The youngest known Cambrian fossils from within the Mount Read Volcanics, western Tasmania

Research Article| March 01, 2015 High-Precision U-Pb Zircon Chronostratigraphy of the Mount Read Volcanic Belt in Western Tasmania, Australia: Implications for VHMS Deposit Formation James K. Mortensen; James K. Mortensen † 1Pacific Centre for Isotopic and Geochemical Research, Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada V6T 1Z4 †Corresponding author: e-mail, jmortensen@eos.ubc.ca Search for other works by this author on: GSW Google Scholar J. Bruce Gemmell; J. Bruce Gemmell 2CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Andrew W. McNeill; Andrew W. McNeill 2CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Richard M. Friedman Richard M. Friedman 1Pacific Centre for Isotopic and Geochemical Research, Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada V6T 1Z4 Search for other works by this author on: GSW Google Scholar Economic Geology (2015) 110 (2): 445–468. https://doi.org/10.2113/econgeo.110.2.445 Article history received: 20 Aug 2012 accepted: 19 Jun 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James K. Mortensen, J. Bruce Gemmell, Andrew W. McNeill, Richard M. Friedman; High-Precision U-Pb Zircon Chronostratigraphy of the Mount Read Volcanic Belt in Western Tasmania, Australia: Implications for VHMS Deposit Formation. Economic Geology 2015;; 110 (2): 445–468. doi: https://doi.org/10.2113/econgeo.110.2.445 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Middle Cambrian Mount Read Volcanics of western Tasmania, Australia, host several world-class volcanic-hosted massive sulfide (VHMS) deposits, representing a wide range of deposit styles. Although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures, rapid lateral and vertical facies changes, and faults with uncertain sense and magnitude of displacement, have made it impossible to correlate stratigraphy across the belt. Previous dating studies in the area have yielded relatively imprecise crystallization ages. We have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy (ID-TIMS) U-Pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the Mount Read Volcanics and underlying mafic-ultramafic complexes. The new data permit detailed resolution of age relationships within the belt. The study establishes an age of 516.0 ± 0.9 Ma for the McIvor Hill gabbro, which is part of a mafic-ultramafic complex interpreted to underlie the Mount Read Volcanics. Magmatism in the central Mount Read Volcanics lasted at least 12.7 m.y., from 506.8 ± 1.0 Ma for a massive dacite unit in the lower part of the Central Volcanic Complex to 496.0 ± 0.9 Ma for a welded ignimbrite in the lower Tyndall Group. Together with previous age constraints, results of the study provide a precise chronostratigraphic framework for magmatism and VHMS deposit formation within the Mount Read Volcanics. The precise age data indicate that, north of the Henty fault, magmatism occurred in three discrete pulses, at least two of which were separated by periods of sedimentation. We demonstrate that VHMS deposits in the Mount Lyell, Roseberry-Hercules, and Que-Hellyer districts, comprising the majority of the known, significant VHMS deposits in the belt, formed within a narrow time interval at ~500 ± 1 Ma, at a relatively late stage in the evolution of the belt. Some of the larger intrusions in the belt (e.g., the Bonds Range porphyry; 500.4 ± 0.8 Ma) were emplaced contemporaneously with VHMS deposit formation; however, other bodies such as the Murchison granite (497.3 ± 0.9 Ma) are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested. These conclusions will aid explorers in targeting VHMS mineralization in the Mount Read Volcanics. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A Cambrian Series 3 (Guzhangian, Boomerangian) fauna is described from Core 12 of the Gidgealpa 1 drillhole from the Warburton Basin, northeastern South Australia. There are four agnostoid taxa: Ammagnostus laiwuensis, Lejopyge calva, L. armata and an unassigned pygidium, and six polymerid trilobite taxa: Fuchouia sp., Dorypyge sp., Solenoparia changi sp. nov., Pianaspis sp., an unassigned cranidium and an unassigned pygidium. This fauna suggests a correlation with the Goniagnostus nathorsti Zone or the lower part of the Lejopyge laevigata Zone (Boomerangian on the northern Australian biostratigraphic scheme), with the latter preferred. These are the first published descriptions of fossils of this age from the Warburton Basin. Xiaowen Sun [Xiaowen_s@yahoo.com.au], Sun Petroleum Geoservices, 4 Jolie Grove, Box Hill, Victoria 3128, Australia; Christopher J. Bentley [ bigfossil@bigpond.com], 30 Albert Street, Clare, South Australia, 5453, Australia; James B. Jago [jim.jago@unisa.edu.au], University of South Australia–STEM, Mawson Lakes, South Australia 5095, Australia.

Guzhangian (late middle Cambrian) agnostoids and trilobites are described from near Riana, northwestern Tasmania. Five agnostoid and five trilobite species are documented. The agnostoids include a new species, which is the oldest known representative of the genus Aspidagnostus. The trilobites include Monkaspidae indet, which probably represents a new genus. The librigena of the species described as cf. Penarosa sp. appears to be unique in that it has a segmented genal spine and possesses closely spaced small, stout spines on its margins. The age of the fauna is probably within the range of Lejopyge laevigata II Zone to lower Acmarhachis quasivespa Zone, which makes it the youngest known Cambrian fauna in the Dial Range Trough. Christopher J. Bentley [bigfossil@bigpond.com]; James B. Jago* [jim.jago@unisa.edu.au], University of South Australia-STEM, Mawson Lakes, South Australia 5095, Australia.

Jago, J.B., Bentley, C.J., Laurie, J.R. & Corbett, K.B., 26 June 2018. Some middle and late Cambrian trilobites and brachiopods from the Adamsfield Trough, Tasmania. Alcheringa 43, 1-17. ISSN 0311-5518.Cambrian Series 3 and Furongian trilobites and brachiopods are described from the Adamsfield Trough in southwestern Tasmania. The oldest fossils are very poorly preserved trilobites, assigned to Asaphiscidae gen. et sp. indet. from within the Island Road Formation a short distance above the unconformity with the underlying Proterozoic Wedge River Beds. A trilobite species from within the isolated Boyd River Formation is referred to Lioparia sp. The Island Road Formation and the Boyd River Formation are stratigraphically equivalent to the Trial Ridge Beds which have previously been dated as belonging to the Lejopyge laevigata Zone. The Trial Ridge Beds are overlain unconformably by the Singing Creek Formation. In the Adamsfield, Clear Hill and Stepped Hills areas, stratigraphic equivalents of the Singing Creek Formation collectively contain the trilobites Pseudaphelaspis sp., Pseudaphelaspis? sp., Prochuangia sp., Mindycrusta sp., Nepeidae gen. et sp. indet., and Olenidae gen. et sp. indet. plus the brachiopods described herein as Billingsella sp. aff. costata, Billingsella sp. A, Billingsella sp. B and a possible member of the Billingselloidea. The Singing Creek Formation has been previously correlated with the Stigmatoa diloma Zone. The genus Lotosoides Shergold 1975 is placed in synonymy with Prochuangia Kobayashi 1935.James B. Jago* [jim.jago@unisa.edu.au] University of South Australia, School of Natural and Built Environment, Mawson Lakes, SA 5095, Australia; Christopher J. Bentley [bigfossil@bigpond.com] 30 Albert Street, Clare, SA 5453, Australia; John R. Laurie [john.r.laurie@gmail.com] Department of Biological Sciences, Macquarie University, NSW 2109, Australia; Keith D. Corbett [keith.corbett@bigpond.com] 35 Pillinger Drive, Fern Tree, Tas 7054, Australia.

In Tasmania shelly fossils are known from Middle and Upper Cambrian sediments of the Dundas Trough, Fossey Mountain Trough, Dial Range Trough, Beaconsfield Trough, Smithton Basin, Adamsfield Trough and from within sediments associated with the Mount Read Volcanics of Western Tasmania. In the Dundas Trough fossils range in age from early Middle Cambrian (Ptychagnostus gibbus Zone) to the middle Late Cambrian (pre‐Payntonian A or B). Late Middle Cambrian fossils occur in sediments associated with the Mount Read Volcanics in two places in Western Tasmania. Late Middle Cambrian fossils only are known from the Smithton Basin and the Beaconsfield Trough. Late Middle to early Late Cambrian faunas are known from the Dial Range Trough; the Adamsfield Trough contains middle Middle to middle Late Cambrian fossils. Tasmanian Cambrian faunas show affinities with those of Queensland, China, the northwest Siberian Platform and northern Victoria Land, Antarctica.

Volcanic-hosted massive sulfide (VHMS) deposits from western Tasmania have significantly higher Au grades than other VHMS deposits from eastern Australia and elsewhere. Huston and Large (1989) have suggested that the dominant controls on Au grades in VHMS deposits are probably temperature and pH of the hydrothermal fluids. However, the concentration of Au in the source rocks being leached by the hydrothermal fluids may also affect the final Au grades in the deposit (e.g., Keays, 1987). The major VHMS deposits of western Tasmania are hosted within the Central Volcanic Complex of the Mount Read Volcanics or overlying andesites of the Que-Hellyer sequence. Other potential source rocks for leaching of metals include late Proterozoic basalts and sediments of the Crimson Creek Formation, ultramafic-mafic rocks from a tectonically emplaced ophiolite complex, and Precambrian schists and quartzites.The background Au concentrations in the least altered Mount Read Volcanics (mainly andesites, dacites, and rhyolites) are in the range 0.9 to 1.3 ppb Au, and the mean values are comparable with those of unaltered modern volcanics of similar composition. Hydrothermally altered equivalents of these volcanics generally have somewhat higher Au concentrations. The Tasmanian ultramafic rocks have significantly lower Au (mean 0.5 ppb) than unaltered periodotite xenoliths, but similar concentrations to ophiolitic ultramafic rocks. High Mg, low Ti volcanics from the western Tasmanian ultramafic-mafic complexes have Au contents (mean 1.6 ppb) similar to the Mount Read mafic rocks. In contrast, relatively high Ti basalts of the Crimson Creek Formation are the most Au-enriched primary volcanics in the area (up to 23 ppb Au). These volcanics were probably erupted during an early rifting phase and may underlie much of the Mount Read Volcanics.Source-rock modeling calculations, based on average Zn concentrations of potential source rocks, suggest that about 70 km 3 of andesitic volcanics would have to be leached to provide the metal content of the Hellyer VHMS deposit. Similar calculations based on Au concentrations suggest somewhat larger leach volumes. Both estimates suggest that some leaching of basement rocks may have occurred during the formation of the VHMS deposits. The relatively high Au concentrations in the Crimson Creek basalts indicate the presence of an anomalous Au-enriched source rock beneath the Mount Read Volcanics which may have contributed to the relatively Au-rich character of the Tasmanian VHMS deposits. However, the temperature, composition (e.g., PH, f (sub O 2 ) , a (sub H 2 S) , m Nacl ), and the consequent solubility of metal species in the hydrothermal fluids will be the major factors which determine the final concentrations of base and precious metals in a specific massive sulfide deposit. Source-rock composition will only be important if the hydrothermal fluid has an appropriate chemistry to allow significant Au solubility.

Late Cambrian (Iverian, Jiangshanian) agnostoids and trilobites are described from two localities in the Professor Range area of western Tasmania. The fossils occur within a thick folded flysch succession of siltstone, mudstone, quartzwacke sandstone and siliceous conglomerate that forms the lower part of the Owen Group correlatives in this area. The flysch succession has a conformable and probable gradational contact with the underlying Mt Read Volcanics and is abruptly overlain by the siliceous pebble-cobble conglomerate correlated with the Middle Owen Conglomerate of the West Coast Range. The stratigraphically lower southern locality contains the agnostoids Pseudagnostus sp. and Agnostinae gen. et sp. indet. plus the polymerid trilobites Parabolina sp., Hedinaspis sp. cf. H. regalis and Eugonocare sp. The northern locality contains the agnostoids Rhaptagnostus sp. and Pseudagnostus sp. plus the polymerid trilobites Hedinaspis sp. cf. H. regalis, Ketyna? sp. 1, Ketyna? sp. 2, and Cermatops sp. It is possible that most specimens of the cosmopolitan genus Hedinaspis belong in the type species, H. regalis.Christopher J. Bentley [bigfossil@bigpond.com], 30 Albert Street, Clare SA 5453; James B. Jago [jim.jago@unisa.edu.au], University of South Australia, School of Natural and Built Environment, Mawson Lakes, SA 5095; Keith D. Corbett [keith.corbett@bigpond.com], 35 Pillinger Drive, Fern Tree Tas 7054, Australia.

The Middle-Upper Cambrian transition in southeastern Newfoundland is shown to be very similar to the transition in Scandinavia and in the English Midlands. The late Middle Cambrian Lejopyge laevigata Zone, which is recorded for the first time from eastern Canada, is contained in the basal part of the Elliott Cove Formation and is conformably overlain by the Upper Cambrian Agnostus pisiformis Zone. The following trilobite species from the Lejopyge laevigata Zone at Manuels River and on Random Island are described: Andrarina costata (Angelin), Paradoxides sp., Lejopyge laevigata (Dalman), and Peronopsis insignis (Wallerius).

The late Middle Cambrian Mount Read Volcanics, Tasmania, Australia, comprise submarine rhyolite to basalt lavas, intrusions, and volcaniclastic rocks which host significant polymetallic massive sulfide deposits. Isotopic U-Pb and 40 Ar/ 39 Ar dating of magmatic zircons and hornblendes, respectively, from different parts of the Mount Read Volcanics yields a concordant age of 502.6 + or - 3.5 Ma (1Sigma ), which constrains the time of volcanogenic massive sulfide mineralization. The Comstock Tuff from the Tyndall Group in the upper part of the volcanic sequence and the Mount Black Dacite from the Central Volcanic Complex have weighted mean 206 Pb/ 238 U ages of 494.4 + or - 3.8 Ma and 494.9 + or - 4.3 Ma, respectively, and may be slightly younger than the other rocks. Fauna in the Mount Read Volcanics is diagnostic of a late Middle Cambrian (middle to late Menevian stage) age, and the isotopic dates therefore imply that the Late Cambrian is younger than 500 Ma. Inherited zircons in the Mount Read Volcanics have ages similar to those found in granites and sediments in the Tasman fold belt, southeastern Australia, which suggests similar source material for the two areas.

The stratigraphically lowest known agnostoids and trilobites from both the Dial Range Trough of northwest Tasmania and from western Tasmania are described. Those from the Dial Range Trough occur in the Isandula Road area and comprise the agnostoid Acadagnostus cf. rakuroensis, and the trilobites Pagetia sp., Nepea cf. nans and Paibianomocare? sp., thus suggesting a range between the Triplagnostus gibbus and Euagnostus opimus zones. Loxonepea is placed in synonymy with Nepea. The Sticht Range Beds of western Tasmania contain only two poorly preserved trilobite specimens referred to Anomocarellidae gen. et sp. indet.; a definite age cannot be obtained from this material. James B. Jago [jim.jago@unisa.edu.au], University of South Australia—STEM, Mawson Lakes, South Australia, 5095, Australia Christopher J. Bentley [bigfossil@bigpond.com], 30 Albert Street, Clare, South Australia, 5453, Australia

The Middle to Late Cambrian Mount Read Volcanics, western Tasmania, comprise compositionally and texturally diverse lavas and volcaniclastic rocks, most of which were emplaced in submarine environments below wave base. The facies architecture reflects the contrasting character and geometry of primary volcanic and volcaniclastic facies which are strongly controlled by eruption style and eraplacement processes. In the northern Mount Read Volcanics the principal volcanic facies are (1) silicic, intermediate, and mafic lavas; (2) juvenile volcaniclastic deposits, generated in association with the extrusion of lava flows, or else produced by explosive eruptions; and (3) synvolcanic intrusions involving silicic and mafic magmas emplaced into unconsolidated host sequences. Distribution and lithofacies characteristics suggest that deposits from both intrabasinal submarine and extrabasinal subaerial and/or shallow-marine volcanic centers are present. The volcanic facies are interbedded with a sedimentary facies association comprising black mudstone and/or graded bedded sandstone of mixed volcanic and Precambrian basement provenance. The predicted facies geometry of the principal volcanic facies has guided our approach to correlation in the Mount Read Volcanics. Mass-fiow-emplaced pumiceous volcaniclastic facies provide the best framework for correlation, because these are produced in large volumes, erupted infrequently, eraplaced rapidly, and are widely distributed. Distinctive units of this type that occur near Hellyer (in the Southwell Subgroup) and 40 km to the south at White Spur-Howards Road (White Spur Formation) could each be part of the same regionally extensive volcaniclastic facies association. A similar volcaniclastic facies association hosts the Hercules and Rosebery massive sulfide deposits and clearly demonstrates that such associations are prospective for this style of mineralization.

Comparison of the 100 Zn/(Zn + Pb) ratio (zinc ratio) of various styles of mineralization in the Mount Read Volcanics of Tasmania indicates that volcanogenic massive sulfide deposits have a restricted range of mean values (60-77) and low standard deviations (less than 15) whereas other mineralization styles have a broader but lower range of mean values (39-61) and higher standard deviations (greater than 26).Thermochemical calculations on lead and zinc transport in saturated ore solutions show that the solution zinc ratio is controlled only by temperature and salinity and is independent of pH, f (sub O 2 ) , and activity of dissolved sulfur. Zinc ratios calculated for saturated ore solutions at conditions typical of the deposition of sphalerite and galena in massive sulfide deposits (250 degrees -150 degrees C and 0.5-1.5 m NaCl) are very similar to those observed in polymetallic massive sulfide deposits, in particular those of the Rosebery, Hercules, and Hellyer deposits in western Tasmania. This similarity, along with the shape of the Pb vs. Zn plots, suggests that zinc and lead (as chloride complexes) were saturated in the metal-deficient (m (sub Sigma metal) < m (sub H 2 S) ) hydrothermal fluid that formed the Rosebery and Hellyer ores.Variations in the zinc ratio between individual massive sulfide deposits in the Mount Read Volcanics indicate that lead-zinc saturation of the ore solution rather than lead-zinc content in the footwall volcanic source rocks was the major control on the zinc ratio of the deposits. However, this is probably not the case for the Pb-poor, Cu-Zn-type massive sulfides (e.g., Cyprus deposits, Canadian Archean deposits, and present-day black smokers), where low concentrations of lead in the basalt-dominated source rocks result in complete undersaturation of lead in the hydrothermal solutions. The low mean values and high standard deviations of Cambrian vein deposits in the Mount Read Volcanics, and in Ordovician and Devonian deposits of the Dundas trough, may be explained by a lack of zinc-lead saturation in the relevant hydrothermal fluids.The characteristic zinc ratio of volcanogenic massive sulfide-style mineralization can be used to distinguish these deposits from other mineralization types in the Mount Read Volcanics. Effective use of this simple technique requires a reasonable number of anomalous samples (minimum = 15) and an unoxidized sample medium (drill core samples are preferable). In combination with geology and lead isotope studies, the zinc ratio provides an excellent early screening of prospects.

The only known Cambrian fauna of the Beaconsfield Trough has a late Middle Cambrian (PtychQgnostus nathorsti or Lejopyge laevigata Zone) age; it occurs 4.5 km south-west of Beaconsfield, northern Tasmania. The poorly preserved fauna includes an orthid brachiopod, plus the trilobites Nepea sp., cf. Erediaspis sp., a probable member of the Damesellidae, a possible solenopleurid, agnostoids and some fragmentary material. The fauna occurs about 140m below the contact with the overlying Tremadocian or Arenigian Cabbage Tree Formation, which suggests that most, if not all, of the Late Cambrian is not represented in the Beaconsfield area.

Volcanogenic massive sulfide deposits in eastern Australia range from less than 0.1 to 110 million metric tons; the median is 2.7 million metric tons. Of the 42 known deposits, 19 are in the Cambrian Mount Read Volcanics of Tasmania: these make up 62 percent of the total tonnage. The major types of deposits in the Mount Read Volcanics are: (1) lead-zinc-rich polymetallic (Pb-Zn-Cu-Ag-Au) ore depositsuch as Rosebery, Hercules, Que River, and Hellyer, and (2) copper-rich massive and disseminated ore depositsuch as Prince Lyell and North Lyell. The polymetallic ores average 21 percent combined Pb + Zn plus 160 g/metric ton Ag and 3 g/metric ton Au and the mean size is about 10 million metric tons; therefore, they represent an extremely attractive exploration target. Mineral exploration in the remote, and previously little-explored, southern part of the Mount Read Volcanics at Elliott Bay has revealed a geologic setting and mineralization styles similar to those in the main part of the volcanic belt between Mount Darwin and Que River. The volcanic pile at Elliott Bay is composedominantly of rhyolitic pyroclastics and quartz porphyry intrusions overlain on the western side by a sequence of epiclastics and marine sediments. The volcanics are dominantly quartz porphyritic whereas feldspar porphyritic assemblages are dominant in the central volcanic sequence which hosts the polymetallic massive sulfide ores to the north. A combination of geologic mapping (with particular emphasis on alteration styles), gold distribution in stream sediments and pan concentrates, and base metal soil geochemistry has proved successful in locating several types of mineralization in various horizons of the volcanic pile. The two major mineralization types, stratiform lead-zinc-silver massive sulfide lenses and strata-bound low-grade gold-carbonate deposits, occur toward the top of the rhyolitic pyroclastic pile 200 to 1,000 m below the contact with the western epiclastic sequence. Formation of the massive sulfide lenses is considered to relate to hydrothermal circulation along fault structures that are interpreted as marginal rift faults which developed early in the formation of the volcanic pile. The strata-bound gold and associated vein-style and disseminated galena-sphalerite occur with high-level quartz porphyry bodies and granitoids emplaced during the final phases of volcanic activity.

The distribution of secondary mineral assemblages in six mineralized areas of the Mount Read Volcanics defines a district-wide zonation which is subconformable with massive sulfide host horizons and the contacts of sill-like granitoids where present. The relationships between alteration patterns and elements of Cambrian geology strongly suggest that the alteration is related to Cambrian hydrothermal circulation responsible for massive sulfide mineralization. There is a later greenschist facies metamorphic overprint. The major components of the principal assemblages are: quartz + albite + chlorite + epidote; quartz + albite + chlorite + calcite; quartz + sericite + chlorite; and quartz + K-feldspar + chlorite. Several complex assemblages involving the above minerals; biotite, magnetite, pyrite, and tourmaline are characteristic of granite contacts. Secondary biotite is also present sporadically in the Rosebery area, probably related to Devonian granitoids at depth. There is a districtwide zonation of sodic + or - calcic versus nonsodic, noncalcic assemblages in felsic volcanic rocks. In preore volcanics, nonsodic, noncalcic assemblages occur immediately beneath the host horizon and may persist several hundred meters before passing into a zone, including granite contacts, where calcium minerals and albite occur sporadically. The nonsodic, noncalcic assemblages are predominantly sericitic, but assemblages with prominent K-feldspar occur locally. In postore volcanics, sericitic non-sodic, noncalcic assemblages may persist up to 200 m above the ore horizon before passing into relatively fresh volcanics with common albite and calcic greenschist minerals. Preore assemblages probably largely reflect downward circulation. Upwelling zones are characterized by intense sericite + chlorite or chlorite development with or without K-feldspar; lateral spreading of solutions immediately beneath the ore horizon is common.Chlorite compositions preserve local patterns due to Cambrian hydrothermal processes, but they do not vary systematically with alteration assemblages throughout the province. Chlorite Mg to Fe ratios appear to have been preserved through metamorphism. A combination of structural and alteration data is useful to mapping in the Mount Read Volcanics. This approach indicates that most syngenetic deposits and sedimentary lenses in the Rosebery-Hercules area may lie on a single host horizon and suggests a continuity of geology from Mount Farrell through Red Hills to Mount Sedgwick.