Ripple marks indicate Mid-Devonian paleo-wind directions in the Orcadian Basin (Orkney Isles, Scotland)

Abstract

Abstract Few climate proxies provide information on paleo-wind directions. However, fossilized bedform elements can provide insight into the direction of flow that formed them. Wave-formed ripple marks, for example, which developed in shallow waters, extend transversely to the wind direction. In this study, Middle Devonian paleo-wind directions are reconstructed by measuring the orientation of 511 fossilized wave ripple marks in the Rousay Flagstone Formation on the island of Westray, Orkney, Scotland. The orientation of ripple marks was measured in four different localities on the island. A chi-squared test demonstrates that these four ripple-mark subsets show the same distribution of ripple mark orientation and thus indicates that the studied ripple marks display the same distribution of paleo-winds. Two dominant ripple mark orientations were observed at all studied localities. The most abundant ripple mark orientation suggests paleo-winds from the present-day North or South. Second most abundant are ripple marks that suggest paleo-winds from the present-day ENE or WSW (70° or 250°). When a 40° clockwise rotation of Orkney since the Middle Devonian is taken into account, the ripple marks suggest that the dominant wind directions were south-southeast (SSE) and north-northeast (NNE) in the tropical setting of the Orcadian basin. The SSE wind-direction is represented by the abundant ripple marks, with a present-day near N–S orientation of the line perpendicular to the crests. These ripple marks are also characterized by the largest wavelengths. The abundance of the latter ripple marks with long wavelengths is the result of a lower water level and thus a larger wave-exposed surface during the dry winter season, from April to July, which was mainly characterized by SSE paleo-trade winds.