Stratigraphy and structure of the Lower Devonian rocks of the Waitahu and Orlando Outliers, near Reefton, New Zealand, and their relationship to the Inangahua Outlier

Abstract

Abstract In the Waitahu River catchment, 1255 m of Lower Devonian Reefton Group sediments are downfaulted into Ordovician Greenland Group rocks to form two outliers of contrasting size (Waitahu and Orlando Outliers). Of the eight formations present, seven have been correlated with those of the neighbouring Inangahua Outlier (in order of younging: Murray Creek Formation, Lankey Limestone, Adam Mudstone, Yorkey Limestone, Ranft Mudstone, Pepperbush Limestone, and Kelly Sandstone) while a new formation is preserved at the top of the sequence (Track Mudstone). The stratigraphic sequence in the Inangahua and Waitahu Outliers is essentially the same, and includes thick sandstones at the bottom and top of the succession, with an alternation of nearshore limestone and offshore mudstone formations in between. The Waitahu sequence is more continuous and is not broken by tectonic slides. Beach to shelf paleoenvironments are inferred, as for the rocks of the Inangahua Outlier, but the Track Mudstone is interpreted as a nonmarine lagoonal deposit. There are, however, significant differences within the Waitahu sequence, such as the absence of Bolitho Mudstone and Forgotten Limestone, both of which are present in the Inangahua Outlier. Thicker Lankey and Yorkey Limestones, bioturbated shoreface silty units in the Ranft Mudstone, replacement of Alexander Mudstone by lower Kelly Sandstone, and the nonmarine Track Mudstone, all suggest that the Waitahu area lay closer to the Early Devonian shoreline than did the Inangahua region. The Devonian outliers are bounded by two types of faults. Some are subparallel to bedding in the Reefton Group and are associated with bedding‐parallel breccia zones, while others are steep, crosscutting faults. The low dip of some boundary faults, and the relationship of cleavage to calcite veins, suggests that the Reefton Group was emplaced over the Greenland Group along low‐angle faults during an early extensional regime. The Reefton Group, together with its bounding faults, was then folded into a tight, south‐plunging syncline during a compressive phase of the ?Middle to Late Devonian Tuhua Orogeny. The structure was further modified by warping and steeply inclined displacements during the Rangitata and Kaikoura Orogenies, so that the outlier became defined by a mixture of bedding‐parallel and crosscutting faults.