Storm‐generated graded beds and debris flow deposits with Ophiomorpha in a shallow offshore Oligocene sequence at Nelson, South Island, New Zealand

An Oligocene (Whaingaroan) succession of sandstones with rare perigenic breccias and fossiliferous detrital conglomerates is well exposed for 1 km along strike and for a stratigraphic thickness of 140 m on a wave‐cut platform at Magazine Point, Nelson. Regional tectonic and stratigraphic relationships are uncertain. All lithologies are lensoidal; virtually no bed extends for more than 200 m. Planar‐laminated to hummocky cross‐stratified, normally graded, mud‐free sandstones 10‐80 cm thick alternate with bioturbated muddy finer sandstones and sandy mudstones 10‐35 cm thick. Ophiomorpha nodosa is the only common trace within the clean sandstones; it is locally abundant in the lower 60 m of the succession and exceedingly rare in the upper 80 m. Thallassinoides , formed by the same progenitor as Ophiomorpha , occurs with more common other indeterminate traces within the muddy units. Benthic microfauna from the muddy intervals indicate “shelf” settings ranging from inner, at the base of the sequence, to outer in the upper 80 m of less varied, less carbonaceous, gravel‐free, more thinly bedded sediments. Redeposited macrofauna of inner shelf character and coalified wood occur locally in the lower 60 m. A paleoenvironment is inferred which lay seawards of the shoreface, below wave base but not at bathyal or abyssal depths. The clean sandstones represent high energy (storm) deposition in channelised scours; the muddy sandstones represent quiet (fair weather) conditions during which muddy sediments were deposited and were homogenised by bioturbation together with the upper portions of the graded units. Breccias, with angular clasts of muddy sandstone up to boulder size, and conglomerates, with rounded detrital gravels of mixed provenance together with mac‐rofossil clasts and angular perigenic clasts, also reflect storm redeposition. One of these conglomerates comprises the natural levees of a noncohesive debris flow. Sandstones intimately associated with this conglomerate are inferred to reflect deposition from the finer tail of the coarse debris flow head.