Print this pageLithocodium-Bacinella Binding of Sub-Basin-Wide Resedimented Deposits in the Oae1a Coeval Aptian Succession of the Western Maestrat Basin (Iberian Chain, Spain)
Organic-rich black shales are the classic and the most studied type of deposit formed during the Early Aptian oceanic anoxic event 1a (OAE1a). These characteristic deposits are important source rocks, which sedimented in deep dysoxic/anoxic ocean bottoms. However, less attention has been paid to their shallower counterparts. In the epicontinental Aptian succession recorded in the Galve sub-basin (western Maestrat Basin), the widely known C-isotope positive excursion that marks the onset of the OAE1a is coeval with an extensive metre-thick (up to 5 m) resedimented horizon mainly made up of coral rubble encrusted by microproblematicum Lithocodium aggregatum Elliott and Bacinella irregularis Radoičić. These deposits resemble present day coral rubble deposits found in reefal complexes in the sense that were basically formed by accumulation of coral debris, which were subsequently bound by microencrusters. In the levels studied, the Lithocodium-Bacinella consortium crusts are more predominant than other encrusters or organic components, and bind together the skeletal fragments in all directions. This underlines the pioneer character of this consortium and suggests that these calcareous microorganisms could have had the ability to develop even in a dim light. The resedimented levels analyzed are interpreted as deposited below fair-weather wave base. Abundant bioerosion, silt-sized quartz grains and large-sized Palorbitolina lenticularis (Blumenbach) with discoidal morphologies, which agglutinate numerous quartz particles, suggest that this lithofacies was formed in relatively deep and/or turbid waters under high trophic levels. The O-isotope curve seems to indicate a potential warming event at the base of the coral rubble levels. Hence, this warming event could have induced severe storms that could have broken into pieces extensive platforms dominated by corals. However, the possibility of acidification of seawater during the OAE1a and/or regional tectonic activity should not be ruled out when accounting for the debilitation and destruction of these coral meadows. The ammonite biostratigraphy analysis carried out in the central Galve sub-basin gives an upper Deshayesites weissi age (Lower Bedoulian) to the resedimented levels studied.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil