Port Phillip Bay Survey 1957–1963. Bottom sediments

Field: Earth and Planetary Sciences (Physical Sciences)

A survej ni the bottom sediments of Port Phillip Bay was undertaken primarily to provide basic data for use in studying the relationships between sedimenl composition and the occurrence of marine animals and plains.Particle size composition is the principal sedimenl character studied.Three textural classifications (grade, predominant traction and textural (lass) are given For each sample, and a map showing the graphic distribution ol the various textural classes is presented,The bottom sediments of Porl Phillip Bay are chiefly sands, silty sands, silly clays and clays.The floor ol the extensive area enclosed within the 10-fathom line in the central pari ol the Baj consists mainly of silly clay in its northern half and of clay in its southern half, Ofl the eastern shore of the Bay, oul to the 6 fathom line, the bottom is mainly sand; and the floor of the Nepean Bay Bar is almosl entirely sand.Westwards of the 10 fathom line to the north wesl shore of the Bay, sediments of liner cram size are more widespread than ofl the eastern shore oul to the 10-fathom line.The bottom sediments oi Geelong Outer Harbour and Inner Harbour (Corio Bay) are chiefly silty claj and clay.Marine skeletal material constitutes a large portion of the sediments In some localities, hi^h prevalence being associated usually with the coarser-grained sediments and relatively Shallow water, Hie mineral and tuck contents of the samples are discussed.In all excepl one sand sample, the predominant detrital mineral is quartz,Rock fragments are present in many sand and silty sand samples obtained near the shore, as well as in some samples collected several miles from the shore.The bottom sediments are considered to have been derived in part from the reworking of detritus originally deposited by rivers before the Bay was Hooded by the sea aboul 6,000 or 7,o(in years ago, and in part from detritus derived since thai time from the rivers discharging into the Bay and from coastal and bottom erosion. INTRODUCTION.Port Phillip Bay is an almost completely land-locked body of water, with its only opening to the sea at Port Phillip Heads which are 1 '/," miles apart.It is about 31 miles long from north to south, and 20 miles wide at the middle, where on the west side an arm (Western Arm) extends W.S.W. for 154 milt ' s t0 Geelong.Port Phillip has a tidal area of 725 square miles, and its water depths range generally from 5 to 13 fathoms, apart from a large area of shoal water in the southern part of the Hay.The latter area constitutes the submerged land surface known as the Nepean Hay Bar (Keble, 1946), which occupies the region south of a line from Rosebud on the Nepean Peninsula to St. Leonards on the Bellarine Peninsula (see Figure 2).Through the shoals a number of channels and tideways run to The Heads.That part of Port Phillip north of the Nepean Hay Bar has been named the Inner Basin (Keble, 1946).Within it there is a large area where water depths range between 10 and 13 fathoms; and, in the central part of the Hay, an extensive; area has an approximately level floor at 78 feet.