Holocene Boulder Beach Eroded from Chromite and Dunite Sea Cliffs at Støypet on Leka Island (Northern Norway)

Abstract

This project examines the role of high-latitude storms degrading a Holocene coast formed by igneous rocks composed of low-grade chromite ore and dunite that originated within the Earth’s crust near the upper mantle. Such rocks are dense and rarely exposed at the surface by tectonic events in the reconfiguration of old ocean basins. An unconsolidated boulder beach occupies Støypet valley on Leka Island in northern Norway, formerly an open channel 10,000 years ago when glacial ice was in retreat and rebound of the land surface was about to commence. Sea cliffs exposing a stratiform ore body dissected by fractures was subject to wave erosion that shed large cobbles and small boulders into the channel. Competing mathematical equations are applied to estimate the height of storm waves impacting the channel floor and cliffs, and the results are compared with observations on wave heights generated by recent storms striking the Norwegian coast with the intensity of an orkan (Norwegian for hurricane). Lateral size variations in beach clasts suggest that Holocene storms struck Leka Island from the southwest with wave heights between 5 and 7.5 m based on the largest beach boulders. This result compares favorably with recent high-latitude storm tracks in the Norwegian Sea and their recorded wave heights. The density of low-grade chromite ore (3.32 g/cm3) sampled from the beach deposit exceeds that of rocks like limestone or other igneous rocks such as rhyolite, andesite, and basalt taken into consideration regarding coastal boulder deposits associated with classic hurricanes in more tropical settings.

Upper Pleistocene and Holocene Storm Deposits Eroded from the Granodiorite Coast on Isla San Diego (Baja California Sur, Mexico)

“…Attention to the phenomenon of coastal mega boulders and their relationship to major storms or tsunami events is a topic of growing interest [8][9][10][11]. Especially in the context of rock density, the data from Isla San Diego provide further insight on comparison of storm beds of Pleistocene and Holocene origins throughout the gulf region [3][4][5][6] with oceanic basalt in the Azores and Canary Islands of the North Atlantic [12,13], as well as rare mantel rocks from storm beds in coastal Norway on the Norwegian Sea [14].…”

Evaluation of Boulder Deposits Linked to Late Neogene Hurricane Events

“…Three contributions to the SI [7][8][9] relate to island deposits in the North Atlantic Ocean. From the Azorean island of Santa Maria [7], comparisons are made among paired CBDs of Pliocene and Pleistocene origin exposed along the southern coast of that island.…”