Fan deltaic‐to‐fluvial sedimentation of the Middle Jurassic Murihiku Terrane, Southland, New Zealand

Abstract

New results on the stratigraphy and sedimentary environment of the Middle Jurassic strata of the Murihiku Terrane in the Waikawa district, Southland, New Zealand, are presented. This area has not been subjected to study for a long time, although the Murihiku Terrane is important to understand the Mesozoic geology of New Zealand. Stratigraphically, from bottom to top, the sequence consists of lithofacies associations A, B, C, and D, distinguished by their lithology. Lithofacies association A (250 m thick) is composed of boulder‐grade conglomerate, coarse‐grained massive sandstone, trough cross‐stratified sandstone, and minor fine‐grained sediments. Lithofacies association B (90 m thick) contains alternating beds of sandstone and siltstone and fossil‐bearing massive siltstone. Lithofacies association C (280 m) consists of poorly to moderately sorted pebble‐cobble conglomerates and trough or planar cross‐stratified sandstones. The characteristic lithofacies of lithofacies association D (>140 m) are planar cross‐stratified sandstone, planar horizontally stratified sandstone, grey siltstone, and carbonaceous beds. The depositional age was deduced from bivalve fossils in lithofacies association B, which indicate a Middle Jurassic (Temaikan: Bajocian‐Callovian) age. Paleocurrent flow directions were from the southwest in lithofacies association A and from the southeast in lithofacies associations C and D. Clast compositions also differed between lithofacies associations A and C; andesite volcanic clasts were dominant in A, and rhyolite volcanic clasts were dominant in C. The major sedimentary environments were terrestrial and shallow marine and were affected by sea‐level changes. The sediments of lithofacies association B were deposited during periods of transgression and high sea level, and the boundary between lithofacies associations B and C is a depositional‐sequence‐bounding unconformity. Fluvial sedimentation became dominant in lithofacies associations C and D. Due to progressive aggradation of the sedimentary basin, the gradient became gentle, and the fluvial pattern changed from a braided to a meandering river. The sediment source, as determined from paleocurrent analyses and clast compositions, was also different from that of lithofacies association A. Thus, the Waikawa district sediments apparently were derived from multiple sources. The provenance of the Murihiku Terrane sediments was probably a primitive volcanic or continental margin arc. Volcanic activity in the hinterland fed large amounts of volcanic detritus to the region by flood, debris flow, and fluvial transport.