Shallow-water turbidites: An example from the Mallacoota Beds, Australia

Abstract

A Late Ordovician turbidite sequence (the Mallacoota Beds) exhibits vertical lithofacies associations that suggest deposition in a “fan-like” environment. However, there are also features indicating occasional, episodic reworking by water currents. Reworking features include both wave-current and unidirectional ripple pavements, sharp-topped turbidites, a difference in current direction between superimposed current rippling and underlying Bouma C division ripples, lensiodal quartzites and occasional HCS beds. Comparisons between the Mallacoota Beds and the depositional products of a variety of open-ocean currents would indicate that storm-related currents were the most likely reworking mechanism. This would indicate that the sequence was deposited within storm wave base. Depositional depths for part of the sequence are estimated at 90?–200 m based on a wave-generated-ripple model. Therefore, these turbidites may have been produced by storm surge with channeling within the turbidites due to the localisation of rip currents during hurricane conditions. A generalised model for the characteristic features of turbidites deposited between fair-weather wave-base and storm wave-base is presented. It is suggested that such turbidites will contain only occasional examples of reworking and that the frequency of such reworking events per stratigraphic interval might be used as a relative measure of water depth throughout a sequence. Moreover, those turbidite sequences that have sharp contacts between the sandstones and the overlying mudstones may have been modified by storms and therefore deposited in shallower water than previously thought.