Bed‐scale quantitative discrimination of hyperpycnites from intrabasinal turbidites—Results from a channelised slope system in the Upper Carboniferous Westward Ho! Formation, United Kingdom

ABSTRACT Features considered indicative of hyperpycnites and intrabasinal turbidites overlap. Outcrop study presented here suggests that the Westward Ho! Formation forms an 800 m high deepwater‐slope system dominated by hyperpycnites. Taking this unit, and other successions where hyperpycnites have been described, as having been deposited solely from hyperpycnal flows and comparing them with examples considered to have been deposited exclusively by intrabasinal turbidity currents, has allowed the first quantitative comparison of hyperpycnites and intrabasinal turbidites at the bed scale. In successions classed as composed of hyperpycnites, on average, 54% of beds have features indicative of hyperpycnal flow, whereas in successions classed as intrabasinal turbidites, only 5% do. Most intrabasinal turbidites, are either sharp based (81% of beds), or erosively based (17%), and characterised by either a graded (60%), or a blocky grain‐size profile (35%), with bed style varying little in different sub‐environments. By contrast, most hyperpycnites (52%) are gradationally based, with bigradational coarsening to fining being the dominant (57%) bed‐scale grain‐size signature. Other hyperpycnites have sharp (38%) or erosive (10%) bed bases. Features indicating pulsed flow are rare, but three times more likely in hyperpycnites (10%). Unlike intrabasinal turbidites, sub‐environments in hyperpycnite‐dominated systems display distinct bed‐scale signatures. For example, in the Westward Ho! Formation, slope‐channel axes are dominated by erosive and sharp‐based, graded and blocky sandstone beds, whereas locations that are more off‐axis or distal are increasingly characterised by gradationally based, bigradational beds. Such facies changes are interpreted to record flows that transform from a wax and wane signature indicative of river floods to surge‐style flows in channel axes. Though more data from high‐quality descriptions of hyperpycnites would be useful, the results point to how facies models could be adjusted to emphasise dominant features of each bed type, and may help discriminate hyperpycnites from intrabasinal turbidites in future studies.