developed as a consequence of prolonged winter storms from prevailing southeast directions, that also resulted in a dominant S-N littoral drift. During sea-level falls it is thought that sediment was derived from both fluvial detritus and longshore drift resulting in a

I. Browne/Marine Geology 121 (1994) 8l 107

predominance of more labile mineral suite which is presented to the coastal process (Force, 1991). Overall the diversity of sediments offshore results from a complex record of depositional and erosional environments arising from glacially induced sea-level oscillations. During the Quaternary the entire shelf experienced a complete spectrum of terrestrial and marine sedimentation (Roy et al., 1992). However (Roy and Stevens, 1981), recognised that there is a compelling argument that geological factors such as morphology, stream drainage patterns and sediment budget strongly influence the nature and preservation of shelf sediments. 3.3. Marine seismic

Shore normal traverses were carried out at intervals of 250, 500 and 750 m for a length of 5 km. The shore-normal traverses were intersected by two or three shore-parallel traverses at intervals of 5 or 2.5 km, respectively. Variations in traverse separation were trailed to test optimum traverse geometry and the "boxwork" pattern of traversing optimised the definition of relict stratigraphy. A critical review of existing exploration technologies and survey methods was undertaken as it was considered that detection of preserved barriers and in particular recognition of beach and berm facies within barriers would be difficult on seismic profiles. As a result of this review a local geophysical contractor was encouraged to develop a digital real-time acquisition system based on personal computer (PC) technology (Lean and Pratt, 1991 ). A number of benefits were anticipated to flow from using a PC based system. The PC system enabled simultaneous operation of several independent geophysical data systems each conforming to separate rates of operation and functioning at different scan times. Each set of geophysical data was subsequently related by reference to a common fiducial recorded on each data set. The technology permits the operator to modify the function of individual geophysical devices and continuously monitor the quality of their output by means of analysing their spectra thereby detecting variations which may result from changes in field conditions or system malfunction. Confident recognition of

85

relatively small scale features on seismic records is both a function of the spatial (vertical) resolution of the seismic pulse plus the manner in which the data is processed and displayed. The required characteristics of the energy source for the seismic system was governed by the desire to resolve sedimentary structures to sub-meter scales which were considered to host heavy mineral accumulations (Morely, 1981 ). For seismic sources the resolvable limit in the vertical plane corresponds to approximately one quarter wavelength of the dominant frequency of the propagated acoustic signal for the medium of transmission (Sheriff, 1985). As loose saturated sand has an acoustic velocity of around 1750 m/s it becomes evident that the required acoustic source would need to have a minimum dominant frequency of about 1000 Hz. Various "high" resolution seismic sources were trailed and a "boomer type" source was subsequently chosen. By design seismic scan times lasted 150 ms for shot intervals of 0.25/s, as this was considered the optimal coverage considering both depth of penetration and horizontal continuity, respectively. Emphasis was therefore placed on acquisition of digital continuous seismic profiling data with the ability to conduct post acquisition data processing to optimise presentation of true scale corrected seismic sections. System operation included data output from various devices onto analogue charts, primarily to verify acquisition and data quality. The digital data were simultaneously recorded onto digital tapes using a helical-scan Hexabyte drive as a mass storage device (Lean and Pratt, 1991). Volumes of data acquisition typically reached 40 Mb/h. 3.4. Data processing

Analogue records were used to monitor data quality but were not considered as a substitute to digital acquisition. Subsequent to field operations and by reference to the analogue records it was possible to select appropriate processing parameters for the digital data in order to achieve maximum enhancement of seismic events. The quality of analogue data usually varies according to the prevailing field conditions. Digital

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data processing overcomes this problem as it is possible to optimise presentation of the seismic data and restore data that may have been acquired in marginal field conditions. Positional information is also simultaneously recorded along with all the geophysical data and makes it possible to reconstruct the seismic record at a true horizontal scale. Standard data processing routines may include amplitude gain functions, deconvolution, band-pass filtering, swell filtering, all of which result in considerable enhancement of the both the lateral continuity and vertical resolution of seismic events. Additional and specific improvements to the data can be further undertaken during the interpretation process using commercial workstation (PC) based software packages which permits the interpreter to focus on particular sites for detailed analysis. Finally the digital data has been archived in a modified SEGY format. For optimum display the data were projected at a vertical scale of 4 ms/cm and 1:2500 horizontal scale. Processing resulted in the production of fully annotated seismic sections which displayed merged navigation data, bathymetric data as well as high sensitivity magnetometer data.

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3.5. Interpretation method Seismic stratigraphic methods were employed to interpret the structures which are apparent on the seismic sections in the window below the seabed and above the Pre-Tertiary basement. Seismic stratigraphy permits sequence analysis to explain reflection seismic events in terms of relict depositional facies. Recent work by Chiocci et al. (1990) and subsequently Baker (1991) shows that seismic sequence analysis can be applied equally to seismic data at the scale of these sections. Seismic stratigraphy has largely evolved by analysis of seismic data acquired for hydrocarbon exploration which define large scale features. Examples describing sequence analysis in relation to high resolution seismic data are not readily available although application of the method should not be confined to differences in vertical resolution and scale. Baker (1991) and Chiocci et al. (1990) show that event attributes and seismic sequence relationships still apply. Recent work by Force (1991) suggests that seismic facies analysis is acknowledged as having applications for basin analysis for mineral deposits.

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The transgressive and regressive environments identified in the above figures and described in terms o f seismic facies are summarised in Tables 1 and 2 using a m e t h o d discussed by R a m s a y e r (1979). The p r o p o s e d age for each seismic sequence derives f r o m w o r k done by R o y et al. (1992) which show that the terrestrial barrier (Fig. 11) dates f r o m 238 to ~ 120 ka and offshore d r o w n e d barriers are younger. The relative level ( R L ) o f each marine incursion has been based on recognition o f foreshore facies and reconstruction o f the eroded barrier sequences and scaling against the shoreface profile model described by H o w a r d et al. (1972). Interpretation suggests that sequential highstand R L ' s are progressively lower

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5. Conclusions (1) Digitally processed high resolution seismic sections have enabled detailed reconstruction o f depositional sequences which are t h o u g h t to represent relict features deposited during Pleistocene marine transgressions and regressions. (2) The use o f seismic sequence stratigraphy has identified relict features attributed to barrier depos-

Table l Relict transgressive sequences Example (Fig. no.)

Sequence

Environment

Seismic facies

Age (kyr)

Suggested RL (highstand)

3

2

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2

< 120?

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8

1

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9

1

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3

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C ON and DWN P and CH and M TE-DWN P and OB TE ON and DWN P and SH TE ON and DWN P and SH TE-ON P

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Terminology after Ramsayer (1979).

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1. Browne/Marine Geology 121 (1994) 81-107

106 Table 2 Relict regressive sequences Example ( Fig. no.)

Sequence

Environment

Seismic facies

Age (kyr)

2

2

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11

1

Foreshore to shoreface Upper to lower shoreface Foreshore to shoreface Foreshore to shoreface Foreshore to shoreface Foreshore to shoreface Foreshore to shoreface Foreshore to shoreface

TE-DWN P and SH C DWN SIG and SH TE D W N P and SH TE and C D W N P and SH TE and C - D W N P and SH C DWN SH TE and C D W N OB and SIG and SH C DWN OB and SIG and SH TE and C D W N OB and SIG and SH

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33 m

+4 m

*Thermoluminescence age dated R o y et al. (1992). Terminology after Ramsayer (1979). Code used in seismic facies mapping: ( A ) - ( B ) / ( C ) .

Upper sequence boundary, (A) : T E = e r o s i o n a l truncation; C = c o n c o r d a n t .

Lower sequence boundary (B) : ON = onlap; D W N = downlap.

Internal sequence conligurations (C) : P = parallel; D = divergent; C H = chaotic; SH = shingled; M = mounded; OB = oblique; SIG = sigmoid progradation.

ition which could serve as potential host sites for economic placer mineralisation. (3) Seismic sequence analysis of offshore sediments in Central and Northern NSW proposes that several types of relict barrier structures and related facies exist and which display various degrees of preservation. During and subsequent to deposition the degree of preservation may be influenced by sediment supply, climatic or wave processes, and relative sea-level position. (4) The presence and degree of preservation of relict barriers and related sediments is not correlatable with the present coastal landscape. In all cases evidence for relict barrier systems exists. Some barrier deposits are relatively unmodified and are overstepped by subsequent marine incursions whereas in some situations relict barrier sediments have been eroded.

Acknowledgements The author acknowledges the support of Cable Sands Holdings Pty. Ltd. for making material available for this paper and particularly the assistance of Joe Stewart for preparing the diagrams and Esme Thomas for typing the manuscript. References Baker, P.L., 1991. Fluid, lithology, geometry, and permeability information from ground-penetrating radar for some petroleum industry applications. Soc. Pet. Eng. Asia Pacific Conf. (Perth, W.A.) pp. 277 287. Bloom, A.L., Broeker, W.S. Chappell, J.M.A., Mathews, R.K. and Mesolella, K.J., 1974. Quaternary sea level fluctuations on a tectonic coast: new ThZ3°/U T M dates from the H u o n Peninsula, New Guinea. Quat. Res., 72:4745 4757. Cappell, J., 1974. Geology of coal terraces, H u o n Peninsula,

Z Browne~Marine Geology 121 (1994) 81 107 New Guinea: A study of Quaternary tectonic movements and sea level changes. Geol. Soc. Am. Bull., 85:553 570. Chiocci, F.L., Orlando, L. and Tortora, P., 1991. Small-scale seismic stratigraphy and paleographical evolution of the continental shelf facing the SE Elba island (Northern Tyrrenian Sea, Italy). J. Sediment. Petrol., 61: 506-526. Davies, P.J., 1975. Shallow seismic structure of the continental shelf, Southeast Australia. J. Geol. Soc. Aust., 22:345 359. Fisher, E., McMechan, G.A., Annan, P.A. and Cosway, S.W., 1992. Examples of reverse time migration of single channel, ground penetrating radar profiles. Geophysics, 57:577 586. Force, E.R.. 1991. Placer Deposits. Sedimentary and diagenetic mineral deposits: A basin analysis approach to exploration. Rev. Econ. Geol. Soc. Econ. Geol., 5:131 145. Hamilton, N.T.M., 1990. Models of Heavy Mineral Deposition. Abstracts of lectures presented at the Thap Sakae field office--February, 1990. Kingdom of Thailand, Dep. Miner. Resour. Thailand. THA/86/018. Mission Rep., February 4 28, 1990. Offshore Minerals Exploration in the Gulf of Thailand. Review of Quaternary Geology of the Coast and Offshore Seabed in Exploration area 2. pp. 49 57. Heward, P.P., 1981. A review of wave-dominated clastic shoreline deposits. Earth-Sci. Rev., 17:223 276. Hobday, D.K. and Tankard, A.J., 1978. Transgressive-barrier and shallow-shelf interpretation of the lower Paleozoic Peninsula Formation, South Africa. Geol. Soc. Am. Bull., 89:1733 1744. Howard, J.D., Frey, R.W. and Reineck, H.-E., 1972. Introduction. Senckenberg. Mar., 4:3 14. Johnson, H.D. and Baldwin, C.T., 1986. Shallow siliciclastic seas. In: H.G. Reading (Editor), Sedimentary Environments and Facies. Blackwell, Oxford, 2nd ed., pp. 229 282. Layton, W., 1967. Completion report on offshore mineral exploration licence for A to P 3020 Murwillumbah. NSW Dep. Nat. Resour., Rep., GS1967 370. Lean, J. and Pratt, D.A., 1991. Multi sensor marine geophysical profiling and digital acquisition using SASI. ASEG Conf. (Sydney, 1991.) MacCulloch, I.R.F., 1969. Final Report. Exploration Licences 60 and 61. Planet Metals. Rep. 822, New South Wales (unpubl.). Marshall, J.F., 1979. The development of the continental shelf of northern New South Wales. BMR J. Geol. Geophys., 4:281 288. McCubbin, D.A., 1969. Cretaceous strike-valley sandstone reservoirs, northwestern New Mexico. AAPG Bull., 53: 2114 2140. McCubbin, D.A., 1982. Barrier island and strand plain facies. In: P.A. Scholle and D. Spearing (Editors), Sandstone Depositional Environments. AAPG, Tulsa, Okla., pp. 247-279. Mitchum, R.M., Jr., 1977. Seismic stratigraphy and global changes of sea level, Part 11: Glossary of terms used in seismic stratigraphy. In: C.E. Payton (Editor), Seismic Stratigraphy--Applications To Hydrocarbon exploration. AAPG Mem., 26: 205-212. Mitchum, R.M., Jr., Vail, P.R. and Thompson, S., 1977a. Seismic stratigraphy and global changes of sea level, Part 2:

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The depositional sequence as a basic unit for stratigraphic analysis. In: C.E. Payton (Editor), Seismic Stratigraphy-Applications to Hydrocarbon Exploration. AAPG Mere., 26: 53-62. Mitchum, R.M., Jr., Vail, P.R. and Sangree, J.B., 1977b. Seismic stratigraphy and global changes of sea level, Part 6: Stratigraphic interpretation of seismic reflection patterns in depositional sequences. In: C.E. Payton (Editor), Seismic Stratigraphy Applications to Hydrocarbon Exploration. AAPG Mere., 26:117 133. Morley, I.W., 1981. A History of the Mineral Sand Mining Industry in Eastern Australia. Univ. Queensland Press. Ramsayer, G.R., 1979. Seismic stratigraphy--a fundamental exploration tool. Proc. Offshore Technol. Conf. 3, Pap. 3568, pp. 1859 1867. Ringis, J. (Team Leader), 1986. Seismic stratigraphy in very high resolution shallow marine seismic data. Proc. Jt. ASCOPE.CCOP Workshop. (1 June 1986, Jakarta, Indonesia.) pp. 115-126. Roy, P.S., 1982. Regional geology of the central and northern New South Wales coast. Heavy mineral exploration of the East Australian Shelf. Sonne Cruise SO-15 1980. Geol. Jahrb. Roy, P.S. and Stevens, A.W., 1981. Geological controls on process-response, S.W. Australia. Proc. 17th Int. Coastal Conf. (Sydney.) pp. 913-933. Roy, P.S., Thom, B.G. and Wright, L.D., 1980. Holocene sequences on an embayed high energy coast: An evolutionary model. Sediment. Geol., 26: 1-19. Roy, P.S,, Zhuang, W.-Y., Birch, G.F. and Cowell, P.J., 1992. Quaternary Geology and Placer Mineral Potential of the Forste~Tuncurry Shelf, Southeast Australia. Geological Survey of New South Wales, Department of Mineral Resources. Final report on the Marine Minerals Investigation - 1990 to 1991 A joint venture Government/ Industry Research Project. Geol. Surv. Rep., GS 1992/201 (unpubl.). Ryler, T.A., 1977. Patterns of Cretaceous shallow-marine sedimentation Coalville and Rockport areas, Utah. Geol. Soc. Am. Bull., 88: 188-188. Sanders, J.E. and Kumar, N., 1975. Evidence of shoreface retreat and in place "drowning" during Holocene submergences, shellfire Island, New York. Geol. Soc. Am. Bull., 56: 65-76. Sheriff, R.E., 1985. Aspects of seismic resolution. In: Seismic Stratigraphy Il, An Integrated Approach. AAPG Mem., 39: 1-10. Short, A.D. and Trenaman, N.L., 1992. Wave climate of the Sydney region an energetic and highly variable ocean wave regime. Aust. J. Mar. Freshwater Resour., 43: 765-791. Swift, D.J.P., 1968. Coastal erosion and transgressive stratigraphy. J. Geol., 76:444 456. Vail, P.R,, Mitchum, R.M. and Thompson, S., 1977. Seismic stratigraphy and global changes of sea level, Part 3: Relative changes of sea level from coastal onlap. In: C.E. Payton (Editor), Seismic Stratigraphy--Applications to Hydrocarbon Exploration. AAPG Mere., 26: 63-81. Wallis, D.S. and Oakes, G.M., 1990. Heavy mineral sands in eastern Australia. In: F.E. Hughes (Editor), Geology of the Mineral Deposits of Australia and Papua and New Guinea. AIMM, Melbourne, pp. 1599-1608.