Structure and Tectonics of the Gunnedah Basin, N.S.W: Implications for Stratigraphy, Sedimentation and Coal Resources, with Emphasis on the Upper Black Jack Group
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University of Wollongong Thesis Collections University of Wollongong Thesis Collection University of Wollongong Year Structure and tectonics of the Gunnedah Basin, N.S.W: implications for stratigraphy, sedimentation and coal resources, with emphasis on the Upper Black Jack group N. Z Tadros University of Wollongong Tadros, N.Z, Structure and tectonics of the Gunnedah Basin, N.S.W: implications for stratigraphy, sedimentation and coal resources, with emphasis on the Upper Black Jack group, PhD thesis, Department of Geology, University of Wollongong, 1995. http://ro.uow.edu.au/theses/840 This paper is posted at Research Online. http://ro.uow.edu.au/theses/840 CHAPTER 4 STRUCTURAL ELEMENTS 4.1 Introduction 161 4.2 Basement morphology 161 4.3 Major structural elements 163 4.3.1 Longitudinal and associated structures 163 A. Ridges 163 i) Boggabri Ridge 163 ii) Rocky Glen Ridge 169 B. Shelf areas 169 C. Sub-basins 171 i) Maules Creek Sut)-basin 171 ii) Mullaley Sub-basin 173 iii) Gilgandra Sub-basin 173 4.3.2 Transverse structures and troughs 174 i) Moree and Narrabri Highs; Bellata Trough 174 ii) Walla Walla Ridge; Baradine High; Bohena, Bando, Pilliga and Tooraweena Troughs 176 iii) Breeza Shelf; Bundella and Yarraman Highs 177 iv) Liverpool Structure 180 v) Murrurundi Trough 180 vi) Mount Coricudgy Anticline 182 4.4 Faults 184 4.4.1 Hunter-Mooki Fault System 184 4.4.2 Boggabri Fault 184 4.4.3 Rocky Glen Fault 186 4.5 Minor structures 186 Please see print copy for image Please see print copy for image P l e a s e s e e p r i n t c o p y f o r i m a g e 161 CHAPTER 4 STRUCTURAL ELEMENTS 4.1 INTRODUCTION It has already been mentioned in the previous chapter that the present Gunnedah Basin forms the middle part of the Sydney - Bowen Basin, a long composite stmctural basin, consisting of several troughs defined by bounding basement highs and ridges. The basin extends along the westem margin of the New England Fold Belt along the Hunter-Mooki Fault System. The basin sedimentary sequence and the basal volcanics overiie the older Lachlan Fold Belt which also bounds the basin to the west. In this study, a structural basin is defined as a remnant structural unit of a former stratotectonic sedimentary (depositional) basin. The structural remnants of the former Sydney - Gunnedah - Bowen Basin can be delineated and subdivided into structural sub-units on the basis of the morphotectonic features of the basement and the tectono-structural elements of the contained sediments. These elements are demonstrated on structure contour and lithofacies maps, including isopach and net sand maps of many genetic sedimentary units within the Gunnedah Basin. Recognition of the morphotectonic features gives an insight into the manner in which the basin developed (see chapter 3) and provides a powerful tool which enables prediction of structure, distribution and geometry of sedimentary facies in areas lacking borehole control (as demonstrated in chapter 5) with great implications to fossil fuel exploration. The structural elements of the Gunnedah Basin, as presented in this chapter and published in Tadros (1988c, 1993f), have been adopted by Scheibner (1993b) in the new structural and tectonic map of New South Wales (see plate 4.1; Scheibner 1993c and in prep.). 4.2 BASEMENT MORPHOLOGY Structure contours on top of the ?Late Carboniferous-Eariy Permian (basal) volcanics, which form the effective basement for the Gunnedah Basin (figure 4.1; see 3-D model - opposite), broadly outline three north-north-westeriy oriented sub-basins lying between meridional basement ridges. The Boggabri Ridge (Russell 1981, Brownlow 1981b) in the east separates the eastern Maules Creek Sub-basin (Hill 1986, Thomson 1986b) from the central Mullaley Sub-basin (nov.) which is separated from the western Gilgandra Sub-basin (Tadros 1993f; Gilgandra Trough of Yoo 1988) by the Rocky Glen Ridge (Yoo 1988). This broad outline of the basement morphology of the Gunnedah Basin has been recently confirmed further in geological results from the AGSO's deep seismic reflection profiling in the northern part of the basin (Korsch et al. 1992, 1993). The profile is east - west oriented and was acquired at about the latitude of Boggabri (figure 3.14). The seismic data suggest that in the northern part of the basin the succession is thin and consists of three sub-basins separated by two ridges. The preliminary 162 GUNNEDAH BASIN - TECTONICS AND STRUCTURE Figure 4.1. Structure contours (m asl) on the top ofthe basal volcanic rocks ofthe Gurmedah Basin, showing longitudinal sub-basins and prominent highs and ridges (from Tadros 1993f) 4. STRUCTURAL ELEMENTS 163 interpretations (Korsch et al. 1992) suggested that the maximum thickness of sediment appears to be greater than 2 km on the western side ofthe Mullaley Sub-basin (i.e. Gilgandra Sub-basin). However, subsequent interpretation ofthe data (Korsch et al. 1993) indicated the presence of only a very thin sedimentary succession in the Gilgandra Sub-basin - in the order of 400 m above ?granitic basement. The structure contours also outline a west-south-westeriy to south-westeriy-trending high on the basal volcanic surt'ace through Boggabri termed the Walla Walla Ridge (Tadros 1988c). Further investigation of basement structure to the north and south of the Walla Walla Ridge, utilising available gravity and magnetic maps and seismic cross-sections, has revealed a basement morphology, which strongly reflects the origin ofthe basin. The longitudinal sub-basins are divided by several of these transverse basement ridges/highs into a series of large troughs oriented parallel to the basin axis (figures 4.1 and 4.2). 4.3 MAJOR STRUCTURAL ELEMENTS The major structural elements ofthe Gunnedah Basin are observable basement morphologic features which have an origin related to deep-seated structures within the upper crust and have influenced the tectonic (and consequently depositional) development ofthe basin throughout its history. In the following sections, the major structural elements are subdivided into longitudinal and transverse structures. The longitudinal structures, the dominant elements, will be dealt with first. 4.3.1 LONGITUDINAL AND ASSOCIATED STRUCTURES A. Ridges i) Boggabri Ridge ?Late Carboniferous to Eariy Permian silicic and mafic volcanic rocks crop out in three distinct areas: west and south of Gunnedah, north of Boggabri (Hanlon 1949a, c; 1950a) and in the Deriah Forest area to the east of Narrabri (Hill 1986; figure 4.3). Hanlon (1950a) suggested that the area in which the Boggabri Volcanics crop out probably represents an old structural high. Russell (1981) concluded that the outcrops form part of a prominent southeriy to south-westeriy-trending basement high coincident with the "Boggabri Anticline" of Rade (1961), who reported dips of 3-4° on the north-eastern flank and 3-8° on the south-western flank, and with an aeromagnetic basement high mapped by Amoseas and referred to by them as the "Boggabri Shelf, Russell (1981) referred to this structure as the Boggabri Ridge, a name also used by Brownlow (1981b), and suggested that the ridge was an original basement topographic high which has subsequently undergone structural modification. Recent borehole data support Russell's (1981) view and indicate that the silicic and mafic volcanic units are continuous in the subsurface between the outcrops, forming a prominent basement ridge extending from southeast of Bellata south through the Deriah Forest area to Baan Baa (Hill 1986), 164 GUNNEDAH BASIN - TECTONICS AND STRUCTURE * r BOWEN BASIN.,''' REFERENCE .'.:^ 10RFE a Fault Volcanic unit ^y boundary .^ • Lineament SHELF V' w:':0-:^-em^:'S::^ / Wmi^Afm0k TROUCSfl NEW ENGLAND FOLD BELT 31 • ••• •• '•^'r^iAxkA^?--^- •TCXDFIAWEENAH,^' ^(jim«,d. \ ..''TROUGH.,-' jrriirunf; \MT FOSTER : .::^. , ^ ^.y\?^-Hi^'f- £J"ON v.^>. SCALE 0 10 20 30 40 50 60 I 1 1 1 I l__l km MACQUARIE TROUGH .., ,, NewcAsn ?;«>' Figure 4.2. Structural subdivision ofthe Gunnedah Basin. Major residual Bouguer gravity lineaments of Scheibner (1993a) are also shown (from Tadros 1993f) 4. STRUCTURAL ELEMENTS 165 [ISO-00' > ISO-30' TN \ \ 1 NEW ENGLAND Narrabri i \ \ s \ I FOLD BELT \ Deriah FwesI i \ \ Area I ^ \ • -/ \ 1 MULLALEY I MAULES X"^ iO-30- SUB - BASIN 30 - 30' Baan Baa 31-00' 31-00- ^EFETiENCE Breeza i Boggabri Volcanics outcrop Werrie Basalt outcrop /^ J Inferred extent of I / the Boggabn Ridge I Caroona SCALE 10 20 km \ \ Quinndi 31' 30' srso- \ \ • \\ ISO-00- ISO-30- \ Figure 4.3. Outcrop ofthe Boggabri Ridge (Ofifenberg 1971; Chesnut et al. 1973; Wallis 1971; Hill 1986) 166 GUNNEDAH BASIN - TECTONICS AND STRUCTURE through Boggabri to south-east of Gunnedah where the ridge is truncated by the Mooki Fault System. It is most probable that the Boggabri Ridge extends north past Moree (shown by broken line on figure 4.1) and joins a longitudinal basement high, identified by petroleum companies from seismic surveys and petroleum wells, known as the Gil Gil Ridge which extends north into the Bowen Basin (shown on fig. 1 of Etheridge 1987). The most prominent of the outcrops are those to the north of Boggabri (figures 4.1 and 4.4). Detailed surface mapping of these outcrops (Tadros 1988c) revealed that the rocks are dissected by two main vertical orthogonal fracture systems which are independent of the flow banding in the lava units (figure 4.5 and photo 4.1). Dissection of the outcrop becomes more prominent where either of the two fracture systems coincides with the direction of the flow banding. The stronger of the two systems trends north-west, is associated with step faults, and has given rise to the elongated morphology of the outcrops which is expressed in the fonm of near-vertical cliffs (e.g. Gins Leap, 4.5 km north of Boggabri, photo 4.2), ridges and valleys (figure 4.5).