Trench‐slope channels from the New Zealand Jurassic: the Otekura Formation, Sandy Bay, South Otago

Abstract

The Otekura Formation (Early Jurassic, Pseudaucella zone) at Sandy Bay comprises part of a 10+ km thick, regressive, forearc shelf and slope sequence, the Hokonui facies belt of the Rangitata Geosyncline. The Otekura Formation is dominantly fine grained, being mostly mudstone, silty mudstone and siltstone. The sediments are volcanogenic throughout. The upper 150 m of the formation contains two 20 m thick, channelized bodies of medium‐thick bedded sandy flysch, each associated with thin bedded muddy flysch interpreted as overbank turbidites. Directional indicators within the channel sequence indicate emplacement from the south‐southwest. In contrast, rare turbidites that occur below the channel sequence, within the background mudstone sediment, were emplaced from the east, i.e. at right angles to the channelized flows. The immediately overlying Omaru Formation contains more abundant macrofossils, intraclastic conglomerates, and appreciable amounts of traction‐emplaced cross‐bedded sand. Bioturbated calcareous siltstones with an in situ molluscan fauna follow (Boatlanding Formation), and are of shelf origin. The Omaru Formation is therefore interpreted as a shelf‐slope break deposit, and the Otekura Formation as an upper slope facies. Reconnaissance studies indicate that the Otekura Formation is underlain by several kilometres of dominantly fine grained, deep water slope sediments, containing occasional sand and conglomerate filled channels similar to those here described in detail from the Otekura Formation. Such channels are inferred to form when a mass‐transported sand, derived from failure higher on the slope, ploughs erosively into the sea floor. After their incision, the channels served for a short time as conduits for downslope transport of sediment, the redeposited deposits of which are found filling each channel. Both channel fills at Sandy Bay are capped by thin‐bedded turbidites inferred to have overspilled from similar channels nearby on the slope.

Late Cretaceous to Miocene seamount accretion and mélange formation in the Osa and Burica Peninsulas (Southern Costa Rica): episodic growth of a convergent margin

“…The pelagic-hemipelagic sediments consist of undated siliceous-calcareous hemipelagic oozes intercalated with fine-grained detrital sediments and deformed lenses of early Eocene radiolarian cherts. We relate the hemipelagic oozes to background sedimentation commonly observed in near-trench environments (Carter 1979). The detritic fraction variously expressed in these sediments has a composition pointing toward a predominant basaltic source and a minor differentiated source that may represent an erosion of the palaeo-Osa Igneous Complex and a subaerial volcanic arc, respectively.…”

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

“…The strata in the north limb consist of Middle Triassic to Middle Jurassic slope-shelf deposits (Carter et al 1978;Carter 1979). Shallower water facies occur on the south limb of the Southland Syncline in rocks of equivalent age (Speden 1971;Boles 1974).…”

Willsher Group and geology of the Triassic Kaka Point coastal section, south‐east Otago, New Zealand

“…This contrasts with a southerly to westerly source and corresponding slope direction for sediments of the Southland Syncline in the Murihiku Terrane (e.g., Speden 1971), although Carter (1979) reported cases of transport at right angles to this, from the east, within a 30-m section of the Early Jurassic Otekura Formation.…”