From carbonate platform to euxinic sea – the collapse of an Early/Middle Devonian reef, Cantabrian Mountains (Spain)

Abstract

The Santa Lucía Formation represents the major phase in Devonian reef development of the Cantabrian Zone (Cantabrian Mountains, northwest Spain). In the present study the transition from the carbonate platform deposits of the Santa Lucía Formation to the overlying euxinic basinal deposits of the Huergas Formation is described. These transitional strata are connected to the Basal Choteč Event and represent a condensed sedimentation of micritic dark-grey and black limestones with an upward increase of dark shale intercalations with iron mineralisation surfaces and storm-induced brachiopod coquinas. The transitional beds are grouped into a new unit, the Cabornera Bed, which consists of limestone, limestone-shale and shale facies associations, representing a sediment-starved euxinic offshore area just below the storm wave base. Four stages in reef decline can be recognised: a reef stage, an oxygen-depleted, nutrient-rich stage, a siliciclastic-influx stage and a pelagic-siliciclastic stage. Additional geochemical and geophysical investigations are needed to verify the results presented herein.

Authigenic Carbonate Burial Within the Late Devonian Western Canada Sedimentary Basin and Its Impact on the Global Carbon Cycle

“…In order for authigenic carbonate to have an effect on global carbon isotope mass balance, it needs to be both voluminous and have negative δ 13 C values (i.e., display a significant isotopic offset from coeval marine carbonates). The Famennian represents an ideal time period to assess this hypothesis owing to the sedimentological conditions that might foster the precipitation of authigenic carbonate: a sharp areal restriction in shelfal carbonate depositional systems (Barnes et al, 2020;Kaiser et al, 2016;Peterhänsel et al, 2008;van Loevezijn & Raven, 2017) and expanded ocean anoxia (Becker et al, 2016;Kaiser et al, 2006;Marynowski et al, 2012;Schieber & Baird, 2001).…”

“…is an example of a time period in the Phanerozoic when authigenic carbonate may have played a significant role in the global carbon cycle. Globally, it was marked by a widespread reduction in shelf carbonate systems and accompanying global rise in siliciclastic deposition (Barnes et al, 2020;Kaiser et al, 2016;Peterhänsel et al, 2008;van Loevezijn & Raven, 2017). The presence of positive δ 13 C excursions in globally dispersed basins (Cramer et al, 2008;Myrow et al, 2011) and the widespread deposition of organic-rich black shales at the Devonian-Carboniferous boundary, often referred to as the Hangenberg Crisis (Becker et al, 2016;Kaiser et al, 2006), suggest a significant perturbation of the carbon cycle.…”