Stratigraphy and structure of the Devonian rocks of Inangahua Outlier, Reefton, New Zealand

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Abstract
The complex tectonics of the Early Devonian Inangahua Outlier makes a stratigraphic succession for the 1450 m thick Reefton Group difficult to establish. Remapping indicates that 4 thin limestone formations (Forgotten Limestone, Lankey Limestone, Yorkey Limestone, and Pepper bush Limestone) alternate with 4 thicker mudstone formations (Bolitho Mudstone, Adam Mudstone, Ranft Mudstone, and Alexander Mudstone). This sequence is in tectonic slide contact with a thick sequence of sandstone beneath (Murray Creek Formation) and a thinner sandstone above (Kelly Sandstone). The ages of the sandstones relative to each other and to the mudstone/limestone sequence are at present uncertain. The tectonic contacts are roughly parallel to bedding, and the sliding has caused intense brecciation of the sandstones and, in 1 case, the mudstones. Murray Creek Formation is a regressive sequence with muddy, laminated, lower shoreface quartzarenites at the base, grading up into beach or upper shoreface orthoquartzites at the top. Kelly Sandstone formation is similar to the top of the Murray Creek Formation. The alternating limestone and mudstone succession represents at least 4 rhythms of transgression and regression. Three muddy shelf biofacies rich in Reeftonella, Acrospirifer, and molluscs, respectively, migrated shorewards with each transgression and sea wards with each regression. The muddy biofacies lay seawards of a coral and stromatoporoid- rich carbonate zone which passed land wards into shelly limestone. The lithology of Yorkey Limestone suggests that the coral and stromatoporoid belt passed into break-point bar sands alongthe Devonian shoreline. Like the muddy biofacies belts, the carbonate lithologies also migrated laterally. A source area to the north seems probable for this part of the Reefton Group. During the Tuhua Orogeny, the Reefton Group was folded, although only part of a single syncline has been preserved in Inangahua Outlier. The form of breccias associated with slide contacts suggests that sliding may have occurred later than the fold event. The folded Reefton Group and tectonic slides were downfaulted into Greenland Group strata along major north-south faults, with associated lesser northeast-southwest transverse faulting, probably during the Rangitata Orogeny. Further movement along the boundary faults occurred after deposition of Cenozoic coal measures, probably during the Kaikoura Orogeny.

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