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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::^ /
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FOLD BELT
31 • ••• •• '•^'r^iAxkA^?--^-
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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). The Namoi River occupies a major
Boggabri Ridge outcrop BAAN BAA \ *" showing fracture system
Generalised trend of Boggabri Fault
Figure 4.4. Boggabri Ridge outcrop (modified from Tadros 1988c, fig. 3 and 1993f, fig. 6.6). Arrows indicate principal directions of fracture systems 4. STRUCTURAL ELEMENTS 167
150° 00"
Outcrop variably obscured by soil and talus
• v/y-- Boggabri Ridge outcrop •; : rS'' showing fracture system
Figure 4.5. Detailed map of fracture systems on the Boggabri Ridge outcrop north of Boggabri (from Tadros 1993f) north-west fracture and conspicuously divides the outcrops of Boggabri Volcanics into eastem and westem groups. The second fracture system, which trends south-west, probably caused the neariy 90° bends in the course of the Namoi River and is occupied by some of its tributaries. Similar fracture systems and relationships between the Namoi River and the outcrops also exist in the area south of Gunnedah (figure 4.4), except that the south-westeriy fracture system in some places is more closely spaced than in the outcrops north of Boggabri. This south-westeriy fracture system coincides with a strong gravity lineament system, which is very prominent over the Gunnedah Basin, shown on figures 3.18 and 3.19.
Eariy Permian sediments lap onto the eastern and westem sides of the Boggabri Ridge (Hanlon 1949a, 1950a; Russell 1981; Tadros 1982; Thomson 1986b; Hill 1986), and thus during the Eariy Permian the ridge would appear to have separated the eastem half of the Gunnedah Basin into the Maules Creek and the Mullaley Sub-basins (figure 4.1). 168 GUNNEDAH BASIN - TECTONICS AND STRUCTURE
Photo 4.1. Boggabri Volcanics outcop near the top of Gins Leap, approx. 4.5 km north of Boggabri, showing strong north-east-trending fractures across flow banding. Photograph taken looking east-north-east
Photo 4.2. Gins Leap, an outcrop of Boggabri Volcanics. Photo taken looking north-westerly 4. STRUCTURAL ELEMENTS 169
Thomson and Flood (1984) and Thomson (1986b) suggested that the Boggabri Ridge was not a continuous high and that gaps existed which enabled sediments derived from the Lachlan Fold Belt region to enter the eastern drainage system. Detailed structure contours on the basal volcanic units (figure 4.1) and isopachs ofthe Eariy Permian Leard, Goonbri, and Maules Creek Formations in the Mullaley Suthbasin (figure 3.21) confirm their views and suggest one possible location for such a gap to the south-east of Narrabri and another probable location to the north of Gunnedah. Isopachs of the overiying marine sequence (figure 3.22) provide further evidence for the existence of these gaps. Tadros (1988c) has pointed out that it is more than just a coincidence that at present there is low topographic relief between the three major outcrop clusters ofthe Boggabri Ridge.
ii) Rocky Glen Ridge
The Rocky Glen Ridge (Yoo 1988; figure 4.1 and 4.2) was originally named "Rocky Glen Shelf by American Overseas Petroleum Ltd (1963) and "Rocky Glen High" by Russell (1981). It is present in the Coonabarabran area as a subcrop of Ordovician - Carboniferous metasediments. Carboniferous granites, and volcanics. Silicic volcanics, which are correlatives of the Boggabri Volcanics (Leitch et al. 1988), consist of ignimbrite and ashfall tuff and form the eastern flank of the Rocky Glen Ridge. Russell (1981) related "a negative feature" on the total gravity map north-east of Coonabarabran to the presence of the silicic volcanics. Drilling by the New South Wales Department of Mineral Resources during 1981-1982, confirmed Russell's suggestion that the "Rocky Glen High" was a prominent basement topographic feature onlapped partially by Pennian sediments. Yoo (1988) proposed that the "Rocky Glen High" may extend south to the "Dunedoo High" and north to Wee Waa to form a north-south-trending ridge (figures 4.1, 4.2, and 4.6a). This ridge is evident on seismic lines (figures 3.15, 3.16 and 4.7) and on aeromagnetic maps. It corresponds with "gravity highs" extending from Dunedoo to Wee Waa (Yoo 1988). The northerly extension ofthe Rocky Glen Ridge to at feast the latitude of Boggabri has recently been confirmed in the preliminary results from the Bureau of Mineral Resources deep seismic reflection profile (Korsch et al. 1992, 1993). The ridge bounds the western margin ofthe Permian sediments in the Mullaley Sub-basin north of Coonabarabran. To the south, the Permian sediments appear to be continuous over the ridge (figure 4.6).
B. Shelf areas The structure contours on the basal volcanics suggest that the western side of the Boggabri Ridge developed into a narrow shelf area which became wider in the north to the west of Boggabri (Baan Baa Shelf, Tadros 1993f) and in the south to the west of Breeza (Breeza Shelf, Tadros 1993f) (figures 4.1 and 4.2). The contours also suggest that the shelf areas developed into the transverse structures ofthe Walla Walla Ridge and Bundella High (Tadros 1993f) respectively.
As is the case with the Boggabri Ridge, structural contours on the floor of the Gunnedah Basin (figure 4.1) also suggest that the eastern margins ofthe Rocky Glen Ridge developed first into broad gently sloping shelf areas before they become steeper towards the main troughs of the Mullaley Sub-basin 170 GUNNEDAH BASIN - TECTONICS AND STRUCTURE
29°S
30°S
- 31°S
- 32°S
148°E I490E ^5Q0£ Figure 4.6a. Total-field Bouguer gravity anomalies and major structural elements in the western Gunnedah Basin (after Yoo 1988, fig. 3) 4. STRUCTURAL ELEMENTS 171
Kir Rolling Downs Group
Juo I Orallo Formation coal measures ^^H Black Jack Qroup
Pilliga Sandstone j Pw] Watermark Formation JURASSIC / jjmpl Purlawaugh Formation l-PP I Porcupine Formation
Garrawilla Volcanics Undifferentiated Early Permian CARBOiNIFEROUS \H:^'\ Felsic volcanics UNDIFFERENTIATED PALAEOZOIC I Pu I M etasediments Figure 4.6b. East - west cross sections across the Roclgr Glen Ridge (after Yoo 1988, fig. 4). For location of sections see fig. 4.6a
(figure 3.4, 4.2 and 4.7). These shelf areas also developed into or joined the major transverse highs and ridges which bound the trough areas. Throughout much of the history of the basin, these shelf areas were stable structures which subsided at significantly slower rates than the neighbouring troughs. The largest of these structures is the Wollar Shelf (Tadros 1993f) in the south, located between the Rocky Glen Ridge in the west and the Murrurundi Trough in the east. The Weetaliba Shelf (Tadros 1993f) is located west ofthe Bando Trough (of Tadros 1988c) and the Wee Waa Shelf (Tadros 1993f) west ofthe Bohena and Bellata Troughs (of Tadros 1988c - see below; figure 4.2).
C. Sub-basins
i) Maules Creek Sub-basin
The Early Permian Maules Creek Sub-basin (Hill 1986, Thomson 1986b) is a remnant structural basin of what was probably a large depositional basin covering an extensive area of the New England region to the east. The Mooki Fault System forms the eastem boundary of the sub-basin on the suri'ace, and the eastern flank of the Boggabri Ridge forms its western margin (figure 4.1). Preliminary results from the Bureau of Mineral Resources deep seismic reflection survey indicate a shallow fault bounding the Gunnedah Basin on the east (?the Mooki Fault), and the Tamworth Belt 172 GUNNEDAH BASDSf - TECTONICS AND STRUCTURE 4. STRUCTURAL ELEMENTS 173 appears to have been thrust over the eastern margin of the Gunnedah Basin (Maules Creek Sub- basin) for at least 6 km (Korsch et al. 1992). However, in subsequent interpretations Korsch et al. (1993) indicated that the sedimentary succession of the Maules Creek Formation and the underiying basal volcanic units appear to extend for at least 15 km to the east beneath the Tamworth Belt (refer to figure 4.13). The slope on the basal volcanics surface, which forms the floor of the sub-basin, generally becomes steeper towards the Mooki Fault System. Thomson (1986b) believed that the Boggabri Volcanics form the effective basement in the Maules Creek Sub-basin.
ii) Mullaley Sub-basin
The Mullaley Sub-basin (Tadros 1993f; figures 4.1 and 4.2) includes the "West Gunnedah Sub- basin" of Hill (1986). Extending over the entire length ofthe Gunnedah Basin, from Moree in the north to the Mount Coricudgy Anticline in the south, the Mullaley Sub-basin is the largest and most prominent of the sub-basins. It is divided by the most prominent transverse high, the Walla Walla Ridge, and by a number of other first-order west-south-west to southwest-trending transverse structural highs or ridges into a series of north-north-west oriented troughs (figures 4.1 and 4.2). These structures have an origin related to deep-seated features in the upper crust and had a remarkable influence on sedimentation throughout the basin's history (Tadros 1988c and chapter 5).
iii) Gilgandra Sub-basin
Although the area west of the Rocky Glen Ridge is largely unexplored, Yoo (1988) indicated that Permo-Triassic sediments of his Gilgandra Trough extend westward to the Mount Forster Structural Zone (of Scheibner in Hamilton 1985b) and northward to the north-easteriy trending Cobar-lnglewood Kink Zone (of Scheibner, in Hamilton 1985b; figures 4.1, 4.2 and 4.6). This interpretation was based on sparse drilling, the presence of a number of elongate Permian outliers west of Mudgee, and the presence of Permian outcrops associated with gravity lows centred south and southwest of the Warrumbungle Range (Yoo 1988, p. 22; figure 4.6).
As with the Mullaley Sub-basin, the Gilgandra Sub-basin is also divided by a transverse basement high (uplifted basement of Yoo 1988, p. 24) into northern and southern troughs (figures 4.1, 4.2 and 4.6).
From total gravity anomalies and a 40 m intersection (DM Worigal DDHl, some 6 km east of Baradrine) of Permian coal measure sediments, Yoo (1988, p. 24) postulated that a thick sequence would have developed in the area north of Baradine where "a negative gravity anomaly of 40 to 50 milligals is recorded". It should be noted that the values quoted by Yoo (1988) are direct readings on the total-field Bouguer gravity anomaly map, and the anomaly is in the order of 10-15 milligals only. Nevertheless, Korsch et al. (1992), from preliminary geological results from the then Bureau of Mineral Resources (now AGSO) deep seismic reflection profiling in the northern Gunnedah Basin, suggested that sediment thickness appears to be greater than 2 km on the "western side of the West Gunnedah Sub-basin (i.e. the Gilgandra Sub-basin). However, as mentioned eariier, in subsequent 174 GUNNEDAH BASIN - TECTONICS AND STRUCTURE interpretations, Korsch et al. (1993) indicated that the sedimentary sequence is just over 400 m thick above ?granite basement. It should be noted however, that the seismic line (figure 3.14) passes over a positive gravity anomaly located to the north ofthe negative anomaly referred to above.
4.3.2 TRANSVERSE STRUCTURES AND TROUGHS
In order to recognise transverse structures and their influence on basin development, reference should be made to chapter 3 (section 3.2.1) for the discussion of the mechanism of extensional/rift tectonics and its application to the Gunnedah Basin. The role that the gravity map plays in identifying these structures has already been highlighted in section 3.2.2.
It should be noted that in contrast to the Bowen Basin, the mapped structural features of the Gunnedah Basin fit the volcanic rift/extension models so well that significant interpretations can be made regarding its stratotectonic development, and in addition, basin morphotectonic features can be extended beyond the limit of available borehole data.
The remarkable correspondence between the discontinuities in the Meandarra Gravity Ridge and the structural highs, and in particular the linear arrangement of the troughs in a constant north-north- westeriy trend, enabled prediction of basement structure within areas of little or no borehole control and provided the basis for subdivision of the Mullaley and Gilgandra Sub-basins into structural subunits (figure 3.17 and 4.2).
i) Moree and Narrabri Highs; Bellata Trough
The Moree High (Tadros 1988c) is the northernmost basement structure interpreted from the total- field Bouguer gravity anomaly map in the Gunnedah Basin. It appears that the high occupies a broad zone centred at Moree. The southern boundary of this zone is located some 15-20 km to the south of Moree and is represented by a strong east-north-east-trending residual Bouguer gravity lineament shown on Scheibner's figures 3.18 and 3.19 (see also figure 4.2). The northernmost limit of this zone most probably coincides with the north-east-trending Cobar- Inglewood Lineament. This lineament is also strongly represented on the residual Bouguer gravity map of Scheibner (figures 3.18 and 3.19) and is taken as an appropriate northern boundary for the Gunnedah Basin.
Contrary to the suggestion by Hill (1986) that the Narrabri High (Exon 1974 amended) does not exist, a basement high is present in the Narrabri - Wee Waa area and is indicated by structure contours on the top of the basal volcanic units (figure 4.1) and by the northward onlap of the Eariy Permian strata ofthe Bohena Trough (Tadros 1988c) to the south (figure 4.8).
The Moree and Narrabri Highs are further defined by a strong transverse discontinuity in the Meandarra Gravity Ridge (figure 3.17) south of Moree and by the closure of the positive gravity anomalies representing the Bellata and Bohena Troughs (Tadros 1988c) respectively. The Narrabri High coincides with the Nandewar Lineament Zone of Scheibner (1973; figures 3.11 and 3.17). 4. STRUCTURAL ELEMENTS 175
8
n Leard-Goontyi & g 3 Porcux>e & r Lower Black ||| ^PPg^^^ck gg [>gby Fru [>X] Napoefby Fm igneous nirusons 3 Maiies Cr.Fms. s dWaiermark Fms. t Jack Gp
Sequerxie erctosed is pariiallv restored lo conioensaie lor sedrrierns eroded Axt\Q the Late Pennan & Early Iriassc Figure 4.8. North - south-east cross-section, Guimedah Basin (after Tadros 1988c, fig. 6). A. Datum base of Black Jack Group (top of marine sequence). B. Datum base of Triassic. C. Present elevation. For location of cross-section see figure 3.17 176 GUNNEDAH BASIN - TECTONICS AND STRUCTURE
The interpretation ofthe Bellata Trough (Tadros 1988c) is aided by results from the Bellata seismic survey (Etheridge 1986), DM Bellata DDH 1 (Etheridge 1987) and the Edgeroi seismic survey (by Namco International Inc. 1964). The results of the Edgeroi seismic survey, which was carried out some 15-25 km to the north of Narrabri, indicate that the strata in the eastern half of the area dip west and north-west towards a trough, the floor of which is more than 1200 m below the present surface. The Bellata seismic survey (figure 4.7) provided clear evidence for the existence of the Bellata Trough, the Rocky Glen Ridge to the west and the intervening broad Wee Waa shelf
These basal structures have been complicated by the Late Permian-Eariy Triassic compressional movement which caused uplift ofthe northern Gunnedah Basin (Tadros 1986b; Tadros, in Tadros et al. 1987a, b; Etheridge 1987), particulariy the area occupied by the Narrabri High and the Bellata Trough, and caused the erosion of much ofthe Late Permian sequence there (figures 3.23 and 3.25). Post-Triassic tectonic movements associated with the emplacement of Tertiary intrusions and volcanics caused further structural readjustments in the northern Gunnedah Basin.
ii) Walla Walla Ridge; Baradine High; Bohena, Bando, Pilliga and Tooraweena Troughs
The Walla Walla Ridge (Tadros 1988c; figures 4.2 and 3.13) is a transverse basement high which divides the Mullaley Sub-basin into two large troughs along a west-south-west-trending line passing north of Boggabri. These troughs were named by Tadros (1988c) the Bohena Trough in the north and the Bando Trough in the south. The area of the Bohena Trough corresponds in part to the "Bohena Basin" (of American Overseas Petroleum Ltd/Mid-Eastern Oil N.L. 1964) which is not an appropriate name because sedimentation was neariy always continuous between the two troughs and they did not exist as completely separate entities during the depositional history of the Gunnedah Basin. The intersection north of Boggabri ofthe Walla Walla and Boggabri Ridges provides the most pronounced surface outcrop of the basal volcanic units in the Gunnedah Basin. The continuation of the Walla Walla Ridge to the south-west across the Rocky Glen Ridge is marked by small silicic volcanic outcrops north of Coonabarabran indistinguishable from the Boggabri Volcanics north of Boggabri.
It is probable that a further south-westward continuation of the Walla Walla Ridge provides the basement high of Yoo (1988), named the Baradine High (Tadros 1993f), which separates the Gilgandra Sub-basin into the northern Pilliga Trough (Tadros 1993f) and southern Tooraweenah Trough (Tadros 1993f; figures 4.2 and 4.7).
Although the Walla Walla Ridge is up to 30 m lower than the present outcrops north of Boggabri, it stands more than 500 m higher than the trough areas (cross-section A, figure 4.8). This can be explained by the greater uplift and thrusting along the eastern margin of the Mullaley Sub-basin at the end of the Permian and during the eariiest Triassic. Successive-cross sections using the top of the Eariy Permian, the top of the Permian and the present elevation as a datum indicate that the Walla 4. STRUCTURAL ELEMENTS 177
Walla Ridge has attained its present height progressively since the Eariy Permian (cross-sections A, B and C, figure 4.8).
Isopach maps ofthe Early Permian Leard, Goonbri and Maules Creek Formations (figure 3.21), the Late Permian Porcupine and Watermark Formation (figure 3.22) and the Triassic Digby Formation (figure 4.9) indicate that the Permian and Triassic sediments are continuous (draped) over the Walla Walla Ridge but are considerably thinner than in the trough areas .
The Late Permian Black Jack Group and the Triassic Napperby Formation, although having been affected by erosion and thus not reflecting their original depositional thickness, particulariy in the northern Gunnedah Basin, show thinning over the Walla Walla Ridge and a significant increase in thickness in the Bando Trough towards the south (figure 4.10). The Walla Walla Ridge, therefore, represents a relatively stable area with less net subsidence than the trough areas (see chapter 5 for detailed discussion).
Hi) Breeza Shelf; Bundella and Yarraman Highs
The structure ofthe area to the south ofthe Bando Trough is not clear due to a lack of borehole data. However, west of Breeza there is a strong indication in the structure contours of a broad and high shelf area on the basal volcanic units adjacent to the Bando Trough named the Breeza SAje/f (Tadros 1993f) (figures 4.1 and 4.2). The contours also suggest that the shelf area developed into a transverse high (Bundella High, Tadros 1993f) which trends south-west to Bundella and appears to extend towards the Dunedoo High. The Bundella High stands some 200-300 m above the floor of the Bando Trough and coincides with a strong north-east-trending residual Bouguer gravity lineament shown on figures 3.18 and 3.19 (see also figure 4.2) extending from north of Coolah through Breeza into the New England Fold Belt. This residual gravity lineament and a parallel one approximately 12 km to the north, define stepwise segmentation of the Meandarra Gravity Ridge with sinistral displacement of some 15 km to the east of the main axis of the ridge. It is most probable that this structure extends south-west to the Ballimore area to provide the southern boundary of the Tooraweenah Trough (figure 4.6).
The transverse trend between the Breeza Shelf and the Dunedoo High coincides with a significant change in the direction ofthe Hunter- Mooki Fault System north of Breeza, and closer to Breeza, on the Breeza Shelf, the structure of the strata above the basal volcanic units has been complicated by compressive lateral deformation associated with the Hunter - Mooki Fault System which caused en- echelon high-amplitude anticlines and associated synclines (Breeza Fold Zone, Tadros 1988c).
There is also a basal structure, the Yarraman High (Tadros 1988c, named after "Yarraman" homestead west of Blackville on the central northern slopes of the Liverpool Range) detected in the Yarraman - Dimby gravity survey (Systems Exploration Corporation Pty Ltd 1975). The survey indicated a relief of some 300 m on the surface of the basal volcanic units. The survey also suggested a possible deepening of the basal volcanic surface to the north-west of Yarraman to greater than 800 m below sea level. However, this trend, if present, should form a closure in the 178 GUNNEDAH BASIN - TECTONICS AND STRUCTURE
Figure 4.9. Isopachs (m) ofthe Digby Formation, Grmnedah Basin (modified from Tadros 1988c, fig. 8). Shading highlights two main depocentres 4. STRUCTURAL ELEMENTS 179
TN
NEW ENGLAND
FOLD BELT
Figure 4.10. Isopachs (m) ofthe Napperby and Deriah Formations, Gunnedah Basin (modified from Tadros 1988c, fig. 10). Shading highlights depocentres 180 GUNNEDAH BASIN-TECTONICS AND STRUCTURE north as it approaches the Bundella High. This assumption is supported by a positive gravity anomaly on the total-field Bouguer gravity map, trending north-north-west in the area between the Yarraman High and the Bundella High (figure 4.1). The southern closure of this gravity high at Yarraman is emphasised by a strong north-east-trending residual Bouguer gravity lineament extending from south of Dubbo to south of Dunedoo to Yarraman. A similar trend was reported by Bradley et al. (1985) as coinciding with the Landsat lineament "on the crest of the Liverpool Range" and they used it to mark the southern boundary of the Gunnedah Basin. In fact, the crest of the larger part of the Liverpool Range is farther to the south and follows a residual Bouguer gravity lineament from north of Ulan to Murrurundi (figures 3.18 and 3.19 and 4.1).
Similar to other transverse basement structures in the Gunnedah Basin, the Permian and Triassic sediments are continuous, but thin over the Bundella High and Yarraman High.
iv) Liverpool Structure
There is a significant discontinuity in the anomalies of the Meandarra Gravity Ridge along the Liverpool Lineament of Scheibner (1973, 1979; figures 4.2 and 3.11). The lineament trends north eastwards along the northem slopes of the Liverpool Range. The discontinuity is emphasised by a strong residual Bouguer gravity lineament which marks the northern closure of a 60 km long meridional positive gravity anomaly with a sinistral displacement of some 20 km to the east of the main trend of the Meandarra Gravity Ridge. Again, the structure in this area is not clear due to lack of borehole data and the thick Tertiary volcanic cover of the Liverpool Range. However, based on the consistent relationship between discontinuities in the anomalies of the Meandarra Gravity Ridge and the morphotectonic elements in the basin, one can predict with reasonable level of confidence that the Liverpool Structure (Tadros 1993f) most probably represents a major crustal fracture approximately orthogonal to main basin axis - that is, a transfer fault along which a very significant amount of displacement between northern and southern basin compartments has taken place. It is also most probable that this crustal fracture was later reactivated and provided a pathway for the Tertiary lavas and intrusions (Martin and Tadros 1990) on the northern and western sides of the Liverpool Range.
v) Murrurundi Trough
The above mentioned 60 km long meridional positive gravity anomaly is present to the west of Quirindi, Murrurundi and Scone. The anomaly corresponds to the north-trending Murrurundi Trough (Tadros 1988c) which has been interpreted from structure contours and isopachs ofthe Permian and Triassic sediments in the area south-west of Quirindi. Sparse borehole data indicate a rapid increase in thickness ofthe Permian and Triassic sequences, in addition to a deepening trough (figures 3.17, 4.9,4.11 and 2.10).
However, Mallett etal. (1988b, p. 7) postulated a "fundamental change in basin characteristics along a north-east trend through Murrurundi". They pointed out that to the north there are linear basins with 4. STRUCTURAL ELEMENTS 181
Figure 4.11. Structure contours (m asl) on the base of the Triassic sequence, Gunnedah Basin, based on borehole data and interpretation of seismic profiles (modified from Tadros 1988b, fig. 11) 182 GUNNEDAH BASIN - TECTONICS AND STRUCTURE basal volcanic rocks similar to the Bowen Basin. To the south, major grabens are not recognised, surface structures dominantly trend north-south paralleling the regional trend in basement, and Permian basal volcanic rocks are less common. They concluded that the Sydney Basin is typical of lower crustal extension and the trace through Murrurundi may represent the change in extension polarity from lower (Sydney Basin) to upper (Gunnedah and Bowen Basins) plate extension.
Tadros (1988c) has shown that the linear arrangement ofthe basins (troughs), which represent half- grabens ofthe original rift, is continuous over the length ofthe Meandarra Gravity Ridge as shown on the total-field gravity anomaly map. The ridge has been transected by numerous north-east-trending transfer faults with variable degrees of displacement. Murray e^ al. (1989) cited the Darting River and Cobar- Inglewood Lineaments as examples. The "trend" at Murrurundi, postulated by Mallett et al. (1988b), shows no evidence of displacement on the total-field Bouguer gravity anomaly map. However, the enhancement achieved by utilising a digital coloured residual Bouguer gravity image (of Murray et al. 1989) has revealed a relatively short lineament coinciding with the Murrurundi trend, but again with no evidence of displacement on the Meandarra Gravity Ridge. This lineament appears to be related to the extrusion of the Tertiary volcanic rocks of the Liverpool Range because it coincides with the axial crest/spine of this mountain range.
In contrast, it will be shown in the next section that the transverse structure (trend) along the Mount Coricudgy Anticline is associated with the largest displacement (sinistral, in the order of 40-50 km) along the ridge. Similariy, surface structures shown by Mallett e^ al. (1988b, fig. 5; see also figure 3.10) change orientation from north-south paralleling the regional trend in the Sydney Basin to a north-easteriy direction in the Gunnedah Basin at the transverse structure along Mount Coricudgy Anticline, not at their "Murrurundi Trend" (p. 7). Permian basal volcanic rocks may appear to be less common in the Sydney Basin than in the Gunnedah Basin, but this can be explained by the fact that fewer intersections of the basal volcanic rocks have been made in the Sydney Basin because of the greater drilling depths required.
Permian basal volcanic rocks are present in the Sydney Basin as far south as Mallett et al.'s (1988b, p. 7) "trend through Sydney". Deep boreholes in the Sydney Basin from East Maitland No. 1 (1 km of pyroclastic and volcanic rocks) to AOG Martindale No. 1A in the west, to Kurrajong Heights No. 1 and AOG Kirkham No. 1 near Camden some 60 km south-west of Sydney all bottom in volcanic units (Qureshi 1984). Also, Permian volcanic rocks occur in the Newcastle region the (Dalwood Volcanics) and in the Muswellbrook district (Gyarran Volcanics).
vi) Mount Coricudgy Anticline
The largest and most significant discontinuity between the anomalies of the Meandarra Gravity Ridge is present to the south of the Liverpool Range (figure 3.17). The discontinuity is represented by a wide zone of low gravity gradient marking the northern closure of a large north-north-east-trending positive gravity anomaly with a very large sinistral displacement, in the order of 40-50 km, to the east of the main trend of the Meandarra Gravity Ridge. The anomaly overiies the Macdonald Trough of 4. STRUCTURAL ELEMENTS 183
Brakel (1984) (originally the "Macdonald Depression" of Mayne et al. 1974) in the Sydney Basin. The south-eastern margin of the low-gravity gradient coincides with the Mount Coricudgy Anticline. Contours on basement in this area (figure 3.17), based on seismic, magnetic and sparse borehole data and on the structure and thickness of the overiying sediments, show rapid deepening of the basin and thickening of the Permian and Triassic sequences south-east towards the Macdonald Trough (figures 4.11 and 4.12). The cross-section (figure 4.12) highlights the significance of the Mount Coricudgy Anticline as a major basement growth feature with present elevation some 2000 m above the floor of the Macdonald Trough. Further, the large displacement on the Meandarra Gravity Ridge strongly suggests that, similar to the Liverpool Structure, the Mount Coricudgy Anticline represents a major transfer fault along which a very large movement between basin compartments has taken place. The trace of the anticline from south of Rylstone to Muswellbrook was originally, and in this author's view, justifiably, defined as the boundary between the structural Sydney and Gunnedah Basins by Bembrick et al. (1973).
NW SE
MARINE SEDIMENTS
> LU 15 ^ O
Z)< O Figure 4.12. North-west - south-east section across the Mount Coricudgy Anticline (after Bembrick et al. 1973, fig. 3; originally modified from Stuntz 1972, fig. 3) 184 GUNNEDAH BASIN - TECTONICS AND STRUCTURE
4.4 FAULTS
4.4.1 HUNTER - MOOKI FAULT SYSTEM
The Hunter- Mooki Fault System forms the present eastern boundary to the Gunnedah Basin and generally trends north-north-westeriy (figure 1.4). In the south, in the Werrie Syncline area, the fault dips 43-48.5°NE (Carey 1934b). In the north, the fault dips approximately 25°E (Ramsay and Stanley 1976) and truncates Eariy Permian sequences of the Maules Creek Sub-basin (Hanlon 1950b, Thomson and Flood 1984, Thomson 1986a, b). Because of its low angle the Mooki Fault System conceals a large area ofthe sub-basin. The geometry ofthe fault as a low angle splay off the Kelvin Fault, in the east, thrusting Carboniferous rocks of the Tamworth Belt westwards over Eariy Permian Maules Creek Formation is cleariy shown on AGSO's deep seismic reflection profile across the Gunnedah Basin (figure 4.13A and the line diagram figure 4.13B). As mentioned eariier, Korsch et al. (1993) indicated that the sedimentary succession ofthe Maules Creek Formation and the underiying basal volcanic units appear to extend for at least 15 km to the east beneath the Tamworth Belt (figure 4.13). The implication of the relationship between the fault and surrounding rocks has been discussed in chapter 3. Movement on the fault must have started after deposition of the Eariy Permian Maules Creek Formation in the mid-Permian period of deformation in the New England region (see chapter 3 for further discussion). The thrust movement continued intermittently through successive compressional phases until the Middle to Late Triassic prior to deposition of the Surat Basin sequence in the Eariy Jurassic .
4.4.2 BOGGABRI FAULT (Tadros 1988c amended)
There is a significant disparity in the vertical elevation of the Maules Creek Formation across the Boggabri Ridge of several hundred metres which can be explained by thrusting of the eastern sub- basin, the Maules Creek Sub-basin, mainly during a compressional event at the end of the Permian to eariiest Triassic. Thrusting was partially accommodated by the Boggabh Fault (Tadros 1988c amended) and resulted in uplift and almost complete removal ofthe Late Permian sediments from a large area of the sub-basin. Only remnants of the marine Porcupine Formation were preserved in the Deriah area in the northern part of the sub-basin. The same compressional movement also caused uplift and tilting in the northern part ofthe Mullaley Sub-basin, terminated the Late Permian coal measure sedimentation and resulted in the erosion of a thick section of the upper Permian sequence. This event is marked by an angular unconformity between the Permian and Triassic sediments. Russell (1981) inferred up-faulting from ERTS-1 imagery (Scheibner 1973). The present configuration, morphology and fracture pattern on the remnant outcrops of the Boggabri Ridge suggest the presence of a thrust fault (the Boggabri Thrust of Tadros 1988c) trending north- north-west, probably along a line now occupied by the Namoi River (figure 4.4).
It appears that the Boggabri Ridge being a longitudinal basement structure (parallel to the rift axis) representing a synthetic fault (surface extension of a detachment fault, see figure 3.8), provided a reactivation surface along which thrust faulting occurred during the compressional movement at the 4. STRUCTURAL ELEMENTS 185
Please see print copy for image
Figure 4.13. (A) Portion ofthe unmigrated deep seismic section BMR. GDI across the Maules Creek Sub- basin and the westem part ofthe Tamworth Belt (from Korsch et al. 1993), showing the succession thickening towards the east and containing local structural complications due to thrust faulting. The Mooki Fault is interpreted as a low angle splay off the Kelvin Fault thrusting Carboniferous rocks ofthe Tamworth Belt westwards over Early Permian Maules Creek Formation. For location ofthe seismic section see figure 3.14
Please see print copy for image P l e a s e s e e p ri n t Figure 4.13. (B) Line diagram ofthe westem part ofthe deep seismic reflection profile BMR91.G01 across c the Gunnedah Basin and the western part of the New England Orogen, showing the o interpreted distribution ofthe Early Permian volcanic pile and the Lachlan Orogen beneath p the Gunnedah Basin (from Korsch cV a-/. 1993). For location of the seismic section y see figure 3.14 f o r i m a g e 186 GUNNEDAH BASIN - TECTONICS AND STRUCTURE end of the Permian - Eariy Triassic. An additional response to these compressional movements in the northem Gunnedah Basin (including the Narrabri and Moree Highs) was a major uplift which caused erosion of much of the Late Permian sequence in the northern Gunnedah Basin (figures 3.23 to 3.25).
4.4.3 ROCKY GLEN FAULT (new)
In contrast to the Mooki and Boggabri Faults, the Rocky Glen Fault has no surface expression as the Gunnedah Basin sequence is almost totally concealed underneath the Surat Basin sediments. Tadros (1988c) invoked a shallow low angle thrust fault along the eastern flank of the Rocky Glen Ridge to explain the offset between the centres of the troughs in the Mullaley Sub-basin and the maxima ofthe positive anomalies ofthe Meandarra Gravity Ridge (see section 3.2.2). He interpreted this offset as caused by displacement of the axis of greatest subsidence (centres of the troughs) towards the Rocky Glen Ridge in the west (and/or caused by an original asymmetry between the area of emplacement ofthe dense igneous material and subsidence where half-grabens were formed).
It was only recently that Korsch et al. (1993) provided evidence for a thrust fault along the eastern flank ofthe Rocky Glen Ridge (figures 3.15 and 3.16). As mentioned eariier, this fault can be traced for at least 4 km through the Gunnedah Basin sequence into the underiying basal volcanics pile. The fault is truncated by the Surat Basin sequence suggesting a Middle Triassic upper limit for the compressive thrust phase.
4.5 MINOR STRUCTURES
The strata within the Gunnedah Basin generally reflect the structure on top of the basal volcanic rocks. West of the Boggabri Ridge the strata dip gently towards the basin axis as shown by the structure contours on the base of the Triassic sequence (figure 4.11). East of the Boggabri Ridge the strata dip gently to the east but steeper dips occur near the Hunter - Mooki Fault System. Steep dips also occur adjacent to intrusions in the Mullaley Sub-basin.
Closed-spaced drilling in the west Boggabri area (Tadros 1982) and in the Maules Creek area to the east ofthe Boggabri Ridge (Thomson 1986b) indicate that localised palaeotopographic variations within the basal volcanic units had an important bearing on the geometry of the overiying sedimentary sequence. It is most evident in the progressive onlap of the Permian and Triassic sediments onto the ridges and highs ofthe basement.
Locally, a number of folds are superimposed upon the regional dip (Hanlon 1949a, b, 1950a; Kenny 1964; Russell 1981; Tadros 1985, Tadros in Tadros et al. 1987b; figure 4.14). The effect of local folds on the regional structure is best represented in the Breeza Shelf area (figures 4.15 and 4.16). Folds were also mapped south of the Liverpool lineament near Quirindi and in the Bohena Trough in the north by petroleum exploration companies using airphoto interpretation and seismic and gravity surveys. In many cases the structures were tested by drilling. Folding in the three areas show 4. STRUCTURAL ELEMENTS 187
150" TN
X c m Narrabri 3) Wilga Park t Anticline \ NEW ENGLAND ^
•t V FOLD BELT Bohena Anticline \
Boggabri
^ \ F v> iGunnedah^ -(ROCKY GLEN RIDGE) -^ * '^'.-A 31°- \ •^ D/u D.:? •^ \ Xu \ Springhurst en Curlewis Dome -\ Anticline •/ Millroy X. -A Nea I Anticline Coonabarabran fl Anticline L Br,eeza # Clift Anticline Watermark* / Anticline
• \ Tribella x/'Caroon'a Anticline X ^:; /^ Anticline REFERENCE
Borehole Mirrabooka -4— Anticline Anticline' 0 10 20 1 [ I Syncline X 150° New Windy Km I Anticline Figure 4.14. Minor stmctures, Mullaley Sub-basin (compiled from Russell 1981; Hamilton 1985a; Hamilton et al 1988; Tadros 1993f, fig. 6.22) GUNNEDAH BASIN - TECTONICS AND STRUCTURE
Figure 4.15. Stmctme contours (m asl) on the base ofthe Hoskissons Coal, Breeza area, showing the influence of local folds on regional stmcture (modified from Tadros 1985, fig. 3)
B(NE)
REFERENCE
Igneous intrusion tc 2 METRES ABOVE S,L § 2 SCftlE ft S 1 2 UJ 5 K CC S O
Figure 4.16. Cross-section, Breeza area (modified ftom Tadros 1985, fig. 2). For location see figure 4.15 4. STRUCTURAL ELEMENTS 189 different orientations. In the Breeza Shelf area near the Hunter- Mooki Fault System (figure 4.14), folding consists mainly of medium to large-amplitude north-east-trending anticlines and synclines developed in response to compressive left-lateral wrenching in front of the thrust (Hamilton et al. 1988). In contrast, folding in the Quirindi area closer to the Mooki Fault System (figure 4.14) consists of large-amplitude north-west-trending anticlines and synclines developed in response to a compressive force apparently opposite to that in the Breeza Shelf area (right-lateral wrenching). Folding in the Bohena Trough in the north, is represented by low-amplitude north-trending parallel anticlines formed in response to east-west compressive movements.
Some folds are diapiric domes related to the emplacement of post-Permian (Jurassic/Tertiary) igneous intrusions, e.g. the Mirrabooka (Alliance Petroleum Australia N.L. 1964) and Wilga Park (Hartogen Energy Ltd) Anticlines, and Springhurst Dome (Tadros 1993f; figure 4.14). It appears that sediments ofthe upper Black Jack Group have been domed up about 90 m by a thick lensoid-shaped igneous intrusion to form the Springhurst Dome. A similar structure was also indicated in drilling some 2 to 6 km to the north-west (Tadros 1985).
Faulting has been detected in drill core in many places in the Gunnedah Basin, particulariy in the south-east near the Mooki Thrust. Thick, brecciated and slickensided zones in borecore, associated with repetition of stratigraphic units strongly suggest that significant thrusting characterises the area near the Hunter- Mooki Fault System. However, the wide-spaced drilling, poor outcrop and thick alluvial cover preclude delineation ofthe exact direcfion and magnitude ofthe faults. Several faults were also interpreted from seismic surveys such as the East and West Bellata Faults near Bellata in the north (Etheridge 1987; figure 4.7). In the north-west, the Wilga Park and Nyora Anticlines are bounded at depth by high-angle reverse faults and their crests are affected by extensional normal faults (figure 4.17a) (Hamilton et al. 1988). ERTS-1 imagery (Scheibner 1973) and also subsequent Landsat images (E. Scheibner pers. comm. 1988) reveal several prominent lineaments which Scheibner suggested could represent thrust faults. Minor faulting appears to be associated in many instances with uplifting due to post-Permian intrusions (Russell 1981). The Wilga Park anticlinal structure is also associated with normal faulting caused by intrusion of a ?Tertiary volcanic plug (figure 4.17b; Hamilton etal. 1988, 1993). Close-spaced drilling by coal explorafion companies in the Maules Creek Sub-basin east ofthe Boggabri Ridge (Thomson 1986b) identified a number of north- westeriy striking normal faults with vertical displacements ranging from 80 to 120 m. 190 GUNNEDAH BASIN - TECTONICS AND STRUCTURE
LINE 85-NGN-15 Wilga Park 1 N Projected
0 L. km
Figure 4.17 (A) Norfii-south seismic line over the Wilga Park gas field (modified from Hamilton et al. 1988, fig. 9 and Hamilton et al. 1993, fig. 19.2a)
LINE 84 - WW7
0 0-5 L -J km
Figure 4.17 (B) North-south seismic line over the Wilga Park structure showing a volcanic plug of probable Tertiary age and associated extensional faulting (modified from Hamilton et al. 1988, fig. 9 and Hamilton et al. 1993, fig. 19.2a) CHAPTER 5
INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL
5.1 Introduction 195
5.2 Early Pennian 195 5.2.1 Leard - Maules Creek depositional sequence 195
5.3 Late Pennian 198 5.3.1 Porcupine - lower Watermari< depositional sequence 198 5.3.2 Upper Watermark - lower Black Jack deposifional sequence 198 5.3.3 Upper Black Jack deposifional sequence 205
5.4 Early and Middle Triassic 206 5.4.1 Digby deposifional sequence 206 5.4.2 Napperby deposifional sequence 206
5.5 Igneous intrusions and extrusions 209 «» i^^:B''-'ii'jikMm ^-ilt^i The morphotectomc elements of the Gunnedah Basin controlled the distribution of igneous intrusions and volcanism. Intrusionofthemajorvolcaniccomplexesofthe Nandewar, Warrumbungle and Liverpool Ranges appears to have followed reactivation of major transfer faults. The photo shows the Ningadhun volcanic plug of the Nandewar Range (SH/55-12,0034D) 195
CHAPTER 5
INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL
5.1 INTRODUCTION
The volcanic rift/extensional mechanism of basin formafion produced major stnjctural elements that influenced sedimentafion over the enfire history ofthe basin. However that infiuence was modified and complicated by the superimposition of subsequent tectonic regimes in the Late Permian and Triassic. It has been demonstrated in chapter 4 that these major stmctural elements consist of longitudinal structures (which represent normal extension faults) and transverse structures (which represent transfer faults; figures 4.1, 4.2 and 3.8).
During the Eariy Permian both transverse and longitudinal structures (transfer faults and nonmal extension faults respectively - figures 4.1, 4.2 and 3.8) acted as prominent topographic features which provided clasfic input to the trough areas (underiain by half-graben structures). Sediments were supplied to the trough areas in the form of alluvial fans and associated braided streams. Further, these stmctures acted as barriers along and across the subsiding half-grabens and gave rise to localised lacustrine environments. In the Late Permian and Triassic, longitudinal stmctures were periodically reactivated as thrust faults resulting in uplift and erosion of much of the upper Permian sequence particulariy in the Maules Creek Suthbasin and the northern Mullaley Sub-basin (north of Walla Walla Ridge). Transverse structures acted as growth features with a remari^able influence on sedimentafion.
In the following secfions, the influence of the basin's morphotectonic and structural elements on development, distribufion and geometry of selected depositional systems and their component facies, as well as the influence of these elements on igneous intrusions and extrusions, will be discussed. This chapter highlights the interrelafionship between basin origin, tectonics and sedimentafion, which is apparent throughout the basin sequence. As such the chapter provides the framework and the basis for sedimentology, stratigraphy and resource exploration in the basin. The upper part of the Black Jack Group is discussed in detail in subsequent chapters.
5.2 EARLY PERMIAN
5.2.1 LEARD - MA ULES CREEK DEPOSITIONAL SEQUENCE
It has already been menfioned (secfion 3.2.5) that basin fill in the Eariy Permian was localised in small rapidly subsiding troughs (figures 3.4A and 3.21). The troughs were separated by highlands and ridges consisfing of silicic and mafic volcanic rocks. Localised distribufion of the lacustrine facies of the Eariy Permian (Stage 3, McMinn 1981d) Goonbri Formafion of Thomson (1986a. b) cleariy demonstrates the structural control on sedimentation. Fine-grained sediments of this facies accumulated in the most 196 GUNNEDAH BASIN - TECTONICS AND STRUCTURE rapidly subsiding depocentres in the Bohena, Bellata and Tooraweenah Troughs and the Maules Creek Sub-basin. The sequence has a maximum thickness estimated at 150 m (Thomson 1986b) to the east ofthe Boggabri Ridge, 105 m in the Bellata Trough and 79 m in the Bohena Trough to the west of the ridge. In the Tooraweenah Trough in the southern part of the Gilgandra Sub-basin, DM Mirrie DDH 1 intersected a 9.5 m secfion of an unnamed mudstone conglomerate sequence equivalent in age (McMinn 19821) to the Goonbri Formation. The sequence appears to have been deposited in an intermontane lacustrine plain (Yoo 1988). The total thickness of this unit is not known as the borehole was terminated before penetrafing the full sequence. It is most probable that similar sediments were also deposited in the Pilliga Trough in the northern part of the Gilgandra Sub-basin and in the Murrurundi Trough in the south-easternmost part ofthe Mullaley Sub-basin.
Although more widespread, the overiying sequence ofthe Maules Creek Formafion also highlights the structural control on sedimentation particulariy in the Mullaley Sub-basin (figure 2.8). Thickness ofthe Maules Creek Formation is 80 to 100 m in depocentres in the trough areas west ofthe Boggabri Ridge, whereas in inter-trough areas along transverse structures, the sequence is very thin or absent (figure 2.8). Thomson (1986a, b, 1993) recognised three zones in the Maules Creek Formation where provenance and architecture vary widely: a northern quartz-rich facies derived from the Lachlan Fold Belt; a southern conglomeratic facies derived from the Boggabri Ridge; and a south-eastern fine grained, coal-rich facies (figure 5.1). This broad zonation compares well with the morphotectonic elements ofthe basin and suggests a strong structural control on deposifion (Tadros 1993g).
The westeriy derived quartz-rich northern zone coincides with the Bohena Trough (figures 4.1 and 4.2). The north-east-trending Walla Walla Ridge was a prominent structural high at this fime and acted as a barrier against the westeriy derived fluvial sediments. The Narrabri High, although probably not as prominent as the Walla Walla Ridge, provided a northern barrier. It appears that the gaps in the Boggabri Ridge (see section on "Boggabri Ridge" above) menfioned by Thomson and Flood (1984) and Thomson (1986b) had already been developed and a considerable quantity of the westerly derived sediment was directed through these gaps to the Maules Creek Sub-basin (figures 5.1, 3.21 and 2.8).
Deposition of the southern conglomeratic facies is confined to the Bando Trough with sediments up to 100m thick in the depocentre. Sediments are largely conglomeratic and of volcanic-lithic composifion. Sedimentafion was mainly by south-westeriy flowing humid alluvial facies derived from the Boggabri Ridge (figure 5.1 and Thomson 1986b).
The south-eastern fine-grained facies is restricted to the Breeza Shelf (figures 5.1 and 4.2). Characteristically, this zone maintained slow stable subsidence with a low rate of sedimentation which favoured a high rate of peat accumulation in a relatively thin sedimentary sequence (Tadros 1985, Thomson 1986b, 1993).
The Maules Creek Formafion in the Maules Creek Sub-basin also shows a rapid increase in thickness over 10 -15 km distance from a few metres on the eastern margin ofthe Boggabri Ridge to more than 800 m near the trace of the Mooki Fault System on the present surface (figure 2.8). Furthermore, the 5. INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL 197
TN ::.\ \ ^:\
I r 1 ^c
> r ' -J\ \ c m \ ^ '^ < i' r- \ 1 XI Narrabri
."H^VfS^^V^^ft. ^ NEW ENGLAND ^:%\:<^i;«i^^f^^.^^MA^EsK a
FOLD BELT
SUB? ^^^•^^BASiNA MULLALE,LALEY;'^;:^V.»1,^;:.f;^^ft;^;'\gpijY |
REFERENCE
Area dominated by westerly derived sands Area dominated by humid alluvial fans derived from Boggabri Ridge SUB - BASIN;. en Slow stable subsidence - -\ low sedimentation rate and vn high peat accumulation rate
Boggabn Ridge
~"'^i'i-^i-'->'!"' r'J>-r..' ^"!.'^'.'x-'^V^\:'.•-•.••••'•-"•.'•-'-•-•-•-••'•!i^S-i-i'^:':• *•;•'-•.--'•;' Dr-/-i/->.-Breeza7 o- -; {,-i-y-*^-'l-,
10 20 -J km
Figure 5.1. Schematic depositional setting for the Maules Creek Formation in the Mullaley Sub-basin (modified from Thomson 1986b, fig. 4.41) 198 GUNNEDAH BASIN - TECTONICS AND STRUCTURE formation continues to thicken underneath the Mooki Fault towards a depocentre in the east. Korsch et al. (1993) indicated that the sedimentary succession of the Maules Creek Formation appears to extend for at least 15 km to the east beneath the Tamworth Belt (figure 4.13) and that the formation reaches 1000 m in thickness.
Sedimentation within the Maules Creek Formafion was by well-defined easteriy and south-easteriy trending channelised braided streams ofthe Donjek type (Miall 1978) towards a depocentre in the east, with sediment load derived from the Boggabri Ridge to the west (Thomson 1986a, b).
5.3 LATE PERMIAN
5.3.1 PORCUPINE - LOWER WATERMARK DEPOSITIONAL SEQUENCE
The change in tectonics in the latest Eariy Permian to eariiest Late Permian which resulted from thermal relaxation of the lithosphere, caused basin-wide subsidence and widespread marine transgression. Although the marine transgression provided a wider coverage for the sediments of the Porcupine and Watermark Formafions than the restricted deposition of the Eariy Permian sediments, the influence of subsidence in the trough areas was also significant, as shown on the isopach maps (figures 3.22 and 5.2). However, as mentioned eariier, basin compartments behaved differently even during basin-wide thermal subsidence because they had variable extension and subsequently variable tectonic and thermal subsidence histories (Murray et al. 1989). Thermal subsidence was greater, and started eariier, in the south and south-east than in the north. This is indicated by the age of the marine Porcupine Formafion, which began in Upper Stage 4 and continued into Lower Stage 5b in the south (palynological ages by A. McMinn 1993). In the north, marine sedimentation started as late as Lower Stage 5b. Marine condifions confinued basin-wide into Lower Stage 5c as indicated by the conformably overiying Watermark Formafion.
The Boggabri Ridge supplied sediment to the lower part of the Porcupine Formation (figure 5.3; Skilbeck and Mcdonald 1993), but depletion of silicic volcanic pebbles in the upper part indicates that the ridge was finally covered by the marine transgression (Thomson 1986b). Other structures, particulariy the Rocky Glen Ridge, appear to have acted in the same way and provided silicic volcanic clasts to the Porcupine Formation to the east (of the ridge). However, relative subsidence of the Rocky Glen Ridge and the flanking shelf areas (the Wollar, Weetaliba and Wee Waa Shelf areas) was significantly slower than the adjacent troughs and resulted in the deposifion of a much reduced thickness of the marine sediments which appear to be totally absent on top of the ridge except to the south of Coonabarabran (figures 3.22, 4.6b, 5.2 and 5.3).
5.3.2 UPPER WATERMARK-LOWER BLACK JACK DEPOSITIONAL SEQUENCE
The change to foreland tectonics provided a new source of sediments for the basin from the overthrusted New England Fold Belt region. Westeriy and south-westeriy trending bed-load streams carried volcanic-lithic detritus from the New England region to the subsiding basin. However, transverse structures (which formed during the extension/rift stage), notably the Walla Walla Ridge, 5. INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL 199
TN
NEW ENGLAND
FOLD BELT
- 31
Figure 5.2. Isopachs (m), Porcupine - Lower Watermark Marine-shelf System (modified from Hamilton 1991, fig. 12) 200 GUNNEDAH BASIN - TECTONICS AND STRUCTURE Please see print copy for image
Figure 5.3. Isopachs (m), lower Porcupine Formation (total Porcupine Fonnation minus tiansition facies of Skilbeck and McDonald 1993; from Skilbeck and McDonald 1993, fig. 12.27) 5. INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL 201 although totally covered by the marine sediments, continued to influence deposition of the overiying sediments during the foreland basin stage. These structures acted as relafively positive features characterised by less net subsidence than the adjacent trough areas (figure 4.8). Hamilton (1985a, 1987, 1991, 1993) discussed the deposifional history of the upper Watermark - lower Black Jack/Ari Sediment for the Upper Watermark - Lower Black Jack Delta Systems was derived by bed-load streams flowing south-west from the New England region. The prodelta facies (figure 5.4), which is the foundafion of delta systems, is absent in the north but thickly developed in the south, particulariy in the south-easternmost part of the Mullaley Sub-basin except around the Breeza Shelf area. Significantly, much of this southern area corresponds to the Bando Trough, whereas the south-easternmost part corresponds to the Murrurundi Trough. Two distinct deltas formed in the upper Watermark - lower Black Jack depositional episode (figure 5.5). The northern delta broadly occupies the Bohena Trough and is slighfiy older than the southern delta. Hamilton (1987, 1993b) suggested that as the main distributary of the northern delta prograded far enough seaward, its gradient and general flow efficiency were reduced to the point where discharge was diverted along a new distributary path with a steeper gradient to the coast. Hamilton concluded that diversion to the south could have been favoured by the regional configurafion of the seaward- broadening and deepening basin. It is evident from comparing figure 5.5 and figures 4.1 and 4.2 that this southern "deepening" basin corresponds to the Bando Trough and that the zone between the two delta lobes corresponds to the Walla Walla Ridge (Tadros 1993g). Basin-wide marine transgression followed and established the Arkamla Shallow-marine System. The net sandstone map (figure 5.6), once again, suggests that two depocentres were present. The northern depocentre is well developed and is thought to have been established eariier than that in the south, which is relafively thinner (Hamilton 1987, 1993b). Note that the thickening of sand along the Mullaley Sub-basin's western margin is related to the fluvial input by streams derived from the Lachlan Fold Belt. The south-easternmost part, which is located on the stable Breeza Shelf area (figure 4.2), remained as a delta plain, not affected by the marine incursion. Delta plains are very sensifive to base level changes, whether caused by tectonic subsidence or eustafic sea level rise, and the fact that the Breeza Shelf area remained as a delta plain during the marine incursion favours a tectonic cause for the base level change. Uplift and tectonic loading of the New England Fold Belt induced basin subsidence and the subsequent marine incursion. The basin's response to tectonic loading was controlled by the inherent basement morphotectonic elements. Basin compartments responded differently to tectonic loading, resulting in variable amounts of subsidence in the trough areas, whereas stable areas, such as the Breeza Shelf, remained comparatively unaffected (Tadros 1993g). Hamilton's depositional model for the Upper Watermark - Lower Black Jack Delta System therefore, strongly highlights the influence of basement morphotectonic elements on sedimentafion. It could be 202 GUNNEDAH BASIN - TECTONICS AND STRUCTURE 150° NEW ENGLAND FOLD BELT 31°- • Borehole 0 25 km I I 1 I I I 150° Figure 5.4. Isopachs (m), prodelta facies, Upper Watermark - Lower Black Jack Delta Systems (modified from Hamilton 1987, fig. 3.19) 5. INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL 203 150" / / ^/ 12 \ if I I 1 "0 Lobate Delta - NEW ENGLAND System FOLD BELT J 3 -31" 31 - LACHLAN FOLD BELT ••.x \ \"-v-k Elongate Delta '! System \ 0 25 km Borehole 150° _| Figure 5.5. Net sandstone (m), lower delta plain facies. Upper Watermark - Lower Black Jack Delta Systems (modified from Hamilton 1985, fig. 6 and 1987, fig. 3.14) 204 GUNNEDAH BASIN - TECTONICS AND STRUCTURE 150" NEW ENGLAND FOLD BELT al as km I Borehole 150° Figure 5.6. Net sandstone (m), Arkarula Shallow-marine System (includes the Westem Bed-load Fluvial System; modified from Hamilton 1985a, fig. 7 and 1987, fig. 3.20) 5. INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL 205 postulated from the different ages of the delta lobes that the troughs (structural domains) subsided in an altemafing step-wise succession at different rates (Tadros 1993g). 5.3.3 UPPER BLACK JACK DEPOSITIONAL SEQUENCE Although foreland loading of the thrust belt in the New England Orogen was the dominant cause of subsidence during deposifion of the upper Black Jack sequence, the inherent, volcanic rift-related basement structural elements also had a significant effect on varying the subsidence rates in the different basin compartments and subsequently on sedimentafion. The total isopach map (figure 6.14) shows a south-south-westeriy basin axis with two depocentres separated by a transverse zone of thin sediment trending south-east through Boggabri. This zone coincides with the Walla Walla Ridge. The northern depocentre coincides with the Bohena Trough and contains sediment up to 90 m thick, whereas, sediment in the southern depocentre overiaps the Bando Trough and is much thicker, ranging from 70 m in the north to over 200 m in the south. Thickness increases rapidly in a south-easteriy direcfion to in excess of 400 m. This rapid increase in thickness is coincident with the Murrurundi Trough. A similar infiuence was also observed on the component systems of the upper Black Jack sequence, although in some cases erosion subsequent to deposition has affected the geometry of the genefic unit. For example, despite the effect of fluvial incision by the overiying Eastern Fluvial System (see chapter 6), the isopach map for the Western Fluvial System (figure 6.21) broadly reflects the influence of basement structure on the development of a transverse thin zone of sediments along the south- easteriy trending Walla Walla Ridge and rapid thickening towards a depocentre in the south-east. A marked change in contour pattern, reflecting a rapid south-easteriy increase in thickness, also exists in the south along a south-easteriy transverse trend through Breeza. This trend coincides with the Breeza - Dunedoo basement structure. A remarkable influence of basement structure on deposifion was also cleariy observed on the Hoskissons Coal despite the effect of other factors such as compacfion within the peat swamp and of the underiying platform (see chapter 8). The coal lithotype profile is thicker and contains addifional plies at the base and top in the trough areas than over the transverse basement structures such as the Walla Walla Ridge. In the central parts of the trough areas the Hoskissons Coal grades into organic- rich mudstone indicating transformafion of the peat swamp into a lacustrine environment (see figure 8.7 and chapter 8 for detailed discussion). The following two chapters provide detailed discussion of deposifional systems and sedimentary facies within the upper Black Jack sequence and highlight the infiuence of basement structure on their distribution and geometry. The influence on coal quality and distribufion is given in chapter 8. 206 GUNNEDAH BASIN - TECTONICS AND STRUCTURE 5.4 EARLY AND MIDDLE TRIASSIC The Triassic sequence provides further evidence of basement control on sedimentation in the Gunnedah Basin. The sequence consists of the Digby Formafion at the base, overiain by the Napperby Formation. Recently Jian and Ward (1993) have studied the sedimentology and depositional evolufion ofthe Triassic sequence in the basin in detail. Jian and Ward (1988) separated out the upper part of the Napperby Formation and named it the Deriah Formation. Continued foreland loading of the thrust sheets in the New England Orogen caused rapid basin subsidence during the Eariy and Middle Triassic. Once again, subsidence rates in the different parts of the basin, and consequently sedimentation, reflect the inherent volcanic rift-related basement structural elements. 5.4.1 DIGBY DEPOSITIONAL SEQUENCE Jian and Ward (1993) divided the Digby Formation into a lower "Conglomerate Interval" representing a major alluvial fan system, and an upper "Sandy Interval" related to a series of fluvial depositional systems. Net conglomerate isopachs for the "Conglomerate Interval" (figure 5.7) show lobate geometry. Jian and Ward (1993) interpreted the sediment distribufion pattern as indicating the existence of two depocentres, a major one in the Gunnedah - Quirindi area in the south-east and a minor one near Narrabri in the north. The Gunnedah - Quirindi depocentre lies in the Bando Trough and consists of two, separate, more localised depocentres. These depocentres highlight the presence of a second-order basement morphotectonic element trending south-west from Gunnedah through Mullaley to the Rocky Glen Ridge (discussed in chapter 3). The depocentre near Narrabri coincides with the Bohena Trough, and the area between the two main depocentres coincides with the Walla Walla Ridge. The net sandstone map for the "Sandy Interval" (figure 5.8) was interpreted by Jian and Ward (1993) as representing the geometry of sandstone bodies built by south-easteriy fiowing streams that joined in the centre of the basin to form a single trunk river system. The south-westeriy trending streams carried lithic detritus from the New England Fold Belt. The streams apparently flowed mainly along the axis ofthe Gunnedah - Quirindi (Bando Trough) lobe of the alluvial fan complex that formed the underiying "Conglomerate Interval". 5.4.2 NAPPERBY DEPOSITIONAL SEQUENCE The isopach map for the Napperby depositional sequence which includes the Napperby and Deriah Formafions (figure 4.10; Tadros 1988c) again highlights the strong influence of basement morphotectonic elements on deposition The strongest infiuences during this period were the Walla Walla Ridge and the Narrabri High. Up to approximately 300 m of Napperby sediments were deposited in the Bellata Trough in the north, whereas only between 60 and 160 m occur on the Narrabri High. The isopach map (figure 4.10) also suggests that the Gunnedah - Mullaley structure, which is a second-order basement morphotectonic element, was more active during this period than during the Permian. 5. INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL 207 Please see print copy for image Figure 5.7. Net conglomerate (m), Conglomerate Interval ofthe Digby Formation (from Jian and Ward 1993, fig. 15.2). Arrows indicate inferred major palaeocurrent directions 208 GUNNEDAH BASIN - TECTONICS AND STRUCTURE Please see print copy for image Figure 5.8. Net sandstone (m), Sandy Interval of the Digby Formation (from Jian and Ward 1993, fig. 15.8). Arrows indicate inferred major palaeocurrent directions 5. INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL 209 The isopach map ofthe NapperiDy Formation, as defined by Jian et al. (1993), indicates the presence of four separate depositional centres (figure 5.9), north and south of Narrabri and south and west of Gunnedah. The depocentres immediately north and south of Narrabri coincide respectively with the Bellata Trough and Bohena Trough. The area between the two depocentres coincides with the Narrabri High. The two depocentres near Gunnedah are located within the Bando Trough and reflect the influence of the above mentioned second order basement morphotectonic element between Gunnedah and Mullaley. The thin sediments between Gunnedah and Narrabri correspond to the Walla Walla Ridge. The sandstone percentage map (flgure 5.10) of the Napperby Formafion shows that all four depocentres were also sites of major sand accumulation (Jian and Ward 1993). The Napperby Formafion consists of three genefic units: "Interval A" at the base, represents a sandy lacustrine fan-delta system, "Interval B" represents an elongate lacustrine - delta system, and "Interval C", a composite lacustrine - delta and fluvial system (Jian and Ward 1993). Of particular interest is the percentage sandstone map for "Interval B" (flgure 5.11), which shows elongate sand-rich bodies trending westeriy and south-westeriy. Jian and Ward interpreted these sand bodies as representing a fluvial-dominated digitate delta system fed mainly by rivers from the New England Fold Belt region. Delta lobes of this system are present in the Bellata, Bohena and Bando Troughs where subsidence rates were highest, while interdeltaic deposits occur over and around the less actively subsiding Narrabri High and Walla Walla Ridge. They suggested that this pattern of distribution refiects the natural tendency for rivers to develop in the resulting topographic and structural lows (Weimer 1984, Alexander and Leeder 1987). Thinning of the Triassic sequence over the Narrabri High and Walla Walla Ridge and the significant increase in thickness in the Bellata, Bohena and Bando Troughs again indicate that the ridges maintained relatively stable, slower subsidence than the neighbouring troughs. 5.5 IGNEOUS INTRUSIONS AND EXTRUSIONS In addition to their influence on the basin sedimentary fill, the basement morphotectonic elements also controlled the distribution of igneous intrusions and volcanism. Major extension/detachment, listric and transfer basement faults, associated with the extension phase of basin development, provided conduits for igneous intrusions and volcanic eruptions. Late Carboniferous to Eariy Permian empfions followed transfer faults and floored the developing Gunnedah Basin with basalt, while silicic volcanism appears to have developed parallel to the basin margins along longitudinal extension/detachment faults. Martin and Tadros (1990) pointed out that the tensional tectonic regime caused by Jurassic break-up of Gondwana reactivated these crustal fractures which formed in the Late Carboniferous to Eariy Permian, providing pathways for the Jurassic Garrawilla Volcanics and intrusions. It also appears that intrusion of the Tertiary volcanic complexes of the Nandewar, Warrumbungle and Liverpool Ranges, which accompanied the Tertiary opening of the Tasman Sea, followed reactivafion of these major transfer faults (figure 5.12). 210 GUNNEDAH BASIN - TECTONICS AND STRUCTURE Please see print copy for image Figure 5.9. Isopachs (m), Napperby Formation (Intervals A, B and C of Jian 1991 and Jian and Ward 1993; from Jian and Ward 1993, fig. 15.15) 5. INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL 211 Please see print copy for image Figme 5.10. Percentage sandstone, Napperby Formation (Intervals A, B and C of Jian 1991 and Jian and Ward 1993; from Jian and Ward 1993, fig. 15.16). Arrows indicate inferred major palaeocurrent directions 212 GUNNEDAH BASIN - TECTONICS AND STRUCTURE NEW ENGLAND FOLD -31° 150° Figure 5.11. Percentage sandstone. Interval B ofthe Napperby Formation (modified from Jian and Ward 1993, fig. 15.19). Arrows indicate inferred major palaeocurrent directions 5. INFLUENCE OF STRUCTURAL ELEMENTS ON THE BASIN FILL 213 REFERENCE Tertiary volcanic complexes Jurassic volcanic complexes Figure 5.12 Tertiary and Jurassic volcanic complexes of the Gunnedah Basin showing broad north-east and/or north-north-west alignment (modified from Tadros 1993g) 214 GUNNEDAH BASIN - TECTONICS AND STRUCTURE This page is blank CHAPTER 6 GENETIC STRATIGRAPHIC ANALYSIS 6.1 Introduction 217 6.1.1 Concepts and definitions 217 Deposifional episodes 217 Seismicand sequence stratigraphy 218 Genefic stratigraphic sequence 220 6.2 Application of Genetic stratigraphic sequences to nonmarine basins 222 6.2.1 Sedimentary processes and peat accumulation ....: 223 6.2.2 Coal seams as Genefic sequence boundaries 223 Coal seam correlafion 227 Coal seam lithotype profiles 227 Ash profiles 228 Time significance of coal seams 228 6.2.3 Application to the Gunnedah Basin sequence 229 6.3 Procedure used in genetic stratigraphic analysis of the Gunnedah Basin 229 6.3.1 Quantitafive facies mapping 231 Applicafion of geophysical log facies 232 6.4 Depositional episodes of the upper Black Jack stratigraphic sequence 234 6.5 Geometry, depositional style and evolution of the upper Black Jack systems 238 6.5.1 Hoskissons Peat-swamp System 238 Depositional setting 245 6.5.2 Western Fluvial and Lacustrine Systems 248 Geometry and depositional evolufion 250 Southern part ofthe Mullaley Sub-basin 253 Hoskissons- Caroona interseam interval 253 Caroona/Hoskissons - Howes Hill interseam interval 255 Howes Hill/Hoskissons - Breeza interseam interval 257 Northem part ofthe Mullaley Sub-basin 259 Terminafion ofthe Westem (Fluvial/Lacustrine) Deposifional Episode 259 6.5.3 Eastem Fluvial System 262 Geometry and deposifional evolufion 262 Breeza - Clift interseam interval 267 Clift - Springfield interseam interval 268 Springfield - Doona seam/top of Black Jack Group interval 271 Terminafion ofthe Eastern Deposifional Episode 274 216 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE This page is blank 217 CHAPTER 6 GENETIC STRATIGRAPHIC ANALYSIS 6.1 INTRODUCTION Defining deposifional units of sufficient extent and appropriate scale for sedimentological and stratigraphic analysis is an increasingly important task in exploration and development of energy resources. This task relies heavily on generafion and analysis of subsurface data. Deposifional systems provide meaningful sections of the basin fill for such analysis; their recognifion and delineation establishes a framework for facies differentiafion and mapping. The concept of depositional systems implies that component facies are spatially related, three-dimensional units, which may be readily described by commonly available types of subsurface data augmented where possible with descripfions of core or outcrop sections (Galloway & Hobday 1983a). This approach to strafigraphic analysis, which will be described in detail later in this chapter, relies heavily on reconstruction of basin morphology and bedding architecture, determinafion of gross lithology, quanfitative delineation of the geometry of framework sandstone units, and recognition of vertical and lateral successions and common facies associations. 6.1.1 CONCEPTS AND DEFINITIONS DEPOSITIONAL EPISODES The genetic approach to stratigraphic analysis evolved from the eariy recognition by stratigraphers of the episodic nature of basin filling (Wanless and Weller 1932, Wheeler and Murray 1957, Sloss 1963). Marine basin margins are characterised by repetitive episodes of progradation punctuated by periods of transgression and flooding ofthe deposifional platform (Galloway 1989). Frazier (1974) introduced the concept of depositional episodes based on extensive three-dimensional stratigraphic studies of quaternary depositional systems of the north-western Gulf Coast Basin, U.S.A. Frazier's concept of depositional episodes provides a basis for recognifion of genefic strafigraphic units within large marine or lacustrine basin fills. The sedimentologic principles of Frazier's concept form a foundation for genetic stratigraphic analysis as summarised by Galloway and Hobday (1983a), Galloway (1989) and below, from Frazier (1974): 1. Terrigenous clastic sediments are allochthonous, and must be transported to the basin margin by rivers which provide the bulk ofthe basin fill. Basins are filled from the margin toward the centre. 2. Basins are filled through a repetitive alternation of depositional and non-depositional intervals. At any specific time, deposition is localised with only minor amounts of sediment accumulating elsewhere. As a result, deposifional units are separated by non-depositional hiatal surfaces. 218 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE 3. The time interval represented by an hiatal surt^ace is different from place to place, but at least one time line can be traced throughout the entire surface. 4. Depositional events are localised pulses of deposition separated from eariier and later pulses by hiatal surfaces of varying duration. Therefore, the temporal extent of any depositional event cannot be correlated with that of another or with any set unit of time. Figure 6.1A shows a deposifional event of Frazier (1974). Each deposifional event produces a surface-bounded genefic stratigraphic unit called a facies sequence which consists of progradational, aggradational and transgressive facies. All facies within each facies sequence are genetically related to a common sediment source. 5. Several depositional events are contained in a depositional episode, which is not a specified interval of time. Figure 6.1 B shows multiple depositional events combine to produce a major physical, genetic stratigraphic unit - the deposifional episode. Each deposifional episode is recorded and defined by a depositional complex constmcted during an interval of sea level stability. The depositional complex, in turn, consists of several facies sequences. During each interval of sea level stability, a basinward progression of facies sequences occurs. Each depositional episode is terminated by a major marine transgression which inifiates a concurrent transgressive phase of deposition (figure 6.1 B). 6. A hierarchy of progradational - transgressive cycles exists in most basins. Multiple events punctuate regional deposifional episodes. Frazier recognised depositional episodes and resultant deposifional complexes as the principal genefic and rock stratigraphic sutxJivisions of basin history and fill. SEISMIC AND SEQUENCE STRATIGRAPHY Concurrenfiy with the development of Frazier's concept of depositional episodes, seismic strafigraphy (Mitchum and Vail 1977, Mitchum et al. 1977) emerged as a result ofthe development of high-quality reflection seismic data acquired for petroleum explorafion in the 1970s. The seismic stratigraphy approach to basin analysis is based on the delineafion and mapping of regional deposifional and erosional surfaces (Mitchum et al. 1977) as depicted from regional seismic sections. The mapped surfaces bound seismic sequences which have become the fundamental element for basin analysis using seismic data. The concept of seismic stratigraphy later incorporated geologic data to become sequence stratigraphy and a plethora of new terminology emerged (Van Wagoner et al. 1985, 1987, 1988). Conceptually, Frazier's deposifional complex is a sequence-stratigraphic unit bounded by surfaces of erosion or non-deposition and their correlafive unconformifies and thus forms a basis for sequence strafigraphy. 6. GENETIC STRATIGRAPHIC ANALYSIS 219 Breeza Coal Mtx Hoskissons Coal Sediment source AtjarxJonment » (X)tentiai for peat Figure 6.1. (A) coal seams accumulatjon. soil development, are conceptually equivalent or erosion. to the hiatal siufaces of Frazier (1974). Localised or Aggradation, lateral accretwDn, (A) mmor progradation. subregional coals can poten tially cap the small-scale facies sequences and depo sitional events, while re gionally extensive coals can bound the depositional Shoreline of maximum transgression episodes (modified from Hamilton and Tadros 1994). Hiatus (B) Temporal and spatial "a' transgression relationships of a depo sitional episode and the phases of its component depositional events (C). Hiatal surfaces bound the depositional episode, which in turn encompasses one major internal hiatal smface (from Frazier 1974). PHASE Transgressive Fades sequence Shoreline of maximum transgression Aggradational (B) Distance basinward ^ Progradational c TRANSGRESSIVE PHASE > 0) MOLTTH c g SHIFTING 'to HIATUS o ^GRADATIONA^jjj:^"*^ a IpHASE ^,;;;JS=^^PR0GRADAT10NAL PHASE o Q OISSTANCE BASINWARD (C) SHORELINE 220 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE GENETIC STRATIGRAPHIC SEQUENCE Frazier's concept of deposifional episodes provided Galloway (1989) with the conceptual basis for its sedimentary product, the genetic stratigraphic sequence, which incorporates and reconciles depositional systems, bedding geometries, and bounding hiatal surfaces within the framework of recunrent cycles of basin margin offlap and flooding: 1) Depositional systems are three dimensional assemblages of process-related facies and provide the stratigraphic record of major palaeogeomorphic elements of the basin. In other words, they are the stratigraphic equivalents of major physical geographic units such as modem rivers or deltas (figure 6.2). Depositional systems grade laterally into adjacent systems forming logical associations of such palaeogeographic elements, and evolve vertically over significant fime spans. They are separated from underiying and overiying systems by hiatal (disconformable surfaces). Genetic stratigraphic packages typically consist of the sediments of several related depositional systems (Galloway 1989). The concept of ancient depositional systems was originally introduced by Fisher and McGowen (1967) for the Wilcox Group. Fisher and Brown (1984) further defined a depositional system as consisfing of a three-dimensional assemblage of sedimentary facies linked genetically by inferred sedimentary environments and deposifional processes. As such, depositional systems are process- response systems and form the fundamental building blocks of the total sedimentary basin fill (Galloway and Hobday 1993a; figure 6.2). The depositional system is generally characterised by a Active depositional system Depositional environments available sediment Stratigraphic depositional system Genetic facies Figure 6.2. An active depositional system consists of a complex of genetically related environments. The sedimentary record of these environments, preserved as genetic facies, constitutes a three- dimensional stiatigraphic depositional system (modified from Galloway & Hobday 1983b, fig. 1) 6. GENETIC STRATIGRAPHIC ANALYSIS 221 particular bedding style or deposifional architecture which is a direct response to the condifions of sedimentation. Additionally, depositional systems have specific geometries and processes of sediment dispersal which are readily detenmined from subsurface data and provide powerful guides for genefic strafigraphic interpretafion. Recognifion and delineafion of deposifional systems are important in establishing a framework for facies differenfiafion and mapping (Galloway and Hobday 1983a). 2) Bounding hiatal surfaces separate stratigraphic packages and record major interruptions in basin depositional history. They represent significant periods of non-deposition or very slow deposition with or without concomitant subaerial or submarine erosion (Galloway 1989). Unconformities are hiatal surfaces that tmncate underiying strata. In marine basin margins, hiatal surfaces are preserved as submarine unconformifies or condensed sedimentary veneers that record maximum marine flooding of the basin margin. Condensed sections are the product of very slow deposifion. Subaerial erosion surfaces, including incised valley systems fonm an important type of unconformity. In such environments widespread palaeosols and coaly zones indicate slow rates of clastic accumulafion. 3) Bedding architecture is used by Galloway (1989) as a general term to describe the geometric relationships between bedding surfaces or the stratification within depositional systems and at bounding surfaces. There is a contrast in geometries of progradafional, aggradafional and retrogradafional sedimentary units. The hierarchy of erosional features, ranging from simple channelling to large scale valley or canyon incision, has been recognised using outcrop and conventional subsurface data (Miall 1985 and chapter 7) It should be noted that the concepts of seismic/sequence strafigraphy and deposifional episodes/genetic sequence stratigraphy recognise that the repetitive stratigraphic architecture of basin fills is a product of interaction between sediment supply, basin subsidence (and uplift) and eustafic sea level change. Both also emphasise attributes of sedimentary bodies and bounding surfaces in subdivision of basin fill on a genefic basis. They differ, however, in the type of boundary used for defining the genetic stratigraphic package as discussed above. This author favours the genetic sequence paradigm of Galloway (1989) because it emphasises preservation of the strafigraphic integrity of three-dimensional depositional systems and does not rely on widespread development of subaerial erosion surfaces caused by eustafic falls of sea level for definifion of sequence boundaries. Further, mapping the genefic sequence allows recognifion of the impact of contemporaneous basement stmctures on sand distribufion and vice versa, e.g. enables prediction of basement structures which were active during deposition. 222 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE 6.2 APPLICATION OF GENETIC STRATIGRAPHIC SEQUENCES TO NONMARINE BASINS This section deals with the utility of regionally extensive coal seams as one type of sequence boundary which has been applied to the upper Black Jack sequence. The concept was developed and published joinfiy in Tadros and Hamilton (1991) and Hamilton and Tadros (1994) (see supporting papers attached to this thesis). In addition, all examples from the Gunnedah Basin, particulariy those from the upper part of the Black Jack sequence including those published in the two joint papers are the work of this author and have been published in detail in the Gunnedah Basin Memoir (Tadros 1993b). They form an essenfial part of this thesis. This secfion is an updated and expanded version of Hamilton and Tadros (1994). The concepts of genefic sequence stratigraphy as outlined by Frazier (1974) and Galloway (1989) and summarised above, have been usefully applied to marginal marine basin setfings where sand-rich progradafional clasfic wedges are separated by thinner, onlapping marine units which can form easily recognised and regionally correlatable sequence boundaries. In non-marine aggradational basins, defining sequence boundaries and establishing a genefic stratigraphic framework for the basin can be difficult because the absence of marine intercalations necessitates different criteria for sequence recognifion (Tadros and Hamilton 1991). It has already been mentioned that Galloway (1989) outlined the importance of deposifional systems and bounding hiatal surfaces as the key elements of marginal marine basin fills. The same key elements divide non-marine, aggradafional basins. It is, therefore, possible to recognise deposifional systems that were deposited during periods of regional palaeogeographic stability and to define the bounding hiatal surfaces separating these genetic stratigraphic packages. The hiatal surfaces record major intermptions in basin depositional history and represent significant periods of non-deposition or very low clasfic accumulafion, with or without concomitant subaerial or submarine erosion (Galloway 1989). Subaerial erosional unconformities in terrestrial basin fills, provide one obvious type of bounding surface between stratigraphic packages, but many other conformable bounding surfaces are present and can be recognised by dramafic changes in depositional style, and in sediment composifion, texture, and dispersal patterns (Reynolds et al. 1989). These conformable bounding surfaces allow subdivision of terrestrial basin-fills into genefic units of common tectonic, climafic and palaeogeographic origin. Tadros and Hamilton (1991) and Hamilton and Tadros (1994) argued that coal seams of regional extent are examples of conformable sequence boundaries because they are the product of prolific peat growth and preservation during periods of widespread, negligible clastic accumulation, which implies major reorganisafion in basin tectonics or substantial climafic change. Conceptually, they are equivalent to the hiatal surface of Frazier (1974) that records the termination of a depositional event or episode (depending on scale; see above) or the condensed sections of Galloway (1989) in marine basin fills (figure 6.1 C). 6. GENETIC STRATIGRAPHIC ANALYSIS 223 Coal seams that are usable as genefic sequence boundaries should be easily recognised and reliably correlated on a regional scale by their characterisfic seam profile, a unique feature of coals that also provides evidence of their time equivalence. Their regional extent suggests an extra basinal control on sediment supply shut-off, such as tectonic reorganisation or climafic change. Coals that can be correlated locally or subregionally by their lithotype profile have fime significance locally or subregionally, and the controls responsible for shutting off sediment supply are autogenic. These coals are not primary sequence boundaries, but because they have time significance, they can be used to subdivide a sequence to map facies distributions locally or subregionally. 6.2.1 SEDIMENTARY PROCESSES AND PEAT ACCUMULATION Models of coal formafion developed as eariy as the 1950s emphasised peat accumulation in low lying swamp or marsh areas adjacent to active sedimentation in shoreline (Young 1955), fluvial-deltaic (Coleman and Smith 1964; Form and Home 1979), and alluvial fan (Howard 1978) depositional systems. Other researchers recognised coals associated with sandy braided rivers (Haszeldine and Anderton 1980), lakes (Ayers and Kaiser, 1984), and eolian dunes (Richardson 1985), such that coals have now been observed in most types of terrestrial settings. McCabe (1984) challenged the convenfional models and argued that, if preserved in the geologic record, peats accumulafing adjacent to acfive sedimentafion in modern swamps would be transformed to carbonaceous shales or, at best, high ash coals. One altemafive model he proposed is that peat formed in raised or floating swamps which, by their physical nature, were protected from clastic deposifion. There are, however, only a few isolated examples of coals derived from floafing peat mats reported in the literature (Spackman et al. 1976; Conaghan, 1982). Also, raised swamps, although more common (Anderson 1964), cannot be considered plausible universal coal deposifional models since these swamps require very specific climafic conditions (Teichmijiler and Teichmuller 1982) and can only support restricted floras (Kosters et al. 1987). Recognising that not all coals can reasonably be interpreted as raised mires, McCabe (1984) offered a further explanafion to account for the apparent anomaly of coal and active sedimentation, suggesting that peat deposifion is not contemporaneous with local clasfic deposifion, in other words, the regional clasfic supply is shut off during swamp development. Although this explanation does not account for the undeniable invasion of elastics into the peat-forming environment (evident from the many published descriptions of seam splits and channel washouts; e.g. Ferm et al. 1979, Flood and Brady 1985), it does allude to the fundamental processes implicit in peat accumulafion and preservation as coal in active deposifional environments. Peats (with the possible exception of an insignificant percentage of allochthonous peats) are a manifestafion of the basic sedimentologic principles of basin filling by siliciclasfic sediments, as outlined by Frazier (1974). That is (1) basin filling is achieved by a repetitive alternation of depositional and non-depositional intervals, (2) clasfic deposifion does not occur everywhere at any time, and (3) clasfic deposition is not continuous anywhere. Peat will accumulate in areas of non-deposition as long 224 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE as there is substantial growth of vegetation and maintenance of the water table at, or above, the peat surface. Opportunity for thick peat development depends on the length of time before clastic sedimentation resumes, and the area of sediment bypass will control the extent of the peat swamp environment. These processes can occur at any scale, from peat infilling of an abandoned fluvial channel to basin-wide blanket peat (figure 6.3). Brief sedimentary incursions into the peat swamp are thus not precluded in this explanation and are incorporated in the coal as clastic splits or represented as channel washouts. Please see print copy for image Figine 6.3. Schematic illustrations of examples, at several scales, of processes capable of shutting off sediment supply and providing opportunity for peat accumulation (from Hamilton and Tadros 1994, fig. 2.). (A) Dismption of sediment supply at a basin-wide scale. Tectonic movement has tilted the thmst beU causing stream capture and shedding of sediment to the north-east into an interthmst beh basin. (B) Sediment bypass at a subregional scale where the axial channel complex occupied the eastem portion ofthe basin and peat accumulated un-intermpted in the west. (C) Localised peat accumulation in a cutt-ofi" meander loop of a moderately sinuous mixed-load fluvial system 6. GENETIC STRATIGRAPHIC ANALYSIS 225 Thus, the processes responsible for shutiing off sediment supply are critical to the distribution and thickness of coals. Autogenic processes, such as crevasse splay abandonment or meander loop cut off, can result in coals of local extent (one to tens of square kilometres; Belt 1993 used the terms autogenic and allogenic rather than autocyclic and allocyclic in order to avoid the notion that cyclicity is present de facto in any of the facies that he described from the Late Cretaceous and Paleocene formafions of Appalachian and Williston Basins, USA, the tectonic and deposifional settings of which, show some similarifies to those of the Gunnedah Basin). Elliott (1974) described coals capping crevasse splay or minor mouth bar-crevasse channel successions that extended for several square kilometres. Coals of subregional extent (tens to hundreds of square kilometres) are the product of larger scale "autogenicity" such as delta switching or river avulsion. Coleman and Smith (1964), for example, observed blanket peats that extend for over 500 km^ in Holocene deposits of the Mississippi delta plain. Mud flats upon which the blanket peats grew were constructed by westeriy longshore transport of fine-grained sediment from the Maringouin delta complex. When the delta system switched to the Teche-Mississippi location, sediment supply to the mud flats was terminated and peat accumulation commenced and continued for as long as peat growth kept pace with subsidence. Coal seams of regional extent (from hundreds to thousands of square kilometres or greater) require processes capable of interrupfing sediment supply at a basin-wide scale. Such disruption in sediment supply can only be achieved through major reorganisafion in basin tectonics or climate. There are several tectonic mechanisms in foreland basin setfings which can control the lateral extent of peat swamps on a basinwide scale. For example, clastic sediments can be excluded by stream capture and shedding of sediments into an intrathmst belt basin within the source area (figure 6.3A; Hamilton and Tadros 1994), or by drainage diversion, within the basin, to a distant embayment leaving much of the depositional basin area available for peat swamp development. Wise et al. (1991), Belt et al. (1992) and Belt (1993) proposed a drainage diversion tectonic model for the Appalachian basin where blind thmst ridges and lithospheric loading combined in a foreland compressive setting (with some degree of similarity to that of the Gunnedah Basin during the Late Permian foreland basin phase). In such a setting, various types of fold-induced topography could produce a series of ridges between the main source of clastic sediments and the adjacent foreland basin. This type of topography would then divert the streams and their clastic loads away from large areas of the depositional basin where peat swamps develop (figure 6.4a; Belt 1993). Clay and fine-grained sediments can be brought into the basin by erosion of an adjacent fold ridge to form stone layers in the coal seams. This mechanism is capable of producing coal seams of regional extent. In the Appalachian basin model, the Upper Freeport coal zone extends unintermpted without coeval channel-belt breaks for more than 150 km parallel to the Allegheny Front (erogenic thrust belt front). When the fold ridges are breached by erosion, coarse clasfics enter the portion of the deposifional basin that was previously protected and form a coarse clastic depositional interval (figure 6.4b; Belt 1993). 226 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE Please see print copy for image 6. GENETIC STRATIGRAPHIC ANALYSIS 227 6.2.2 COAL SEAMS AS GENETIC SEQUENCE BOUNDARIES Regionally extensive coals bear all the characteristics essential to a genetic sequence boundary. They are lithologically distinctive and therefore readily recognised. Moreover, the seam lithotype profile is a feature unique to coals that enhances reliability of regional correlation and the establishment of the seam's regional coverage. Coal seams have time significance and are potenfially dateable. They are biochemical sediments that record gaps in the accumulation of clastic sediments and, therefore, significant interruptions in the sediment record that signify major reorganisations in basin tectonics or climate. COAL SEAM CORRELATION Reliable and confident coal seam correlafion is essenfial in establishing the seam's regional coverage and accordingly its utility as a genetic sequence boundary. Coal seam correlation can be achieved using seam lithotype and ash profile analysis, which are applicable to most data sets in coal-bearing sedimentary basins. Lithotype profile analysis can be successfully applied to outcrop exposures, fresh mine walls, subsurface cores, and geophysical well logs. Coal is particulariy susceptible to processes of physical erosion, and seam exposures in creek sections or retreating coastal headlands as well as many highway and railway cuts are generally of excellent quality. Mine walls, especially open cut longwalls, are obvious candidates for analysis. In the subsurface, coalfield explorafion routinely requires continuous coring through seams for bulk samples to assess coal properties. Geophysical logs accompany the cored secfion to detect core loss. Gamma ray, neutron, and density logs always complement the lithotype profiles and are cleariy sensitive to minor fluctuations in mineral matter within the coal. Resisfivity logs also respond to variations in coal seam character. Coals that are often overiooked in prolific petroleum-producing basins have potential for analysis because of their geophysical well log response. Coal seam lithotype profiles Coal seam lithotype profiles are a funcfion of the coal's constituents. The most elementary constituent of coal is the maceral, and there are three maceral groups in the petrographic classificafion of coals: vitrinite, liptinite, and inertinite (Stach et al. 1982). Macerals can be grouped together to fomn microlithotypes, which are microscopic layers thicker than 0.05 mm and distinguished by their maceral components and proportions. Microlithotypes can in turn be grouped together to form macroscopically identifiable layers called lithotypes. The vertical arrangement of successive lithotypes, then, is the profile or signature of a coal seam (figures 10.3, 2.16, 10.14 and 10.20). Correlafion of coal seams using lithotype profiles (often referred to as brightness profiles) is a well-established technique in the coal industry. Because it is composed of microlithotypes (and consequently of macerals), the lithotype profile is a function of many variables, mainly the original peat-forming plant material and the physical and chemical conditions of the peat swamp as well as the extent of clastic or volcanic sediment influx, climate and diagenesis during burial and preservation. These variables are to some extent, interrelated. Widespread fluctuafion in any variable can profoundly affect the swamp and be preserved in the 228 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE resultant lithotype profile, which is indeed a detailed record of the history and development of the peat swamp, (see chapter 9 for further discussion). Ash profiles A secondary feature of coal seams that facilitates their cortelation is the ash profile Coal is not composed entirely of organic material. There is a significant inorganic fraction of primary or secondary minerals. The non-combustible residue of these minerals form ash on combustion ofthe coal; the minerals themselves may occur either as discrete or disseminated matter. Discrete mineral matter occurs as macroscopically visible layers or lenses and is introduced into the peat primarily by sedimentary processes, such as overtDank flooding, crevasse splays, or airfall tuffs. However, thin partings may also develop from the extreme degradafion of the peat and subsequent release and concentrafion of plant-derived mineral matter (Ronton and Cecil 1979; Ronton et al. 1979). The disseminated component consists predominanfiy of inherent or primary mineral matter that was formed eariy during diagenesis from inorganic matter in the coal-fonming plants, by ionic exchange from pore waters, or by chemical precipitation (see Cecil et al., 1980). Very fine grained detrital fragments (adventitious mineral matter) can also contribute to the disseminated mineral matter in coal. MaCTOscopically visible layers of discrete mineral matter that originate from extreme degradation of the peat, or from airfall tuffs, can display considerable lateral confinuity and become an integral part ofthe lithotype profile (see chapters 8 and 9 for detailed studies). The disseminated mineral matter, although microscopic, is liberated and recorded during tests routinely carried out to determine coal properties such as coking potential and calorific value. Thus, if the coal seam has been analysed, it is possible to construct an ash profile that includes the disseminated mineral fraction, which is an important aspect of coal seam character and is useful in correlation. TIME SIGNIFICANCE OF COAL SEAMS Conceptually, the coal seam is equivalent to the hiatal surface of Frazier (1974), which records the termination of a deposifional event or episode (depending on scale). Coals of local or subregional extent are examples of smaller scale hiatal surfaces and are likely equivalents to the surface that caps the facies sequence of Frazier (1974) and separates individual depositional events (figure 6.1 A). In contrast, the regionally extensive coals are larger scale and most likely equivalent to the surface that bounds the sedimentary complex of a depositional episode (figure 6.1 B). In his model, Frazier (1974) emphasised that all points on a hiatal surface do not represent the same durafion of time but that one instant in fime is common to all points. This concept is embodied in coal seam character (figure 6.1 C). Although it can be demonstrated that most coals are to some extent diachronous (i.e peat accumulafion commenced eariier, or confinued longer, in some areas of the peat swamp), there are numerous essenfially isochronous events occuning throughout the development of a peat swamp that are recorded by the coal's lithotype profile. Airfall tuffs ejected from contemporaneous volcanism, for example, have better chance for preservation in peat swamps than in dynamic clastic deposifional environments (figures 10.3 and 10.24) . The tuffs are preserved as thin, extensive tonstein layers 6. GENETIC STRATIGRAPHIC ANALYSIS 229 (Spears and Kanaris-Sotiriou 1979, Ryer et al. 1980) . Widespread layers of fusinite maceral have been attributed to fires that burned through the swamp, and a general abundance of fusinite can indicate dry periods during peat swamp history (Cohen et al. 1987). Excessively wet years can fiood the peat swamp and raise the pH of the swamp waters, enhancing in situ bacterial degradafion of the peat. Extremely degraded peat releases inherent mineral matter to the peat swamp that is preserved in the lithotype profile as extensive, thin partings (Ronton and Cecil 1979). It should be emphasised that correlafion of lithotype profiles must be achieved before fime equivalence of a coal seam can be demonstrated. Diachronous (fime-transgressive) coal seams do not share a common temporal lithotype profile signature throughout their spafial distribufion and, therefore, cannot be used as genefic sequence boundaries. 6.2.3 APPLICATION TO THE GUNNEDAH BASIN SEQUENCE This new concept of ufility of coal seams as genefic strafigraphic sequence boundaries is particulariy applicable to regionally extensive coal seams in the upper Black Jack sequence in the Gunnedah Basin. These seams have been used to subdivide the sequence into deposifional units of disfinctly different sediment composition, texture and bedding architecture (Tadros 1986b, Tadros and Hamilton 1991). In addition to the regionally extensive coal seams, the upper Black Jack sequence also contains coal seams of subregional extent, and although not representing sequence boundaries, these seams have time significance locally. They have proved useful in finer subdivision within the larger genetic stratigraphic packages, and detailed lithofacies mapping of the sedimentary section between the seams has made it possible to reconstruct the evolution of the upper part of the Black Jack fluvial systems (Tadros and Hamilton 1991, Tadros 19931) (discussed in detail later). 6.3 PROCEDURE USED IN GENETIC STRATIGRAPHIC ANALYSIS OF THE GUNNEDAH BASIN Galloway and Hobday (1983a) proposed an integrated hierarchical approach to genetic stratigraphic analysis, which first defines regional attributes of genefic sequences before attempting site specific interpretations. They consider this approach affords the greatest potenfial for successful interpretation of depositional sequences, and application of that interpretation to resource discovery and development. Most aspects of their method have been applied to the study of the Gunnedah Basin sequence in general (presented in the Gunnedah Basin Memoir, Tadros 1993b) and the upper Black Jack sequence in particular, for this study. In their preferred method of genetic stratigraphic analysis, Galloway and Hobday (1993a) emphasised the importance of carrying out several levels of increasing detail of description and interpretation. The more completely each level can be concluded, the more likely unique, predictive interpretations can be made in the next lower level. This analysis should describe and interpret several aspects of basin geology, including: 230 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE first, the framework ofthe depositional basin, including its size, shape, and major sources of sediment; second, the principal genetic stratigraphic units or depositional episodes, and an analysis of their deposifional architecture. Regional well logs or seismic secfions are useful tools at this stage. Defining the three-dimensional geometry of framework facies outlining major depositional systems and their component facies is the third level of analysis. This is achieved by the use of isolith and sandstone percent, and various quantitative facies maps and cross-sections. The fourth task is to recognise the recurrent vertical sequences, both within the larger genetic units as well as within specific facies. Geophysical well logs are an especially effective tool for revealing vertical changes in lithology or bed thickness. The fifth level of analysis incorporates interpretations of lateral and vertical facies associations and their cross-secfional geometry. Detailed correlation and mapping of selected representative bore hole logs and measured sections constitute important tasks at this level. The final step requires outcrop or core material to carry out interpretation of internal bedding and sedimentary structures, texture and bedding features of main facies. Availability of a large number of fully cored bore holes with regional coverage, many of which are complemented with geophysical logs, provided data on the vertical and lateral distribution of facies sequences. This data base has been utilised for the first five levels and the majority of the interpretation for the final level of analysis. A solitary outcrop of the bed-load facies of the Westem Fluvial System at Mount Watermark, west of Breeza, has been mapped and studied in detail to complete the analysis. Lack of suitable outcrop for rocks of the Eastern Fluvial System restricted the study of that system to drill core and geophysical well logs. Initially, the upper Black Jack sequence was divided into four major genetic elements, the Hoskissons peat swamp, the Lacustrine, the Western fiuvial and the Eastern Fluvial Systems, based on contrasts in lithology, deposifional and tectonic setfing, on palaeogeographic relafionships and on recognifion of major sequence boundaries. As menfioned eariier, coal seams of regional extent have time significance and serve as genetic sequence boundaries. These seams separate genetic sequences of distincfiy different depositional setting, bedding architecture, and sediment composition, and demonstrate the utility of coals in defining the basin's genefic strafigraphic framework. Deposifional systems served as mapping units, which defined bounding surfaces for the contained genetic facies and made it possible to construct the Late Permian deposifional and tectonic history of the basin. Recognifion and mapping of deposifional systems then allowed establishment and or refinement of coal seam correlafions, particulariy those of sub-regional extent. Coal seams of sub-regional extent also have time significance and have been used to subdivide the sequence and to map facies distributions locally or subregionally. Lithofacies mapping ofthe genetic units provided an insight into the evolution of the upper Black Jack fluvial systems. The mapping enhanced depocentres, revealed areal and strafigraphic distribution of sand bodies within the genetic units and ultimately allowed recognition of the impact of contemporaneous basement structures on sand distribution (see chapter 5). 6. GENETIC STRATIGRAPHIC ANALYSIS 231 6.3.1 QUANTITATIVE FACIES MAPPING Most clastic depositional systems are characterised by specific geometries and processes of sediment dispersal, and almost all bed-load transport processes leave a deposifional record of the path of the sediment dispersal system (Galloway and Hobday 1983a). For example, bed-load sand and channel gravel dominate channel deposits and thus form the skeletal framework of the fluvial system (Galloway and Hobday 1983a). Recognifion and mapping ofthe geometry, composifion and internal stmctures of channels provide valuable infonmafion about the deposifional architecture of the system, and ulfimately an understanding of the sedimentary environments and associated processes. Selecfion of appropriate quantitafive geological maps is fundamental to the understanding and recognifion ofthe principal depositional systems and associated facies. Galloway and Hobday (1983a) emphasised the importance of genetic interval isopach maps, such as net and percentage sandstone maps and sand/shale ratio maps, in genetic stratigraphic interpretations. These map types outline the main depocentres for both total sediment and for the bed-load (sand) fraction, and display the distribufion, trends and areal patterns ofthe framework sand facies (Galloway and Hobday 1983a). Galloway and Hobday (1983a) also emphasised that the utility of the maps is increased if basic sedimentological concepts and genetic models are incorporated in the contouring style. Contouring should emphasise the continuity, rather than the isolation, of sand bodies because bed-load sediment by-pass and thick isolated pods or lobes of sand are rare in clastic deposifional systems. Sands are likely to be preserved as a series of interconnected bodies whose trends reflect the direction of sediment transport. Exceptions have strong implications for the interpretation of deposifional processes and depositional systems. Subsequent basin history should be thoroughly investigated in all cases and fully accommodated in any interpretation as post-depositional erosion and tectonic deformation will modify the geometry of a facies (or a group of facies/depositional system) and inhibit the use of its overall shape as a diagnostic depositional feature. Igneous intmsions have no depositional significance and their thickness must be subtracted from the genefic units before contouring. Map resolufion is maximised when genefic packages are finely divided. However, it is not always possible to finely subdivide the sequence and some genefic packages may include more than one depositional system. In such thick sequences, the detailed geometry of a specific sand body may be obscured, and map resolufion reduced, because of partial superimposition of sand units deposited in different environments and stratigraphic levels. However, vertical persistence of depositional environments in a rapidly subsiding basin results in stacking of similar facies and preservation of framework trends. Such facies mapping can, in fact, enhance depocentres, reveal areal and stratigraphic distribution of sand bodies within the genetic units and allow recognition of the impact of contemporaneous structures on sand distribution (Galloway and Hobday 1983a). Map resolution is also affected by data distribution. Since the principal target of the coal drilling programmes in the Gunnedah Basin was the Late Permian coal measures, density of borehole 232 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE Intersections is greatest for the upper Black Jack sequence and units stratigraphically higher. They are adequately covered by a networi< of fully cored boreholes. However, because density of borehole intersections is generally influenced by economic considerations, such as depth to the target seams, therefore, data distribution for the upper Black Jack sequence, and units stratigraphically higher, is greatest where the target seams are shallow and towards the subcrop zone. Basically, this author achieved definition of the bed-load framework for the upper Black Jack sequence by the constmction of genetic interval isopach maps, net and percentage sandstone maps, adopting the Galloway and Hobday (1983a) approach to contouring and utilising geophysical logs in genetic facies analysis. Genefic interval isopach maps were constructed by contouring the thickness intersected in each borehole, excluding igneous intmsions which are common within the upper Black Jack sequence and have no genefic deposifional significance (see appendix 2). Facies boundaries were defined from detailed analysis ofthe drill core and geophysical logs (appendices 3 and 4). Net sandstone maps are based on the lithological descriptions of each borehole. Net sandstone values were obtained by adding the total thickness of sandstone beds/layers within each genetic unit. The sandstone content of interbedded sequences is usually expressed as a percentage (e.g. sandstone/siltstone 60:40), which can easily be converted to absolute thickness by multiplying the unit thickness by the percentage of the sandstone (appendix 3). Percentage sandstone was obtained by dividing the net sandstone value by the thickness ofthe genetic unit for each borehole. APPLICATION OF GEOPHYSICAL LOG FACIES Geophysical gamma and electric well logs are useful in the detenmination of basic lithology and vertical sequence, in recognition and mapping of log facies and in interpretation of depositional environments. Upward-coarsening and upward-fining textural patterns of aggradational, progradational and lateral accretion bedding geometries are readily recognised on electric and gamma logs and are extensively used in petroleum exploration drilling, e.g. the Frio Formafion ofthe Texas Gulf Coast Basin (Galloway et al. 1982, Galloway and Hobday 1983a). Gamma and neutron (and density) logs are extensively used in coal measures exploration because of their sensitivity to minor fiuctuations in mineral matter within the coal. They are equally useful in delineafing major genetic stratigraphic units (or deposifional systems) based on contrasts of regional extent in the geophysical log facies. Hamilton (1987), applied geophysical well logs to genetic facies analysis in the Gunnedah Basin. His approach compares with the study on the Frio Formation of the Texas Gulf Coast Basin (Galloway et al. 1982, Galloway and Hobday 1983a) except that the Frio Formafion study utilised electric log facies. Hamilton identified 16 log facies types; 9 aggradational, 5 progradafional and 2 mixed log facies to encompass all variations in genetic facies of the Gunnedah Basin sequence (figure 6.5). He defined the principal deposifional systems of the basin by contrasts in geophysical log facies and associated log patterns and presented the results on four regional cross-sections. 6. GENETIC STRAHGRAPHIC ANALYSIS 233 AGGRADATIONAL LOG FACIES PROGRADATIONAL LOG FACIES G Nl MIXED LOG FACIES G = Gamma log N = Neutron log G N Figure 6.5. Geophysical log facies in the Gurmedah Basin. Letters a-p distinguish differences in log pattems for aggradational, progradational and mixed log facies (modified from Hamihon 1987, fig. 3.9; and 1991, fig. 10) In this study, the author adopted the above approach but expanded the application to accommodate new data from 40 boreholes drilled in the Narrabri drilling programme (Tadros, in Tadros et al. 1987) and to incorporate improved and detailed interpretations of the upper Black Jack sequence (as well as modified boundaries in the Triassic and Jurassic successions). The results are presented on modified regional cross-sections (figures 6.6 - 6.9; enclosures) designed to incorporate a large number of the available geophysical well logs and to provide the widest basin coverage. The cross-secfions show the new interpretafions of the upper Black Jack sequence together with changes in the Permian/Triassic unconformity boundary (amongst other changes). The new regional cross-sections show gamma and neutron logs on either side ofthe lithological log for each borehole. Both gamma and neutron logs are arranged so that they show contrasfing responses in opposite direcfions to emphasise lithologies e.g. high neutron and low gamma response for sandstone). The location ofthe cross-sections are shown on figure 6.10. The author and D. Hamilton have worked closely together on definifion and delineation of the Triassic/Jurassic boundary and some of the results are incorporated in Hamilton (1991). The regional cross-secfions provided a broad framework for the establishment of the basin's new strafigraphy given in chapter 2. Detailed analysis of drill core and geophysical borehole data (appendices 3 and 4) allowed delineafion of four deposifional systems within the Upper Black Jack deposifional episodes (figures 6.11 and 6.6 - 6.9) : the Hoskissons peat swamp system at the base, overiain by the Lacustrine System in the east and the Western Fluvial System in the west; the Eastern Fluvial System occurs at the top. 234 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN Ref No. Borehole name .1 2 DM Narrabri DDH 2 10 DM Blake DDH 1 11 DM Turrawan DDH 2A 26 DM Gorman DDH 1 28 DM Turrawan DDH 1 37 DM Tullamullen DDH 1 NEW ENGLAND 60 DM Walla Walla DDH 1 78 DM Denison West DDH 1 79 DM Denison DDH 1 FOLD BELT 85 DM Benelabri DDH 2 88 DM Hall DDH 1 89 DM Brigalow DDH 2 90 DM Brigalow DDH 1 91 DM Coogal DDH 1 92 DM Benelabri DDH 1 93 DM Gunnedah DDH 1 94 DM Borah DDH 1 100 DM Girrawillie - Bulga DDH 1 108 DM Brown DDHl, 1A 111 DM Nea DDH 2 RIDGE 116 DM Clift DDH 3 117 DM Clift DDH 5 119 DM Clift DDH 2 120 DM Howes Hill DDH 1 Coonabarabran ^/ 122 DM Clift DDH 4 126 DM Wallala DDH 1 128 DM Terrawinda DDH 1 .J29_130 129 DM Tinkrameanah DDH 1 130 DM Wilson DDH 1 131 DM Bomera DDH 1 138 DM Wallala DDH 3 143 DM Cookabingie DDH 1 165 DM Morven DDH 1 cv Figure 6.10. Borehole and cross section locations for figures (enclosures) showing ganuna and neufron log response ofthe rocks and the depositional systems in the Gunnedah Basin (modified from Hamilton 1991, fig. 2) 6.4 DEPOSITIONAL EPISODES OF THE UPPER BLACK JACK STRATIGRAPHIC SEQUENCE The top of the Black Jack stratigraphic section is mari Deposifion of the upper part of the Black Jack stratigraphic sequence represents the last phase in the Penmian depositional history of the Gunnedah Basin and signifies a major change in tectonics and sedimentation in the basin. The preceding Upper Watermari< - lower Black Jack Depositional Episode (see chapter 2 and Hamilton 1993a, b) was terminated by rapid lowering of sea level and cessation of deposition of the Arkarula Shallow-marine System. 6. GENETIC STRATIGRAPHIC ANALYSIS 235 Figure 6.11. Depositional systems, upper Black Jack Group Rapid lowering of sea level after deposition of the Arkamla Shallow-marine System provided appropriate conditions for widespread accumulation of the peats of the Hoskissons Peat-Swamp System and the start of a new phase in the history of Black Jack deposition characterised by terrestrial sedimentation and the total absence of marine infiuence (Tadros 1986b). Further, basin-wide accumulation of the Hoskissons peat, in itself, represents a significant period characterised by almost total non-deposifion of terrigenous elastics except along the basin's westem margin. The Hoskissons Coal interilngers with quartzose channel fills of the Westem Fluvial System only in the westernmost part of the Mullaley Sub-basin and is intergradafional with the overiying lacustrine sediments in the east. Tectonic loading of the cmst beneath the basin caused regional subsidence, an end to accumulation of the Hoskissons peat and the establishment of the lacustrine conditions in the east concomitant with structural readjustment and uplift on the eastem edge of the craton, along the western 236 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE basin margin. Stripping of the cratonic Lachlan Fold Belt region supplied dominantiy bed-load quartzose sediments of the Western Fluvial System, which were carried by south-easteriy contributory channel systems via an axial drainage complex into the lake (Tadros 1986a, b). The lake was subsequently infilled by sediments ofthe Western Fluvial System The 'western bed-load fluvial system" ol Hamilton (1987), which underiies the Hoskissons Coal in the western and north-westem parts ofthe Mullaley Sub-basin, is contemporaneous with the terminal phase of the Arkamla Shallow-marine System and represents eariy pulses of the Lachlan Fold Belt-derived sediments of the Western Fluvial System of the Upper Black Jack deposifional sequence. The lower ^western bed-load fluvial system" (of Hamilton 1987) and the upper Westem Fluvial System are separated by the Hoskissons Coal except in the westernmost margin of the sutnbasin where, locally, they may appear confinuous and have either interfingured with, or completely replaced, the coal. The "western bed-load fluvial system" of Hamilton (1987) was confined to the westem and north-western parts ofthe Mullaley Sub-basin (figure 6.13), whereas in the upper part of the Black Jack stratigraphic sequence, the Westem Fluvial System ultimately extended basin wide, with its axial channel complex reaching the eastern margin ofthe sub-basin (see detailed discussion below). Deposition of the Western Fluvial System was followed by an almost basin-wide peat accumulafion of the Breeza Coal Member in the southern half ofthe Mullaley Sub-basin, north ofthe Liverpool Range, and its equivalents in the north, prior to renewed tectonic acfivity in the New England Fold Belt region. As a result, the westeriy-sourced sediment supply diminished gradually and was replaced by volcanic- lithic detritus of the Eastern Fluvial System, from the east (Tadros 1986b, 19931). Sediments of the Eastern Fluvial System were carried westeriy and south-westeriy into the axial drainage complex resulting inifially in intermixing with detritus derived from the west. The easteriy sourced sediments ultimately dominated the sub-basin. South-westward migration of the axial drainage complex occurred during localised infiux from the New England tributaries such as in the upper part of the sequence in the south-central and south-eastern areas. Widespread silicic volcanism in the New England Fold Belt region contributed large amounts of pyroclastic detritus to the basin-fill. A major phase of lateral compression and thrusting ofthe New England Fold Belt onto the craton caused structural readjustment and uplift, particulariy in the north, and ended Permian deposifion in the basin. The dominant criterion for differentiating the Western and Eastern Fluvial Systems is a shift in source areas from the Lachlan Fold Belt region to the New England Fold Belt region. This shift in source areas is evident from the contrast in lithology between the quartz-rich sediments of the Western Fluvial System and the volcanic - lithic tuff and tuffaceous-rich sediments of the Eastern Fluvial System. The change in lithology is also associated with a change in sediment transport direction from dominanfiy south-east to south and south-west as shown on the lithofacies maps. Also coincident with this change in source areas and at the intert^ace between the two contrasting lithologies is the presence of the widespread Breeza Coal Member in the south and its equivalents in the north. 6. GENETIC STRATIGRAPHIC ANALYSIS 237 150 LACHLAN NEW ENGLAND FOLD BELT FOLD BELT -31^ 31 - 0 25 km L_ Borehole 150° Figure 6.13. Net sandstone (m), "westem bed-load fluvial system" (modified from Hamilton 1985, fig. 8; 1987, fig. 3.23 and 1993, fig. 13.9) 238 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE 6.5 GEOMETRY, DEPOSITIONAL STYLE AND EVOLUTION OF UPPER BLACK JACK SYSTEMS Isopach, net sandstone and percentage sandstone maps (figures 6.14 - 6.16) were constructed for the enfire upper Black Jack sequence, including the Hoskissons Coal. The deposifional grain of the framework sand facies dominates the contour patterns despite irregular stacking of sand bodies within the thick genetic package which causes decreased map resolution and loss of details of framework geometry. The net sandstone and percentage sandstone maps reveal elongate strike-fed sand bodies trending south-easteriy along the basin axis (figures 6.15 and 6.16), joined to the east and west with dip-fed elongate, narrow sand bodies. The maps also emphasise a dominant fiuvial character for the sediments of the upper Black Jack depositional episodes, with the main south-easteriy sand bodies representing a major axial trunk channel complex fed (either contemporaneously or at different times) by easteriy and westeriy contributory channels. Regional genetic interval lithofacies maps (including net sandstone, percentage sandstone and isopach maps) were then constructed for the four major depositional systems of the Upper Black Jack depositional episodes. In addifion, detailed mapping ofthe geometry and distribution of lithofacies between specific coal seam intervals was also carried out (figure 6.12). Six coal seams in the upper Black Jack Group in the southern part of the sub-basin, together with the Hoskissons Coal, provided bounding surfaces for smaller genetic stratigraphic intervals. Maps were constructed for the key interseam intervals and integrated with data on sediment composifion and sedimentological analysis to investigate the relative contribution of the Lachlan and New England Fold Belts to the evolution of the upper Black Jack fluvial systems. The depositional evolution of the upper Black Jack systems is summarised as a series of schemafic 3-D palaeogeographic reconstructions (figure 6.17 C - F) together with the precursor environments which formed the later phases in the evolution of the Upper Watermark - Lower Black Jack Deposifional Episode of Hamilton (1987, 1990; figure 6.17 A , B). 6.5.1 HOSKISSONS PEAT-SWAMP SYSTEM The Hoskissons Peat-swamp System was the precursor environment to coal formation, which gave rise to the economically important Hoskissons Coal. This coal formafion contains neariy half (13.7 billion tonnes) of the total coal resources ofthe Gunnedah Basin. The Hoskissons Coal is present over much ofthe Mullaley Sub-basin. The eastern subcrop extends in a north-north-westeriy direction from east of Breeza to Narrabri. The coal ranges in thickness from less than 1 m in the west to 12 m in the north and to more than 17 m in the south-east. It is poorly developed in the central-south of the sub-basin and is split in some western areas (figure 6.18). 6. GENETIC STRATIGRAPHIC ANALYSIS 239 TN NEW ENGLAND FOLD BELT LACHLAN FOLD BELT / (ROCKY GLEN RIDGE) Coonabarabran REFERENCE . Boretiole 0 10 20, 60 70 80 90 100 120 140 160 400 Figure 6.14. Isopachs (m), upper Black Jack depositional systems (modified from Tadros 19931, fig. 14.2) 240 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN - NEW ENGLAND FOLD BELT LACHLAN FOLD BELT / (ROCKY GLEN RIDGE) Coonabarabran REFERENCE . Borehole 0 10 u Figure 6.15. Net sandstone (m), upper Black Jack depositional systems (modified from Tadros 1993i, fig. 14.3) 6. GENETIC STRATIGRAPHIC ANALYSIS 241 150° TN X ^% f/ To c m 03 NEW ENGLAND FOLD BELT "(ROCKY GLEN RIDGE) Coonabarabran . Borehole 0 10 20 Figure 6.16. Percentage sandstone, upper Black Jack depositional systems (modified from Tadros 19931, fig. 14.4) 242 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE Figure 6.17. Schematic three-dimensional palaeogeographic reconstmctions of major phases in the evolution of the Upper Black Jack Depositional Episodes (C-F) and the precursor environments which formed the late phases ofthe Upper Watermark - Lower Black Jack Depositional Episode (A & B) (modified from Tadros 19931, fig. 14.6) A. Arkamla Shallow-marine System B. "Western bied-load fluvial system" C. Hoskissons Peat-swamp System 6. GENETIC STRATIGRAPHIC ANALYSIS 243 REFERENCE CHANNEL BACK BARRIER/LAGOON n DELTA FRONT n BARRIER BEACH EARLY PERMIAN ALLUVIAL DEPOSrrS BASAL VOLCANICS ACTIVE VOLCANO MIXED-LOAD FLUVIAL FACIES ALLUVIAL FAN EASTERN (BED-LOAD) FLUVIAL FACIES I LAKE MARGIN PEAT SWAMP WESTERN (BED-LOADt FLUVIAL FACIES D. Lacustrine and Westem Fluvial Systems E. Eastem Fluvial System, channel fill/bed-Ioad facies F. Eastem Fluvial System, flood plain facies 244 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN X cz H m NEW ENGLAND FOLD BELT LACHLAN FOLD BELT f^ (ROCKY GLEN RIDGE) Coonabarabran Figure 6.18. Isopachs (m), Hoskissons Coal (modified from Tadros 1993i, fig. 14.7) 6. GENETIC STRATIGRAPHIC ANALYSIS 245 Palynological evidence suggests that the Hoskissons peat accumulation commenced approximately synchronously throughout the basin (Hamilton 1987). The Hoskissons Coal closely overiies the first appearance of Dulhuntyispora parvithola in the upper strata of the short-lived Arkarula Shallow-marine System. The first appearance of this spore marks the palynological zone boundary between Lower and Upper Stage 5. The rapid lowering of the sea level after deposition of the Arkarula Shallow-marine System was associated with equally rapid establishment ofthe Hoskissons peat swamps. Further, Hunt et al. (1986) suggested that the peats of the Bayswater seam (Sydney Basin) and its correlatives (including the Hoskissons Coal) accumulated as a neariy continuous blanket approximately synchronously over most of the Sydney and Gunnedah Basins, implying tectonic stability with Utile or no subsidence. The Hoskissons Peat-swamp System consists principally of peat-swamp facies and minor fluvial sediments except along the western basin margin where fiuvial facies form a significant part of the Hoskissons Coal. Despite local variafions, the Hoskissons Coal has a characteristic macroscopic lithotype profile with an equally characterisfic maceral composifion profile: both can be traced throughout the Mullaley Sub-basin and the coal has been used as a marker horizon (figures 6.19, 9.6, 9.7 and 8.1). This persistence of the coal lithotype and maceral composition profiles suggests that there may have been major depositional controls on coal formation and coal quality (see chapter 8). Typically, the Hoskissons Coal consists of upper and lower sections separated by a persistent tuff or tuffaceous claystone marker averaging 0.1 m in thickness (figures 6.19 and 9.6). The lower section is generally low in discrete and disseminated mineral matter, with a tendency for increased brightness towards the base. The upper section is generally high in discrete and disseminated mineral matter, with an increase in brightness towards the top. Over much of the eastern half of the sub-basin the top of the Hoskissons Coal passes gradually into organic-rich mudstone. Along the western margin of the sub-basin, sandstone and siltstone layers up to 0.3 m thick and interbeds up to a few metres thick variably split the coal into two or three seams. Locally, particulariy in the centre and south, organic-rich mudstone and "canneloid" coal up to 1.3 m thick have also been observed at various levels within the upper section. Detailed study of vertical and lateral variations in maceral composition of the Hoskissons Coal and their relationship to the macroscopic lithotype profile of the seam are provided in chapter 9. DEPOSITIONAL SETTING It has been mentioned eariier (section 6.4) that towards the end of the Arkarula Shallow-marine Depositional Episode rapid lowering of the sea level terminated marine conditions and provided an extensive platform of marine-reworked deltaic sediments. The platform was rapidly colonised by vegetation and vast peat swamps were established (Becketi et al. 1983), forming the Hoskissons Coal (figure 6.17 A-C). 246 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE DM TRINKEY DDH 1 0 100 707.09_ d-J ram REFERENCE COAL-BRIGHT COAL-BRIGHT WITH DULL LAYERS COAL-INTERLAYERED DULL AND BRIGHT COAL-MAINLY DULL FREQUENT BRIGHT LAYERS COAL-DULL r WITH MINOR BRIGHT LAYERS GOAL-DULL COAL-STONY SILTSTONE CLAYSTONE CARBONACEOUS CLAYSTONE >»: TUFF AAAAij SEAM ERODED -3 CANNEL COAL -2 •-0 715.16 % ASH (a. d.) Figure 6.19. Lithotype and ash profiles, Hoskissons Coal (modified from Tadros 19931, fig. 14.18) 6. GENETIC STRATIGRAPHIC ANALYSIS 247 Lowering of sea level was accompanied in the north-westem half of the Mullaley Sub-basin by deposifion ofthe "western bed-load fluvial system" which was derived from the Lachlan Fold Belt region (figure 6.17 A, B). Toward the sub-basin centre, the channels debouched into bay environments of the shallow-marine facies and were reworked by tidal processes and wave action to form small wave- dominated deltas (figure 6.17 B). A thin veneer of siltstone was deposited in a shallow bay or lagoon which graded into marsh and peat swamps as regression continued (Hamilton 1987). Therefore, the Hoskissons Peat-swamp System overiies nearshore, barrier beach and lagoonal deposits in the south and centre of the basin. Based on the associated underiying facies, Hamilton (1985a, 1987) suggested that the coal formed "mostly" as a back barrier- lagoonal peat. However, Tadros (1986b) recognised the strong fluvial influence on the Hoskissons Coal and particulariy on its upper part. He noted that the Hoskissons Coal extends over large areas in the north and west of the sub-basin where it overiies and interfingers with the westeriy-derived fluvial sediments, which suggests that the peat swamps must have extended far inland, northwards and westwards, and were subject to the influence ofthe Western Fluvial System (Tadros 1986b; figures 8.2 and 6.17 D). The lower secfion of the Hoskissons Coal formed in regressive environment particulariy in the central and southern areas. The peat swamps were surrounded by highlands in the east and west, and inifially by a regressive marine environment in the south. Hamilton (1987) suggested that the sea level fall may have been irregular with intervening short-lived stillstands rather than a gradual regression, based on the presence of shoreline sands beneath the Hoskissons Coal in the south. With each addifional drop in sea level, a new extensive platform was exposed which probably led also to the development of new shoreline sands farther to the south. The peat swamps rapidly encroached over the freshly exposed platform (and the old shoreline sands). At each stage, the shoreline sands acted as efficient barriers which protected the outer edge of the peat swamps from inundafion by the sea unfil the next sea level fall. The barriers also helped maintain the water table at a level high enough for the formafion and preservafion of the peat. Tadros (1986b) compared the lower part of the Hoskissons Coal with the extensive, thick, low ash low sulphur Tertiary lignites of the Lower Rhine Bay, Germany, which were fornied also as back barrier peats (Teichmuller and Teichmuller 1968). It will be demonstrated in chapter 9 that the water table was high and resulted in the formation ofthe vitrinite-rich plies at the base ofthe Hoskissons Coal. Tadros (1986b) indicated that the back barrier - lagoonal interpretation of Hamilton (1985a, 1987) for the environment of formation of the Hoskissons peat holds only for the lower section of the formation, particularly in the centre and south of the sub-basin. Diessel (1986) who applied coal petrographic indices to samples from the mining section (i.e. lower section) ofthe Hoskissons Coal in the Gunnedah Colliery, confirmed that the coal formed in a back barrier environment under regressive condifions. (see chapter 9 for detailed petrographic study). In the northern and westem areas of the sub-basin, the Hoskissons Coal in general and the upper section in particular exhibit significant fluvial character. Sediments of the Western Fluvial System are 248 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE interspersed with the Hoskissons Coal in the north and split it in the west indicating that fluvial deposifion was contemporaneous with peat formation (figure 6.18) (Tadros 1988a and chapter 8). Vitrinite content increases upwards in the upper section of the Hoskissons Coal suggesfing a gradual raising of the water table as a result of continued subsidence, which ultimately provided accommodation for large thicknesses of peat now represented by up to 17 m of coal (figure 6.18). The presence of organic-rich mudstone and "canneloid" coal at various levels within the Hoskissons Coal indicates varying rates of subsidence within the peat swamp. The progressive change of coal to organic-rich mudstone characteristic of the top of the Hoskissons Coal in the eastern half of the Mullaley Sub-basin, indicates the final transformation of the peat swamps into a Lacustrine System. fTadros 1988b; refer to detailed discussion of other palaeo environment indicator macerals of the Hoskissons Coal in chapter 9). Termination of peat swamps by drowning under lacustrine conditions is common and is known in some modem analogues. Spackman et al. (1966) cited an example from Mud Lake near Ocala in Central Florida in which a layer of algal mud (sapropel - a lake deposit) caps a 10 m thick peat. 6.5.2 WESTERN FLUVIAL AND LACUSTRINE SYSTEMS Along the westem and the north-western margins of the sub-basin the Hoskissons Coal is interbedded with, and overiain by, the quartz-rich sandstones ofthe Western Fluvial System (figures 6.11 and 6.12). Over the eastern half of the sub-basin, the quartz-rich sandstone is separated from the Hoskissons Coal by lacustrine sediments of lake margin and lake basin facies (see chapter 7). At the lake margins (high sand areas in the percentage sandstone map for the Lacustrine System in figure 6.20), and to a lesser extent in the lake basin, sediments of the Western Fluvial System are, in part, laterally intergradafional with the lacustrine sediments. It should be emphasised that the genetic unit used to constmct the isopach, net sandstone and percentage sandstone maps for the Western Fluvial System is from the top of the Hoskissons Coal to the base of the Eastem Fluvial System (at the top of the Breeza Coal Member and its equivalents). These maps, therefore, also include sediments of the Lacustrine System over the eastem half of the sub-basin. However, the Lacustrine System and its component facies were also studied separately in chapter 7 because of its depositional and tectonic significance and its unique sedimentary characteristics. The Lachlan Fold Belt derived sediments of the Westem Fluvial System consist dominantly of quartz- rich sandstone. Although similar in composition, sedimentary structures and style of sedimentafion to the sandstone ofthe "westem bed-load fiuvial system" ofthe lower Black Jack depositional sequence of Hamilton (1987), sediments ofthe Western Fluvial System of the upper Black Jack sequence signify a major change in deposifional and tectonic setting in the basin. This change is represented first by a period characterised by almost total lack of deposition of terreginous clastic material during the accumulation of the Hoskissons peat, except along the western basin margin, followed by an increase in the supply ofthe quartz-rich sands and progressive expansion ofthe Western Fluvial System until it 6. GENETIC STRATIGRAPHIC ANALYSIS 249 Figure 6.20. Percentage sandstone. Lacustrine System, upper Black Jack Group (modified from Tadros 19931, fig. 14.11) 250 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE eventually covered the entire sub-basin. This change was in response to tectonic loading of the eastern edge ofthe craton which resulted in a "forebulge" along the western basin margin. In the east, the quartz-rich sandstone is separated from the Hoskissons Coal by the lacustrine sediments. GEOMETRY AND DEPOSITIONAL EVOLUTION Sediments of the Western Fluvial System (including the minor thickness of the Lacustrine System) are generally around 20-35 m thick except in the south and south-east where thickness rapidly increases to in excess of 90 m (figure 6.21). The lower bounding surface of the Western Fluvial System in the upper Black Jack sequence is the lower surface of the first sandstone to have split or replaced the Hoskissons Coal. Definifion of the upper bounding surface is somewhat difficult because ofthe local development of zones of intermixing with easteriy sourced lithologies. However, recognifion ofthe upper bounding surface was made easier by the presence of the regionally extensive Breeza Coal Member and its equivalents, associated with the change from dominanfiy westeriy sourced to dominanfiy easteriy sourced lithologies (figure 6.17 D, F). The percentage sandstone map for the interval between the Hoskissons Coal and the Breeza Coal Member (and its equivalents in the north; combined Western fiuvial/Lacustrine Systems; figure 6.22) gives improved resolution of lithofacies distribution and geometry over that for the total upper Black Jack depositional sequence shown in figure 6.16 The Howes Hill Coal Member (and its equivalents in the north - see chapter 8), which underiies the Breeza Coal Member, coincides with the maximum upper extent of the lake sediments in the east and thus provides a convenient upper surface for the Lacustrine System. The sandstone geometry of the interval between the Hoskissons Coal and the Breeza Coal Member (combined Western fluvial/Lacustrine System; figure 6.22) displays a prominent fiuvial style with strongly dip-oriented and confiuent contour patterns. The major depositional elements recognised within this system are the south-easteriy flowing contributory streams, expressed on the maps by high sandstone percentages of elongate sand bodies emanating from the Lachlan Fold Belt in the west and a trunk channel complex flowing south and south-easteriy along the sub-basin axis. The streams were drawn to the east and south-east by the rapid subsidence which initially caused the transformafion of the Hoskissons peat swamps into the Lacustrine System, and finally the infilling ofthe lake. The detailed history and evolufion of the Western Fluvial System can be interpreted from analysis of a succession of smaller genefic lithofacies intervals bounded by coal seams correlated over broad areas in the southern half of Mullaley Sub-basin. Percentage, net sandstone and isopach maps were constructed for three interseam intervals defined by four seams (including the Hoskissons Coal) in the southern half of the sub-basin. Seam correlafion in the north was difficult on the sub-regional scale because peat accumulation was generally limited to discrete interchannel areas (see chapter 7). 6. GENETIC STRATIGRAPHIC ANALYSIS 251 TN NEW ENGLAND FOLD BELT LACHLAN t) FOLD BELT p Aio- '{ROCKY GLEN RIDGE) Figure 6.21. Isopachs (m), Westem Fluvial System, upper Black Jack Group (modified from Tadros 19931, fig. 14.16) 252 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN 150° / .«/ CO -s NEW ENGLAND FOLD BELT LACHLAN o\ L FOLD BELT / ^ (ROCKY GLEN RIDGE) Coonabarabran . Borehole 0 10 20 Figure 6.22. Percentage sandstone. Western Fluvial and Lacustrine Systems. The inset shows palaeocurrent rose diagram for cross-bedding within the Clare Sandstone (Westem Fluvial System) on an outcrop near the top of Mount Watermark, west of Breeza. The arrow outline points to Mount Watermark. For data see appendix 3. (modified from Tadros 19931, fig. 14.17) 6. GENETIC STRATIGRAPHIC ANALYSIS 253 Southern part ofthe Mullaley Sub-basin The Caroona, Howes Hill and Breeza Coal Members, together with the Hoskissons Coal, fonmed bounding surfaces for three genefic lithofacies intervals in the southem part of the Mullaley Sufc)-basin (figure 6.12). In these maps the areal extent of the interseam interval is determined by the seam with the smaller area. The upper two seams, although apparently areally limited to the southern half of the sub-basin, were contemporaneous with counteqparts in the north (see figure 6.12 and chapter 8) and provided basinwide bounding surfaces for the upper part ofthe Westem Fluvial System. The bounding coal seams provided a sharply defined change on the geophysical log facies, from the characteristic very low gamma and neutron responses of the coal seams to the blocky low gamma, high neutron response of sandy channel fills or the heterogeneous, moderately serrated log pattem representing channel margin and flood-plain deposits. Hoskissons - Caroona interseam interval The Caroona Coal Member is confined to the south-eastem comer of the sub-basin (figure 10.8) and is separated from the Hoskissons Coal by channel margin and flood-plain deposits of the Westem Fluvial System. The two seams neariy merge in the west, thus bounding a sediment wedge which attains a maximum thickness of 35 m in the south-east (figure 6.23). The percentage and net sandstone maps for this interseam interval (figures 6.24 and 6.25) define a narrow south-westeriy to southeriy trending high-sand zone consisting of quartz channel fill with a characteristic aggradational log signature (log facies type "d", figure 6.5). This zone corresponds to the axial channel complex of the Western Fluvial System. The Caroona seam is split along this zone. Figure 6.24 also shows a TN N NEW ENGLAND -:^. ^4,. Gunnedah N^ FOLD BELT % REFERENCE - Borehole 10 20. km Figure6.23. Isopachs (m), Hoskissons - Caroona interseam interval (fromTadros 19931, fig. 14.18) 254 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN X NEW ENGLAND *v\ . 1'. iGunnedah X FOLD BELT % cI i h - z LIMIT OF OUTCROP h •ZONE 0- LACHLAN FOLD BELT ^1 O (ROCKY GLEN RIDGE) ^ #V i^ / Coonabarabran x .<# .^^ REFERENCE . Borehole 10 20, =1 km Figure 6.24. Percentage sandstone, Hoskissons - Caroona interseam interval (from Tadros 19931, fig. 14.19) TN ^\ X" NEW ENGLAND i \ "is ' Gunnedah ^s FOLD BELT ^o^\ LIMIT OF k OUTCROP •ZONE ui LACHLAN FOLD BELT a. (ROCKY GLEN RIDGE) ^ ^ / Coonabarabran .4<^. •O, . REFERENCE Borehole 10 20 km Figure 6.25. Net sandstone, Hoskissons - Caroona interseam interval (reaches 22m in the east; from Tadros 19931, fig. 14.20) 6. GENEHC STRATIGRAPHIC ANALYSIS 255 marked decrease in sand percentage towards the west (and also a small area in the east). Low sand areas are characterised by moderately serrate heterogeneous log facies type "e" (figure 6.5) characteristic of fine-grained sediments of channel margin and inter-channel/floodplain facies. The change from the Hoskissons Coal to the fine-grained sediments and to the Caroona seam is very sharply defined on the geophysical logs. The low gamma and neutron responses of the lower and upper bounding seams are in contrast to the signature of log facies "e" which has a tendency to shift towards high gamma and neutron responses. Caroona/Hoskissons - Howes Hill interseam interval The Howes Hill Coal Member is more extensive than the underiying Caroona seam and covers the south-eastem quarter of the sub-basin (figure 10.13). The architecture ofthe sediments for the interval between the Caroona/Hoskissons Coal and Howes HiM Coal Member (figures 6.26-6.28) defines a south-easteriy trending thick sand zone which is 5-10 km wide and consists mainly of stacked quartzose channel fills. The log facies of this zone is characterised by a succession of blocky aggradational sand units (log facies type "d", figure 6.5) with its sharply defined low gamma and high neutron responses. This zone corresponds to the axial channel complex of the Westem Fluvial System. The percentage sandstone map (figure 6.27) shows that, in the Caroona area, the axial channel complex shifted laterally some 10 km to the west over the Caroona peats and attained a stronger south-easteriy trend. Lateral shifting in channel position appears to have been caused by rapid compactional subsidence of the Caroona peats and the silts and clays of the lower intervals, including the Hoskissons peat. Ferm ef a/. (1979 p 605) demonstrated that channels tend to select TN X NEW ENGLAND -^ "4- ^. Gunnedah \ FOLD BELT •*'O\, <^iS,>' \ Ui\ LACHLAN FOLD BELT "1 (ROCKY GLEN RIDGE) f/ >/ Coonabarabran ^ -5^d^ ^^ REFERENCE . Borehole 0 10 2,0 u km Figure 6.26. Isopachs (m), Caroona (and/or Hoskissons) - Howes Hill interseam interval (from Tadros 1993i, fig.14.21) 256 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN is X NEW ENGLAND '* \ 80 IGunnedah \ FOLD BELT i M --, LIMIT OF --.^ OUTCROP ),-r- ZONE LACHLAN FOLD BELT "1 • ^>A\*'60 . \ (R «/^\\4i,'\ y , VJ ^Bree^e ' u ^ / Coonabarabran < REFERENCE . Borehole 0 10 20 km Figure 6.27. Percentage sandstone, Caroona (and/or Hoskissons) - Howes Hill interseam interval (from Tadros 19931, fig. 14.22) TN X NEW ENGLAND ^ Gunnedah ^v FOLD BELT i %: \z. 12 14 16 ^> LACHLAN FOLD BELT o/ (ROCKY GLEN RIDGE) i^v Coonabarabran / f '<>^ x^t REFERENCE 2 4 6 Borehole 0 10 20 L. =1 km Figure 6.28. Net sandstons, Caroona (and/or Hoskissons) - Howes Hill interseam interval (from Tadros 19931, fig. 14.23) 6. GENETIC STRATIGRAPHIC ANALYSIS 257 areas of thickest peat where compactional subsidence attracts active channel flow; whereas Brown (1969, cited by Galloway and HotxJay 1983a p266) indicated that the clay-rich substrate causes rapid subsidence of channel fill sands and produces vertical stacking of channel sands along a persistent path. In the Mullaley Sub-basin, lateral shifting in channel position may have also been controlled by shifts in the basin's axis. The Howes Hill Coal Member is absent along this axial channel zone. Along the margins of the axial channel complex the seam is disrupted by splitfing and shows deteriorafion in coal quality caused by influx of sediments introduced during flooding events which are well represented by aggradational log facies type "e" (figure 6.5). Along the eastem half of the sub-basin to the west of the axial channel complex, interchannel areas developed into an extensive lake system as a result of rapid subsidence. Expansion of the Western Fluvial System by the confluence of east-flowing tributaries and the south-easteriy flowing axial channel complex infilled the Lacustrine System (figures 6.20 and 6.22). Infilling of the lake was by crevasse splays and by channels debouched directiy into the lake to form lacustrine deltas (figure 6.20). As a result, fluvial and lacustrine sediments interfinger, particulariy at the lake margins and locally show well-developed progradational log facies (log facies type "m", figure 6.5). Howes Hill/Hoskissons - Breeza interseam interval The Breeza Coal Member extends over much of the southem sub-basin area (figure 10.18) and, as mentioned eariier, this seam was contemporaneous with counterparts in the north; these are the Denison and Turrawan seams. Together they provide (almost) a basinwide upper bounding surface for TN N NEW ENGLAND \ • i \ \ Gunnedah \ FOLD BELT LACHLAN FOLD BELT (RCX;KY GLEN RIDGE) Coonabarabran 10 20 30 Figure 6.29. Isopachs (m), Howes Hill (and/or Hoskissons) - Breeza interseam interval (from Tadros 19931, fig. 14.24) 258 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TiN LACHLAN FOLD BELT (ROCKY GLEN RIDGE) Coonabarabran 0 u 0- Figine 6.30. Percentage sandstone, Howes Hill (and/or Hoskissons) - Breeza interseam interval (from Tadros 19931, fig. 14.25) TN NEW ENGLAND Gunnedah N. FOLD BELT LACHLAN FOLD BELT (ROCKY GLEN RIDGE • Coonabarabran REFERENCE Borehole 0 10 20 =l_ =1 km Figure 6.31. Net sandstone (m), Howes Hill (and/or Hoskissons) - Breeza interseam interval (from Tadros 1993i, fig. 14.26) 6. GENETIC STRATIGRAPHIC ANALYSIS 259 the upper part of the Western Fluvial System. The isopach, and percentage and net sandstone maps for the interval between the Howes Hill and Breeza Coal Members (figures 6.29-6.31) show that in the area south of Gunnedah, the axial channel complex of the Westem Fluvial System has moved some 15-20 km to the east, trending south to Breeza thence in south-south-westeriy direction for a short distance before it swings back to the southeast into the Caroona area. The path of the axial channel through the Caroona area is superimposed on top of that which was occupied by the axial channel during the Caroona - Howes Hill interseam interval. The maps also show a number of south-easteriy trending tributary streams along the westem basin margin, and significant clastic deposition by crevasse splays off the axial channel complex west of Breeza. Geophysical logs display an aggradational blocky pattern (log facies type "d", figure 6.5) characteristic of channel fills for the axial channel complex, and log facies type "e" for channel margin and floodplain sequences. In areas where the Howes Hill Coal Member is absent, lake facies may be present between the Hoskissons Coal and Breeza Coal Members and are represented by progradational log pattern type "m" (flgure 6.5). The axial channel complex maintained its position during accumulation of the Breeza seam peat, and caused seam splitting in the south-east where quartzose channel tills were deposited. In the west, quartzose channel fllls from south-easteriy flowing tributaries also caused seam splitting. In the central area, sandy crevasse splay deposits and fine-grained laminated lacustrine sandy silts and clays split the seam (see chapter 8). Northern part ofthe Mullaley Sub-basin Improved resolution of the framework geometry, which was made possible in the southern part of the sub-basin by detailed seam correlation, could only be partially achieved in the north because the seams cannot be correlated over large areas. Peat accumulation within interchannel areas isolated by the axial and tributary streams was independent of neighbouring peat swamps, and coal seam characteristics rarely correlate across the fluvial channels (see chapter 8). However, renewed tectonic activity and the introduction of volcanic - lithic detritus from the New England region to the drainage system in the basin, provided a genetically and lithologically different sequence to the underiying Western Fluvial System. Further, the Denison and Turrawan seams in the north occupy a strafigraphic posifion beneath this tectonically controlled change in the north similar to that of the widely correlatable Breeza Coal Member in the south. Although the Denison and Turrawan seams, most probably, lack physical continuity to the south, they display a gross lithotype profile broadly similar to that of the Breeza Coal Member. This, together with their similar strafigraphic position beneath the tectonically controlled change, suggests that they were contemporaneous with the Breeza Coal Member and thus can be used as a valid bounding surface between the two genetic sequences, the Western and Eastern Fluvial Systems. TERMINATION OF THE WESTERN (FLUVIAL/LACUSTRINE) DEPOSITIONAL EPISODE Sediments of the Western Fluvial and Lacustrine Systems were subjected to considerable intraformational and post-depositional erosion prior to deposition of the eastem fluvial sediments. 260 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE Borehole evidence indicates that in most cases where sections of the western fluvial sequence (and occasionally the upper parts of the lacustrine sequence) are absent the sediments are always erosively overiain by the eastern (bed-load) channel facies. Exceptions are where mixing of the easteriy and westeriy derived sediments occurs in the south-east. In the northern part of the sub-basin, isopachs of the upper western fluvial sequence (above the Lacustrine System; figure 6.32), show contour patterns and rapid thickness changes which suggest significant fluvial incision into the upper strata of the underiying lacustrine sediments. The contours strongly suggest that the fluvial incision was by south- westeriy flowing channels which were responsible for deposition of the sediments of the Eastern Fluvial System. Figure 6.32 also shows a rapid change in thickness of the upper western fluvial sediments in the southern part of the basin from an average of 50 m to as low as 5 m along a transverse south-westeriy trend. Incision along this trend, although deep, had not reached the underiying lacustrine sequence because of the high subsidence rates which allowed build-up of such a great thickness (up to 63 m) of the upper western fluvial sediments, particulariy along the main axial channel. Deposifion of the Western Fluvial System gradually diminished when the supply of the quartz-rich sands declined due to changes in the base level and the decreased rate of erosion in the Lachlan Fold Belt region. The Breeza Coal Member (and its equivalents in the northern part of the basin) represents a significant period of minor terrigenous clastic deposition in the sub-basin. The widespread distribution of the seam suggests that the basinwide peat accumulafion was controlled by a major tectonic event, which marks the end of the Western Depositional Episode. The Breeza Coal Member coincides with a significant change in provenance from dominantly westeriy sourced quartz-rich sediments to easteriy sourced volcanic - lithic detritus, suggesting a corresponding change in tectonic regime. However, some areas along the axial drainage system show evidence of intermixing of the two lithologies for a brief period after cessation of peat formafion of the Breeza Coal Member (photos 7.11 and 7.12). Intermixing is most evident in the south-east where the boundary between the two systems isgradafional. The sequence illustrated in photo 7.11 and figure 7.23 from DM Wallala DDH 3, located close to the axial channel complex in the south-east, shows a typical gradational change in lithologies. At the base is a coarse-grained quartz-rich sandstone typical of the Western Fluvial System. Towards the top (photos 7.11 and 7.12), the quartz-rich sandstone is intermixed with an increasing proportion of lithic sandstone which finally dominates. The Breeza Coal Member, which is generally situated below the transitional zone, is split by fluvial channel deposits, and is pooriy developed in this borehole and in other boreholes along the main axial channel. Generally, the eastern conglomerate units become poorer in quartz progressively upwards. Intermixing probably occurred during the transition from craton to orogen-controlled tectonics. It also may have resulted from erosion (and reworking) of the previously deposited western fluvial sediments by streams ofthe Eastern Fluvial System in the north. 6. GENETIC STRATIGRAPHIC ANALYSIS 261 ISO" TN / / c H S 5 \Oo m C9 / Narrabri « / \' NEW ENGLAND \ii 0- \ FOLD BELT ^ \ LACHLAN o^ / FOLD BELT / -31° (ROCKY GLEN RIDGE) ,\ a: LU (/3 UJ Coonabarabran ,/ / ^-i-! REFERENCE . Borehole 0 10 20, km 150" Figure 6.32. Isopachs (m), upper part of the Westem Fluvial System (interval between the Howes Hill and Breeza Coal Members and their equivalents) showing incision due to erosion by the south westerly flowing chaimels ofthe Eastem Fluvial System. Erosion is most pronoimced in the north where subsidence rates were low, with possible occasional uplift or stillstand. In the south-east where subsidence was rapid, a thicker sequence accumulated and was littie affected by erosion (modified from Tadros 19931, fig. 14.27) 262 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE 6.5.3 EASTERN FLUVIAL SYSTEM Renewed tectonic activity in the New England region heralded a major change in the depositional setfing ofthe Gunnedah Basin. The New England region became the major source of sediments to the basin and provided the detritus for the Eastern Fluvial System. This change is indicated by the lithofacies maps, particulariy the net sandstone, percentage sandstone and percentage conglomerate ofthe total net sand maps (figures 6.33-6.35) These New England derived sediments were carried by bed-load and mixed-load fluvial and alluvial fan systems, and consisted of axial and tributary channel fill, channel margin and fioodplain facies. Sediment composition is volcanic - lithic and ranges from fine-grained sandstone to pebble conglomerate, with interbedded claystone, siltstone and coal (photo 7.29; figures 7.21 and 7.23). Tuff and tuffaceous sediments, as well as carbonaceous claystone and coal, are also abundant, particulariy in the top part of the sequence. Over much of the sub-basin, the sediments of the Eastern Fluvial System are separated from the underiying quartz-rich sandstone of the Western Fluvial System by the Breeza Coal Member and its equivalents, or locally by fine-grained laminated fioodplain sandy siltstone and claystone (figure 6.17E, F). GEOMETRY AND DEPOSITIONAL EVOLUTION The isopach map for the Eastern Fluvial System shows that the sequence (figure 6.36) is generally thinner in the north and west, where it ranges from 10 m to 60 m, than in the south, ranging from 80 m to 180 m. Thickness increases rapidly in a south-easteriy direction to in excess of 300 m. The map also shows a thin transverse zone trending south-west through Boggabri coinciding with the Walla Walla Ridge (chapter 4). The base of the Eastern Fluvial System is the top of the underiying Western Fluvial System, and was described in detail in the previous section. The upper boundary is the regional unconformity between the Permian Black Jack Group and the Triassic Digby Formation. The upper boundary is recognised by the marked contrast between the highly serrate log facies ofthe tuffaceous coaly sequence (figure 7,21 and log facies type "f, figure 6.5) and the overiying blocky, aggradational log patterns of the Triassic Digby conglomerate facies (figure 6,36 and log facies type "a", figure 6.5). Even where the Permian and Triassic conglomerates are in contact, their log patterns are quite distinct despite similarity in their appearance in core (figure 7.6). Close examination indicates that, although framework grains may in some cases be similar, matrix is predominantly argillaceous and rich in tuff and pyroclastic detritus in the Permian conglomerates, but sandy lithic in the Triassic conglomerates. The Digby conglomerate facies are therefore sharply defined by their lower gamma and higher neutron log responses than their Permian counterparts. Broad similarity in appearance in drill core and lack of spore assemblages in coarse lithologies ofthe conglomerate have led some authors in the past to incorrectly define the Digby Formation as partly Permian in age. As for the Western Fluvial System, the detailed history, depositional style and evolution of the Eastern Fluvial System has been interpreted from analysis of a succession of smaller genetic lithofacies 6. GENETIC STRAHGRAPHIC ANALYSIS 263 150° TN / so/ I 1SI NEW ENGLAND % FOLD BELT LACHLAN o\ • I / FOLD BELT / 4' -31° • \ (ROCKY GLEN RIDGE) Coonabarabran REFERENCE . Borehole 10 30 40 Figure 6.33. Net sandstone (m), Eastem Fluvial System, upper Black Jack Group (modified from Tadros 19931, fig. 14.29) 264 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE NEW ENGLAND FOLD BELT LACHLAN FOLD BELT (ROCKY GLEN RIDG Coonabarabran REFERENCE . Borehole 10 Figure 6,34. Percentage sandstone (m), Eastem Fluvial System, upper Black Jack Group (modified from Tadros 19931, fig.14.30 ) 6. GENETIC STRATIGRAPHIC ANALYSIS 265 TN 150° / I 'I m CO I ^1° Narrabri • \' • 601 , • 50j 70 . IZONEOF- • 40| \ ,50 ',SUBCROP\ \ i\ NEW ENGLAND FOLD BELT (ROCKY GLEN RIDGE) 31°- Coonabarabran . Borehole 0 10 20 Figure 6.35. Percentage Conglomerate (m). Eastern Fluvial System, upper Black Jack Group (modified from Tadros 19931, fig.14.31 ) 266 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN 150° / i ,«/ X •V c z \0^ H m 3J 1 Narrabri \ \ ZONE OF \ SUBCROP \ \ •> . -J* NEW ENGLAND \\ 1^ 10 20 30 40 5Q 50 ^Q FOLD BELT LACHLAN Boggabri -n > FOLD BELT / V Gunnedah \ ^A^ „,, (ROCKY GLEN RIDGB s^4^31_j Coonabarabran Figure 6.36. Isopachs(m), Eastem Fluvial System, upper Black Jack Group (modified from Tadros 19931, fig, 14.35) 6. GENETIC STRATIGRAPHIC ANALYSIS 267 intervals bounded by coal seams correlated over broad areas in the southem part of the Mullaley Sub- basin. Percentage and net sandstone and isopach maps were constmcted for three interseam intervals defined by three seams, together with the Breeza Coal Member, in the southern half of the sub-basin. Correlafion of seams within the Eastern Fluvial System in the north was more difficult than for seams within the Western Fluvial System because not only was peat accumulation generally limited to discrete interchannel areas, but also disrupted by influx of large quantities of pyroclastic detritus and airfall tuff (see chapter 8). Breeza - Clift interseam interval The architecture of the sediments between the Breeza and C//7if Coal Members (figure 6.37) indicates that a major change in sedimentation followed accumulation of the Breeza seam peat. The percentage sandstone map for this interval displays a well-defined south-easteriy trending axial channel complex which has shifted from its previous position laterally to the west some 40 km in the north (from Gunnedah to Mullaley), and 20 km in the south (from Breeza to west of Caroona; see figure 6.30 for comparison). Lateral shifting of the axial channel complex was accompanied by the development of a number of very strong and cleariy defined westeriy to south-westeriy trending large tributary streams. There are strong indications that these tributaries emanated from the New England Fold Belt region in the east and flowed south-westeriy, pushing the axial channel complex basinward. At least five to six main entry points where these large tributary streams joined the main axial channel complex can be TN ^^p \ i \ '\ \ A LACHLAN FOLD BELT (ROCKY GLEN RiDGE) i • ?7 iS^ / Coonabarabran S ^^. REFERENCE . Borehole 0 10 20 km Figure 6.37. Percentage sandstone, Breeza - Clift interseam interval (from Tadros 19931, fig. 14,36) 268 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE identified on the percentage sandstone map (figure 6.37). Borehole intersections of the Breeza to Clift interseam interval show an abrupt change in sediment composition from quartzose to volcanic - lithic conglomerate and sandstone in the main south-westeriy trending tributary streams, and a transition from quartzose to mixed quartz/volcanic lithic, and finally volcanic-lithic sandstone and conglomerate along the main axial channel complex (photo 7.15). The net sandstone and isopach maps for the Breeza to Clift interseam interval (figures 6.38 and 6,39) show that the sediments thin, and the sand component decreases markedly in the interchannel areas, particulariy towards the south-west and in the north. Sediments in the interchannel areas are dominated by fine-grained laminated fioodplain/lacustrine and marginal sandy crevasse splay deposits, and they progressively become finer and thinner, and finally pinch out towards the central parts of the interchannel areas. Consequently, the Breeza and Clift Coal Members coalesce into one seam in these areas; however they do not lose their character. Clift - Springfield interseam interval The Clift and Springfield Coat Members could only be identified in the eastern and central parts of the southern sub-basin area (figures 6.40-6.42 and chapter 8), and in the western and south-western areas sand trends could only be mapped for the total interval ofthe Eastern Fluvial System. South-westward expansion of the tributary streams continued throughout the accumulation of the Clift Coal Member peat and caused shifting ofthe main axial channel westward as well as seam splitfing by volcanic - lithic channel fills in the north and south-west. The percentage and net sandstone maps for the Clift - Springfield interseam interval (figures 6.40 and 6.41) show contributory drainage patterns TN NEW ENGLAND 4-P.. i <\^ . \ FOLD BELT \ \ \ o- LACHLAN FOLD BELT O (ROCKY GLEN RIDGE) V i^ / Coonabarabran s ^^ ^. REFERENCE Borehole 0 10 20 =1 km 10 40 Figure 6,38. Net sandstone, Breeza - Clift interseam interval (from Tadros 19931, fig, 14.37) 6, GENETIC STRATIGRAPHIC ANALYSIS 269 TN X T NEW ENGLAND Gunnedah >v \ 1 "^'.c \ -kA 0- LACHLAN FOLD BELT (ROCKY GLEN RIDGE) •:/ iS^/ Coonabarabran ,<3 ^' r^$> REFERENCE . Borehole 10 20, km 20 30 40 Figure 6.39. Isopachs (m), Breeza - Clift interseam interval (from Tadros 1993i, fig. 14.38) TN rso 70 \ NEW ENGLAND •% I^J^' _.^..o; •: BGunnedah X FOLD BELT i 'o^Or S' LIMIT OF m \ 3 v20-^v-30 OUTCROP I ' - - ZONE LACHLAN FOLD BELT (RCX;KY GLEN RIDGE) V iS^ Coonabarabran . REFERENCE Borehole 10 20, =J km Figure 6.40. Percentage sandstone, Clift-Springfield interseam interval (from Tadros 19931, fig. 14.39) 270 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN \ NEW ENGLAND is . / \ Gunnedah \ FOLD BELT i ^A, LIMIT OF • I tm A OUTCROP 33 ZONE 0- LACHLAN FOLD BELT o/ (R(X;KY GLEN RIDGE) i^ Coonabarabran / ^^£ ^ . REFERENCE Borehole 0 10 20 L. =1 km Figure 6.41. Net sandstone, Clift - Springfield interseam interval (from Tadros 19931, fig. 14.40) TN 4, Y NEW ENGLAND ^ *\ Gunnedah %. FOLD BELT i 'c> <^S',\ \ ui (3- LACHLAN FOLD BELT o (ROCKY GLEN RIDGE) f/ iS'/ Coonabarabran ^, REFERENCE Borehole 0 10 20 m u 1 1 k Figure 6.42. Isopachs (m), Clift - Springfield interseam interval (from Tadros 19931, fig. 14.41) 6. GENETIC STRATIGRAPHIC ANALYSIS 271 recognised by the confluence of south and south-westeriy oriented sand-rich trends separating discrete interchannel areas. Springfield Coal Member- Doona seam/top of Black Jack Group interval Silicic volcanism in the New England Fold Belt region during the later part of upper Black Jack deposition provided large amounts of pyroclastic debris, airfall tuff and tuffaceous sediments to the basin. The influx of large quantities of pyroclastic detritus dismpted peat accumulation, caused loss of seam character and rendered correlation difficult (see chapter 8). Nevertheless, because of the limited area over which the Doona seam can be correlated, two sets of maps were constructed, the first for the Springfield - Doona interseam interval (figures 6.43 - 6.45), and the second for the interval between the Springfield Coal Member and the upper bounding surface of the Black Jack Group (figures 6.46-6.48). Both sets of maps show progressive expansion of the south-westeriy trending tributary streams, probably caused by massive sediment influx from the New England region, and further shift in the main axial channel westward. \ LIMIT OF TN \OUTCROP X \/' ZONE "-!-. \ ^' ^ i t><>> , \r>> \ ^ AI Ul LACHLAN FOLD BELT (i- (ROCKY GLEN RIDGE) 1 • ^ #V ^ / Coonabarabran f !^. r^^. REFERENCE Borehole 0 10 20 L. km Figure 6.43. Percentage sandstone, Springfield - Doona interseam interval (from Tadros 19931, fig. 14.42) 272 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN X \ NJEE W ENGLAND ^4- Gunnedah N> FOLD BELT %. \\ I] LACHLAN FOLD BELT (ROCKY GLEN RIDGE) 1 V ^ / Coonabarabran .«f / x^! REFERENCE , Borehole 0 10 20 =1—1 =1 km Figure 6.44. Net sandstone (m), Springfield - Doona interseam interval (from Tadros 19931, fig. 14.43) TN \ NEW ENGLAND hsv> Gunnedah \ FOLD BELT \ %\ A uia-' LACHLAN FOLD BELT (ROCKY GLEN RIDGE) •:/ is' Coonabarabran / ^^d . REFERENCE . Borehole 0 10 2,0 km Figure 6.45. Isopachs (m), Springfield - Doona interseam interval (from Tadros 19931, fig. 14.44) 6, GENETIC STRATIGRAPHIC ANALYSIS 273 TN \ i "^ • LACHLAN FOLD BELT (ROCKY GLEN RIDGE) • Coonabarabran REFERENCE . Borehole 20 30 40 50 50 40 0 10 20 =J km u Figure 6.46. Percentage sandstone, Springfield Coal Member - top of Black Jack Group (from Tadros 19931, fig. 14.45) TN N NEW ENGLAND \ \ Gunnedah ^s FOLD BELT ]• •%A, 201 Ih LACHLAN FOLD BELT 0, O (ROCKY GLEN RIDGE) f/ ^ Coonabarabran / -S^d^ 4^ REFERENCE 10 20 30 40 . Borehole 0 10 20 =1 km Figure 6.47. Net sandstone, Springfield Coal Member - top of Black Jack Group (from Tadros 19931, fig. 14.46) 274 GUNNEDAH BASIM - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE TN X NEW ENGLAND -:v. X i ^^is % LACHLAN FOLD BELT (ROCKY GLEN RIDGE) ^ / Coonabarabran <<^"f j^% O. <£"6>' c^. REFERENCE Borehole 50 70 90 140 0 10 20, =1 km u Figure 6.48. Isopachs (m), Springfield Coal Member - top of Black Jack Group (from Tadros 19931, fig. 14.47) TERMINATION OF THE EASTERN DEPOSITIONAL EPISODE Extensive Late Permian plutonism and associated felsic volcanism in the New England Fold Belt (Scheibner 1976) contributed large quantities of volcanogenic material to the basin during the Late Permian. Subsidence in the northem part of the basin was intenrupted towards the end of the Late Permian by a period of stmctural readjustment and basin tilting, particulariy in the north and north-east. As a result, the north-eastem side of the basin was uplifted and a thick section of the Permian sediments was eroded (figures 3.23 and 3.24). Subsequently the Gunnedah Basin received lithic sediments initially from the New England erogenic region in the east followed by quartzose sediments from the Lachlan Fold Belt region in the west. These sediments prograded over most of the Gunnedah Basin in a southeriy direction and formed the unconformably overiying Digby Formation during the Eariy Triassic (Tadros, 1986b). In the southem Gunnedah Basin, the rapid subsidence during the Late Penmian was intermpted only for a short period, during which an unknown portion of the upper Black Jack Group was eroded. Subsidence resumed in the Eariy Triassic and the basin in the south also received the easteriy and westeriy sourced sediments ofthe Digby Formation (Tadros, 1986b; Jian, 1991). 6. GENETIC STRATIGRAPHIC ANALYSIS 275 The unconformity between the Penmian and Triassic sediments in the Gunnedah Basin is evident from the erosive nature and the angular relationship of the contact between the Black Jack Group and the overiying Digby Formation (Tadros 1986b, 1993e; Tadros in Tadros et a/. 1987b). Sections across the area show that the Digby Formation tmncates the Black Jack Group progressively from west to east, leaving a wedge of Black Jack sediment thinning to the east (figures 3.23 and 3.24; see chapter 3 for details). Palynological evidence (McMinn 1985c), also indicates a hiatus between the Black Jack Group and Digby Formation in the Narrabri area in the north, with the gap equivalent mostly to Upper Stage 5 (the Protohaploxypinus microcorpus assemblage) and the Eariy Triassic (Lunatisporites pellucidus assemblage^. 276 GUNNEDAH BASIN - SEDIMENTOLOGY - UPPER BLACK JACK SEQUENCE This page is blank, \ LIMIT OF k OUTCROP I \ 1-i— ZONE LACHLAN FOLD BELT 0- •^1 (ROCKY GLEN RIDGE) o ?7 iS^/ Coonabarabran <<^,