Systematics, palaeoenvironments and stratigraphy of the Silurian trilobite Dalmanites wandongensis Gill, 1948 and its bearing on the structural geology of the Kilmore area, Victoria

Abstract

One of the most prolific trilobites in the mid-Palaeozoic of central Victoria is the dalmanitid Dalmanites wandongensis Gill, 1948. Extensive museum collections permit a revised diagnosis and a redescription of the species, and an interpretation of its distribution. The affinities of D. wandongensis lie with South American species assigned to Dalmanites and the closely related Kasachstania rather than northern hemisphere taxa including the D. caudata group. It occurs between Heathcote and Yan Yean in central Victoria, in late Wenlock and early Ludlow (Silurian) strata of the Dargile Group, on the eastern margins of the Melbourne Zone; its greatest abundance is between Kilmore and Wandong, where it is documented from over fifty localities, occurring rarely with other trilobite taxa. At Heathcote, Upper Plenty, Yan Yean and Kilmore East D. wandongensis stratigraphically underlies a more diverse trilobite fauna dominated by the encrinurid Cromus simpliciculus. This trilobite faunal succession defines a significant macrofaunal biostratigraphic datum in the sequence, complementing brachiopod and graptolite zonations. This datum is coincident with a subtle but widespread change in lithology that defines the lithostratigraphic boundary between the Yan Yean Formation and the Melbourne Formation. In the structurally complex and poorly exposed sequences of the Kilmore East area the distribution of D. wandongensis is used as a primary tool in mapping the distribution of these units. Outcrop patterns reveal the presence of two large scale east-northeast-oriented faults traversing the area that support a model of deformation for the Melbourne Zone in which transverse shearing is significant. Taphofacies and biofacies distribution in the Yan Yean Formation indicate a deep-water shelf depositional environment, at depths below maximum storm-wave base in the Kilmore-Wandong area, deepening eastwards, with outer shelf environments between Yan Yean and Christmas Hills.