Contourites in laminated black shale facies of the Aldeatejada Formation (Precambrian/Cambrian boundary range province of Salamanca, western Spain)

Abstract

Laminated black shale facies and phosphorite conglomerates that appear in the lower part of Aldeatejada Fm. (Precambrian/Cambrian boundary range, Province of Salamanca) are interpreted as contourites. In modern seas, contourites form beds related to stable currents of generally low near-bed velocity (2-15 cm/s) that flow coast- or slope-parallel like the actual Gulf Stream. These currents rework or deposit clay, silt or very fine sand depending on fluctuations of flow speed or position of the current. During periods of elevated velocity, features like non-deposition, erosion and formation of conglomerates occur. Such conglomerates appear in the Aldeatejada Fm. and are referred to as gravel-lag contourites. The countourite-producing current is considered as a shelf-impinging oceanic current which carried phosphate-rich water due to offshore upwelling. The current triggered high organic productivity and probably alimented anaerobic bacterial mats wich are preserved in the black shales.