THE RECORD OF THE RHEIC OCEAN CLOSURE IN MID-CARBONIFEROUS CALCI-MUDSTONES OF NORTHERN SPAIN

The continental collision of Gondwana and Laurentia during the assemblage of Pangea closed the Rheic subequatorial seaway connecting the Panthalassa and Palaeotethys Oceans in the mid-Carboniferous, affecting the global... [ view full abstract ]

The continental collision of Gondwana and Laurentia during the assemblage of Pangea closed the Rheic subequatorial seaway connecting the Panthalassa and Palaeotethys Oceans in the mid-Carboniferous, affecting the global oceanic circulation and climate. In the Variscan marine foreland basin of the Cantabrian Zone (N Spain), which represents the remnant of the Rheic Ocean, dark and laminated calci-mudstones with associated evaporites accumulated over wide sectors of the basin during Serpukhovian-Early Bashkirian times. Deposition of these sediments was affected by the main tectonic events recognized in the Variscan Orogen: a) the onset of Gondwana crust deformation, which documents the beginning of the continental collision in eastern Pangea during Late Visean times; and b) tectonic deformation in the internal areas of the Variscan Orogen with the emplacement of major tectonic units in Spain and France, which might have caused a limitation of the connection of the marine Cantabrian foreland basin with the Paleotethys, leading to basin restriction and anoxic conditions in the basin.
The Serpukhovian-Lower Bashkirian dark calci-mudstone of the Cantabrian foreland basin, 150−400 m thick, consist of two stratigraphic units: the San Adrian Mb. (Pendelian-lower Arnsbergian) and the Barcaliente Fm. (Chokierian-Alportian), which are separated by upper Arnsbergian condensed pelagic sediments (the Millaró beds). Both stratigraphic units, which interfinger with orogen-sourced siliciclastic turbidites towards the foredeep, display a remarkable uniformity over thousands of square kilometres. They consist of deep-subtidal deposits, comprising slightly burrowed to laminated calci-mudstones with very scarce biota alternating with metre-thick beds of well-laminated, organic-matter-rich, marly calci-mudstones barren of biota. Celestite evaporites occur in sections close to the foredeep, both in laminated calci-mudstones and as detrital grains in centimetre-thick turbidite beds deposited from dilute gravity flows. In the uppermost part of the Barcaliente Fm., evaporites increase significantly and laminated carbonate−evaporite facies with probable gypsum and celestite moulds underlie a decametre-thick intraclastic breccia interval (Porma Breccia). These breccias cover thousands of square kilometres and might record the mobilization and dissolution of thick, laterally extensive evaporite deposits. The studied strata most likely records anoxia and hypersaline conditions along very wide areas of the foreland basin documenting restricted conditions. This interpretation is well supported by sedimentological, paleontological and geochemical data.