2 Otway Basin Stratigraphy

VIMP Report 23 Early Cretaceous stratigraphy along the northern margin of the Otway Basin, Victoria G.J. Parker November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ontents 1 Introduction 2 2EMHFWLYHV 2 Otway Basin Stratigraphy 7 &DVWHUWRQ)RUPDWLRQ &UD\ILVK*URXS (XPHUDOOD)RUPDWLRQ &UD\ILVK*URXS(XPHUDOOD)RUPDWLRQ%RXQGDU\ 6KHUEURRN*URXS 7HUWLDU\ 3 Conclusions 15 References 16 Appendix 1 18 :HOO)RUPDWLRQ7RSVOLVWLQJ Appendix 2 19 :HOOGHVFULSWLRQVKHHWV Victorian Initiative for Minerals and Petroleum (VIMP) report series 69 List of Figures 1 Line of stratigraphic well correlation 5 2 Tullich 1 to Warracbarunah 2 regional well correlation (back pocket) 3 Stratigraphic column for northern Otway Basin units 6 4 Geological cross section from Tullich 1 to Warracbarunah 2 7 5 Time - rock relationship diagram for the northern Otway Basin 9 6 Seismic line OPX86A - 41 across Moyne Falls 1 10 7 Eumeralla / Crayfish boundary vs hughesii / wonthaggiensis palynological zones 14 2 NORTHERN MARGIN - OTWAY BASIN Abstract A regional well correlation across the northern margin of the Otway Basin has shown that the earliest stratigraphic unit in the basin, the Casterton Formation, shows different lithological and age characteristics between half grabens. Overlying this, the Early Cretaceous Crayfish Group units cannot be reliably correlated across the basin. Three broad subdivisions can be observed and another informal one is proposed here. However, detailed stratigraphic analyses of the Crayfish Group can be achieved within individual half grabens, each with its own unique section of Early Cretaceous units. The overlying Eumeralla Formation is continuous across the Otway Basin but mappable units within it are sparse and no regional subdivision can be made. Late Cretaceous and Tertiary units were not studied. A succession of palynological zonation schemes has had a large impact on stratigraphic classification in the basin. Problem areas still exist, such as the C. hughesii / F. wonthaggiensis Zone boundary. This boundary is affected by extensive erosion in the western Otway Basin where it lies within the Crayfish Group NORTHERN MARGIN - OTWAY BASIN 3 1 Introduction 1.1 Objectives 6WUDWLJUDSKHUVZRUNLQJLQWKH2WZD\%DVLQ In order to identify and perhaps solve some of the KDYHIDFHGDZLGHJDPXWRIFRQIOLFWLQJ problems in Otway Basin stratigraphy it was VWUDWLJUDSKLFQRPHQFODWXUHDQG necessary to gain a regional point of view. To do ELRVWUDWLJUDSKLFLQIRUPDWLRQRYHUWKH\HDUV this a regional well log correlation was constructed 6HYHUDOSUREOHPVKDYHGHYHORSHGZLWKWKH beginning from Tullich 1 in the far west of the VWUDWLJUDSKLFQRPHQFODWXUHLQFOXGLQJDQH[FHVV Victorian Otway Basin and terminating at RIURFNXQLWQDPHV 7KRPVRQ :DONHU Warracbarunah 2 at its eastern end. The line of 0RUWRQHWDO PDQ\RIZKLFKKDYHEHHQ section can be seen in Figure 1 and the well UHGHILQHGPDQ\WLPHV6RPHXQLWVDOVRKDYH correlation in Figure 2. The details of the VHYHUDOLQIRUPDOQDPHVDQGLQVRPHFDVHVWKH construction of the section are described in the VDPHQDPHKDVEHHQDSSOLHGWRYHU\GLIIHUHQW section entitled 'Methodology'. XQLWVE\YDULRXVZRUNHUV More detailed objectives are as follows: The reasons for such problems are varied and are not discussed in detail in this report. However, · Revision of formation tops using all available two contributing factors are the introduction of new data (see Appendix 1 for final formation tops): stratigraphic names and of new palynological zonation schemes. Several palynological zonation · Examination of the relationships between schemes are currently used for Cretaceous units depocentres along the northern rift margin: in the Otway Basin today (Alley, 1993). Although improved zonation schemes are continually In the Late Jurassic to Early Cretaceous the emerging the differences between them can be Otway Basin consisted of a series of rift quite significant. basins stretching from offshore South Australia to the eastern edge of the basin. In the past there has been a lack of Little is known about the nature of deposition communication between government bodies and in these early depressions. The earliest with industry. This has created an environment identified sedimentary unit, the Casterton where stratigraphic classification of Otway Basin Formation (Wopfner et al., 1971) is thought to units has developed independently between occur only in these depressions. However, Victoria and South Australia. Part of the work of very little is known about this formation the National Geoscience Mapping Accord (NGMA) because relatively few wells have reached it. has been an attempt to reconcile many of the The units now defined as Crayfish Group differences in rock unit nomenclature across the (Morton et al., 1994) have also been border and close consultation between state and deposited here. However, it is uncertain if industry bodies has been crucial to its progress. formations within the group can be correlated throughout the basin or if the group was laid This report discusses some of the work done by down in a series of discreet depocentres, each the Geological Survey of Victoria as part of its with a unique sedimentary style. commitment to the NGMA. The well correlation (Fig. 2) was constructed in an effort to create a · Combination of Victorian and South Australian regional picture from which many of the problems rock unit names into a single stratigraphic in Otway Basin stratigraphy could be outlined. scheme: Though the correlation concentrates on the Victorian side of the basin, due recognition and Several problems were identified regarding incorporation of work performed in South differences between named formations in Australia, particularly Mines and Energy South South Australia and in Victoria before the Australia (MESA), was crucial in this endeavour. project commenced. This is particularly so for the Crayfish Group units, especially the This report pays particular attention to the nature prospective Laira and Katnook formations. and distribution of Early Cretaceous units. As a result, the section has been constructed to follow · Examination of lithostratigraphic and those Early Cretaceous rifts on the northern palynological data and how they relate: margin containing thick Casterton and Crayfish formations. Late Cretaceous to Tertiary units are One of the most problematic areas of Otway not examined in detail. Basin stratigraphy is the Crayfish Group / Eumeralla Formation boundary and its 4 NORTHERN MARGIN - OTWAY BASIN relationship with the Cyclosporites hughesii/ stratigraphic and lithological descriptions from well Foraminisporis wonthaggiensis palynological completion reports and examination of cores and zonation boundary. In some cases the cuttings. available data place the rock unit boundary between the palynological zones. In others Additional palynological datings were obtained the Zone boundary lies above or below the where considered necessary. rock unit boundary. One of the aims of this report is to identify such problems and their When the formation tops were finalised, several causes, particularly with reference to the problems were identified and are discussed later. various palynological zonation schemes. Appendix 1 gives a listing of the final formation tops for each well. 1.2 Methodology The final well log correlation was then reproduced on Geolog using the new formation tops (Fig. 2). Construction and interpretation of the well log A stratigraphic column (Fig. 3) and a geological correlation was performed as follows: interpretation incorporating the structural elements map from Perincek et al. (1994; Fig. 4) are also A well log section display was produced on enclosed. Geolog 5 software with a datum at sea level (Fig. 2). This consisted of the following petroleum, stratigraphic investigation and groundwater wells in the order in which they appear: Tullich 1 Bus Swamp 1 McEachern 1 Heathfield 1 Casterton 2 Casterton 1 Mocamboro 11 Hotspur 1 Greenbanks1 Moyne Falls 1 Hawkesdale 1 Woolsthorpe 1 Ballangeich 1 Garvoc 1 Stoneyford 1 Tirrengowa 1 Warracbarunah 2 Well description sheets for each of these can be found in Appendix 2. Existing formation tops were marked on the sections. These came from several sources. Those in the eastern Otway Basin came from Geological Survey of Victoria (1995); many of those in the western part of the basin came from Ahmad Tabassi (pers comm.) All well completion report formation tops were incorporated across the basin. All available palynological information was then transcribed onto the section. An attempt was then made to correlate the formations in the

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