Volume 86 Part 3

Journal of theSemeniuk: Royal Society Holocene of Western sedimentation Australia, and Supplement stratigraphy, to Volume Canning 91Coast Part 1: 53-148, 2008 Holocene sedimentation, stratigraphy, biostratigraphy, and history of the Canning Coast, north-western Australia V Semeniuk V & C Semeniuk Research Group 21 Glenmere Road, Warwick, W.A., 6024 Manuscript received December 2005; accepted December 2006 Abstract Holocene environments along the Canning Coast are of four types: (1) rocky shores of Mesozoic to Tertiary rock and semi-indurated desert sand dunes; (2) rocky shores of Quaternary limestone; (3) sand-dominated flats and beaches/dunes along open coasts; and (4) protected mud-dominated embayments, lagoons and bays. Sediments vary markedly in origin, grainsize (boulders to mud), and composition (quartz sand, shells and their fragments, foraminifera, ooids, beachrock intraclasts, lithoclasts, and carbonate mud). Twelve new Holocene stratigraphic units are described from this region: Shoonta Hill Sand, Eighty Mile Beach Coquina, Cable Beach Sand, Port Smith Sand, Race Course Plains Coquina, Sandfire Calcilutite, Cape Gourdon Formation, Barn Hill Formation, Church Hill Sand, Horsewater Soak Calcarenite, Kennedys Cottage Limestone and Willie Creek Calcarenite. The current depositional nature of most of these units indicates their recent age, but radiocarbon dates confirm the Holocene age for all units, with the oldest date being 7450 14C yrs BP and the youngest 900 14C yrs BP. The three limestones (Willie Creek Calcarenite, Kennedys Cottage Limestone, and Horsewater Soak Calcarenite) and inland occurrences of Sandfire Calcilutite are the oldest Holocene formations in the region. For the contemporary units, the Port Smith Sand generally underlies all unconsolidated Holocene formations. The Sandfire Calcilutite, Cable Beach Sand, and Eighty Mile Beach Coquina are laterally equivalent representing a change from low energy progradational to high energy progradational to high energy dominantly erosional. Shoonta Hill Sand caps those sequences formed under high energy conditions, but also overlies Sandfire Calcilutite where retrograding. The Barn Hill Formation, Church Hill Sand, and Cape Gourdon Formation are developed where coastal erosion cuts into Pleistocene dunes, mixing it with carbonate grains; here, coastal sedimentation involves interactive accretion of beaches, dunes and alluvial fans. Mangroves and fossil molluscs (involving various types of biocoenoses and thanatocoenoses) were used to biostratigraphically characterise the formations, determine depositional environments, and assess the extent of coastal reworking. The mollusc fauna signal that there have been changes in substrates, and the regional stratigraphy points to alternating erosion and deposition and to major substrate changes in tidal zones. This is reflected in the thanatocoenoses on beaches and within polymictic shell gravels of the Eighty Mile Beach Coquina. Radiocarbon dating of stratigraphic interfaces and tide level markers shows MSL was+2m between 7500 and circa 5000 yrs BP, + 1.5 m between 4500 and 2500 yrs BP, and at its present position by 2100 yrs BP. The post-glacial marine transgression flooding into the western margin of the Great Sandy Desert 7500 yrs BP created a complex jagged and locally rocky coastline with numerous embayments that has since been smoothed out. Three types of shore were developed initially: large funnel-shaped embayments, small embayments, and rocks and desert dunes exposed at the coast. Witha+2msealevel, waves entered small embayments more pervasively, and many embayments were sand-dominated, but with accretion of barriers (later to be lithified), and concomitant fall of sea level, there was more effective protection of embayments, and carbonate mud began accumulating in their interiors. For deeply indented coasts, accumulation of mud had been prevailing, but with a falling sea level, the coast prograded and the seafront became more wave exposed, resulting in alternating coastal advance and retreat, forming mud ribbons, barriers, cheniers, and shell concentrates. Keywords: Canning Coast, Canning Basin, Holocene sedimentation, Holocene stratigraphy, Holocene biostratigraphy, sea-level history, Holocene palaeogeography © Royal Society of Western Australia 2008 53 Journal of the Royal Society of Western Australia, Supplement to Volume 91 Part 1, 2008 Introduction sedimentation. It also assists in reconstructing the palaeogeography of the Canning Coast during the The Canning Coast in north-western Australia, Holocene. In a larger multidisciplinary perspective, the fronting, to a large extent, the western margin of the information presented in this paper can form the Great Sandy Desert, and comprising the Quaternary foundation to coastal hydrogeology useful to the study of coastal fringe of the Canning Basin (Semeniuk 1993a; and coastal ecology, the framework for studying mangroves Figure 1), presents a complex of sedimentary in relation to their habitats, a framework to environments and facies that are the result of understanding Holocene middens, and a basis for coastal autochthonous to allochthonous carbonate sand conservation in geoheritage. sedimentation, allochthonous carbonate mud sedimentation, and allochthonous reworking of quartz- There are also a number of Pleistocene stratigraphic dominated desert dunes. As a coastal sector, one of its units along the Canning Coast equivalent to the Holocene characteristic features is the negligible fluvial input from units to be described in this paper; these include the hinterland. As such, coastal sedimentation, where limestone lenses in red/orange sand, indurated (marine) dominated by mud, is composed of carbonate mud. calcilutite, aeolianite, and calcreted shelly calcarenite. The Sedimentologically and compositionally it is a mixed focus in this paper, however, is on the Holocene carbonate and siliciclastic complex. The entire Canning stratigraphic units. The Pleistocene stratigraphic units Coast also has been subject to a variable sea-level history will be the subject of a later report. that has resulted in changes in coastal configuration and Geographic names mentioned throughout the text are hence in sedimentation style. As a consequence, a variety shown in Figure 2. of Holocene stratigraphic units occur along this coastal zone, reflecting variation in sedimentary patterns, accretionary and erosional coastal history, and relative Materials and methods sea level changes. These stratigraphic units have accumulated, and are accumulating, in various shore Regional traverses and transects were undertaken to environments that are generating distinct sedimentary determine types and distribution of stratigraphic units. bodies which, to date, have not been formally described The location of these traverses and transects are shown or defined. in Figure 3A. Stratigraphic data for this paper derive Sedimentary environments along the Canning Coast from four types of field situations: 1. natural cliff occur in a macrotidal setting which varies in tidal range exposures of dunes, limestones, and some muddy from6minthesouth to 10.5 m in the north (Calder 1979; sequences; 2. natural tidal creek bank exposures of Anon 1991). Hence the individual, distinct, tidally-related muddy sequences; 3. coring and augering at many sites; lithosomes are of reasonable consistent lithology and and 4. trenches. Locations of study sites such as cliffs, thickness to be clearly and readily recognisable and auger or drill sites are shown in the various regionally. Since most occur along the entire circa 600 km stratigraphic profiles presented later. length of the coastal zone of the Canning Basin they can Sediments and rocks are described in the field be justifiably assigned formational status. In addition to sequentially from large scale to small-scale in terms of their distinct lithology, the stratigraphic units have been geometry of stratigraphic units, structures, fabric of and currently are related to tidal levels. As such, these sediments, texture, and composition. Samples in the stratigraphic units also are useful as regional palaeo- laboratory were examined under binocular microscope, environmental indicators, and the identification of these and the carbonate nature of whole samples (e.g., muds), Holocene Formations, and their assignment to standard and the carbonate nature of individual grains were coastal sequences, helps unravel the sedimentologic determined by reaction with drops of HCl under the patterns and Holocene coastal history in this part of stereoscopic microscope. Some 100 selected samples of Western Australia, and helps reconstruct Holocene limestone and sand were prepared as thin sections and coastal palaeogeography along the Canning Coast. examined under a petrographic microscope. Selected The objectives of this paper are manifold. They are: to mud samples were examined under a Scanning Electron provide a Quaternary stratigraphic framework for the Microscope (SEM) at the CSIRO laboratories, Kensington. Canning Coast region; describe the coastal settings host Samples of mud were also separated into size fractions of to the Holocene stratigraphic sequences; describe the < 4 µm, 4–20 µm, 20–40 µm, and 40–60 µm for X-ray modern sedimentary environments wherein the various diffraction analyses to determine mud-sized mineralogy. Holocene lithotopes occur and the lithosomes have been Some of these XRD results are reported here, but the generated; describe and formally define the Holocene

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