The Geology of the Blue Mountains - with reference to Cox’s Road The upper Grose Valley from Baltzer’s lookout Col Bembrick May, 2015 1 Contents: Introduction 3 Geomorphology and Structure 5 Rock Units and their Origins 9 Igneous Rocks 22 Glaciation 23 Conclusion 24 Acknowledgements 24 References 25 Glossary 28 Figure 1 The Sydney Basin 3 Figure 2 The Geological Time Scale 5 Figure 3 The Lapstone ‘Monocline’ Structure 8 Figure 4 Rock Strata and their environments 9 Figure 5 Geological map of the Blue Mountains 12 Figure 6 Coal Measure deltas 15 Figure 7 Mt Banks cliff-face strata 16 Figure 8 Volcanic neck development 22 2 Introduction: This article has been written as a contribution to Greening Bathurst’s Cox’s Road Dreaming Project – part of the 2015 Bathurst Bicentenary celebrations. It is not intended to be a detailed, technical compendium on the geology of the Blue Mountains, but rather a brief summary of the current state of knowledge of the area’s geology. It is hoped that this will be of interest to the tourist, students and other travellers who may require more in-depth information without the need to consult the more detailed geological literature. Wherever possible, reference is made to parts of the Cox’s Road (and other modern roads) where some geological observations are relevant, even to some comments made by William Cox himself. It is worth remembering however, that for the most part, Cox and his men would have been totally ignorant of the geological significance of the unfamiliar terrain through which they travelled and certainly would have been amazed at the ages of the rocks that they traversed. Although lacking the extensive spread of the coastal exposures, the dissected plateau of the Blue Mountains gives some spectacular opportunities to observe the geological sequence (if only from a distance at times) preserved along the western margin of the Sydney Basin. This summary deals only with the Permian, Triassic and younger strata and concentrates on the terrain between Lithgow/Hartley and the Nepean River. Another article in the Cox’s Road Dreaming project covers the country between Bathurst and Mt York (Marshall, 2015) – largely granite and sediments older than the Blue Mountains strata. The early geological investigators in this region are well summarised by Pickett & Alder (1997) in the publication “Layers of Time”. A few of the more famous ones will be mentioned in this article. It appears that Sir Thomas Mitchell (1792-1855) was the first to describe some rocks of Mt Tomah and Mt Hay as “trap rock” – i.e. basalt. Others, such as Count Strzelecki (1797-1873), had commented on the basalt caps of the higher hills of the Blue Mountains, and in 1845 he published a geological map outlining what we now call the Sydney Basin (Pickett & Alder, 1997).(fig 1) Figure 1 – location and extent of the Sydney Basin. 3 The geology of the Blue Mountains records much of the Early Permian (295Ma) to Middle Triassic (240Ma) (fig2) sedimentation on the stable, western margin of the Sydney Basin. Historically, most of the mapping in the Blue Mountains has been along the western margin, where the economic imperative has been to map the Illawarra Coal Measures for commercial exploitation. The rugged and highly dissected nature of the plateau has precluded, until relatively recently, detailed mapping of much of the Blue Mountains terrain. Initially, the western margin of the Sydney Basin developed on the broadly folded and metamorphosed sediments of the Lachlan Fold Belt, where the last stage of folding and granite intrusion occurred during the Kanimblan Orogeny, after which the area stabilised during the earliest Carboniferous - around 340 Ma. A period of uplift and erosion followed, but highlands of moderate relief remained, with mountain glaciers developing in the cold climate, with the south pole situated some 1500km to the south. At this time in the Late Carboniferous (about 300 Ma), the Sydney Basin proper did not exist, but valley glaciers and streams delivered coarse sands and conglomerates from the west to the east, across the terrain which was to become the Sydney Basin. Subsequently, in the Early Permian (about 280 Ma) the eastern margins of the Lachlan Fold Belt began to subside and the deposition of sediments began on a shallow, broad continental shelf, in a cold water, marine environment. These widespread deposits of sandstones and siltstones became the Shoalhaven Group strata, with sands of the Snapper Point Formation derived from the Lachlan Fold Belt sediments to the west. The Permian and Triassic sequence on the western margin of the basin records the westward spread of the Permian sea and its later retreat, leading to the onset of coal swamp sedimentation. This phase of deposition ended as it was overwhelmed by coarser sediments from the north and northwest and this is dramatically evident in the bold sandstone cliff-lines of the western Blue Mountains. To the east the overlying Hawkesbury Sandstone provides evidence of the change of sediment source area as sands were derived from the south and south-west. Permian and Triassic sedimentation then continued for over 50 Million years as described later in this article, with major volcanic events such as the Jurassic diatremes (about 175 Ma) and the Miocene basalt flows (about 16 Ma) occurring much later in the geological history of the area. 4 Figure 2 – Geological Time Scale – age of base of unit in millions of years (Ma). Geomorphology and Structure: Originally called mountains by the colonists, the Blue Mountains are in fact a dissected uplifted plateau. Gentle uplift and warping together with some (relatively minor) faulting has exposed the area to some millions of years of erosion by water and given the “mountains” their spectacular gorges and cliffs we admire today. Charles Darwin (1809-1882) crossed the area via Cox’s Road in 1836 and speculated on the origin of the vast canyons and gorges as he viewed the scene southwards from near “The Weatherboard” – present day Wentworth Falls. He invoked the ocean as the powerful erosive force to carve out these valleys. However, it was Charles Wilkinson (1843-1891, the first Director of the Geological Survey of NSW), who first put forward the correct explanation, which is that the gorges have been eroded over geological time by the streams we see today (Pickett & Alder, 1997). As the Blue Mountains represent the stable western margin of the Sydney Basin (Branagan, Herbert and Langford-Smith, 1976), obvious structures are rare, most being expressed by gentle warps or monoclines, generally oriented north-south. The most obvious is the Lapstone Monocline (fig3 - now Lapstone Structural Complex) which forms the eastern boundary of 5 the plateau. Other less obvious structures, to the west but parallel to the Lapstone structure, include the Mt Tomah Monocline and (further west again) the Bodington Monocline (Holland, 1974). It appears that these structures have been intermittently active since the time of Permian sedimentation as borehole data indicate that the sediments thicken to the east across the structural trend. The Mt Tomah and Bodington Monoclines are responsible for the topographic division of the plateau into what we term today the “lower Blue Mountains” and the “upper Blue Mountains”. While these are simple monoclines, the Lapstone structure (fig 3) is a complex of steeply west dipping reverse faults and monoclinal flexures (Herbert, 1989). There is still considerable debate as to the age and timing of development of this structure. However, it appears that much of the activity on this major structure took place in the early Tertiary (Fergusson et al., 2011), and thus the uplift of the plateau appears to date from this period. Recent palaeomagnetic studies (Schmidt et al., 1995) have suggested that most of the movement is younger than 90Ma (mid-Cretaceous). An even earlier initiation of the Lapstone Monocline is proposed by Pickett and Bishop (1992), who suggest a pre-Jurassic age for the major movements on this structure. It appears the jury is still out on this one. The Lapstone complex (fig 3) is believed to be controlled by re-activation of faults in the basement rocks beneath the Sydney Basin. Evidence for movement on these old structures can be seen close to the western margin of the basin. For example, faults exposed in the Triassic Mt York Claystone at the top of Victoria Pass (also to be seen along Berghofer’s Pass) indicate post-Triassic movement on a basement structure which runs north-south in this area and is largely responsible for the straight course of Lawson’s Long Alley. The basement fault can be seen on satellite photos and is expressed as closely spaced joints and fractures on the sandstone plateau surface. These fracture zones have been known to cause considerable problems in underground coal mines in the region. 6 The multiple faults in the Mt York Claystone near the top of Victoria Pass are and expression of the underlying basement fracture zone, which has been active in post-Triassic times – in the lower photo the small faults or fractures have been outlined in black. (CSB). 7 Figure 3 – Lapstone ‘Monocline’ Structure, showing the Rickabys Creek high level gravels, of probable Miocene age. (adapted from Branagan & Packham, 2000). 8 Rock Units and their Origins: Along the stable western margin of the Sydney Basin, Permian sediments rest unconformably on the basement rocks of the Lachlan Fold Belt and the Bathurst/Kanimbla granites. The granite is well exposed in the Hartley and Lett River area, while older folded sediments can be seen in the road cuttings near Mt Lambie. Figure 4 – The geological strata (oldest at the base) in the Blue Mountains and the likely environments in which they formed (covering a time span of approx.