Resedimentation and Diagenesis, Including Silicification, of Barremian-Aptian Shallow-water Carbonates from the Galicia Margin, Eastern North Atlantic, at Ocean Drilling Program Site 641

Abstract

Barremian through uppermost Aptian strata from ODP Hole 641C, located upslope of a tilted fault block on the Galicia margin (northwest Spain), are syn-rift sediments deposited in the bathyal realm and are characterized by rapid sedimentation from turbidity currents and debris flows. Calcarenite and calcirudite turbidites contain shallow-water carbonate, terrigenous, and pelagic debris, in complete or partial Bouma sequences. These deposits contain abraded micritized bioclasts of reefal debris, including rudist fragments. The youngest turbidite containing shallow-water carbonate debris at Site 641 defines the boundary between syn-rift and post-rift sediments; this is also the boundary between Aptian and Albian sediments. Some Aptian turbidites are partially silicified, with pore-filling chalcedony and megaquartz. Adjacent layers of length-fast and-slow chalcedony are succeeded by megaquartz as the final pore-filling stage within carbonate reef debris. Temperatures of formation, calculated from the oxygen isotopic composition of the authigenic quartz, are relatively low for formation of quartz but are relatively warm for shallow burial depths. This quartz cement may be interpreted as a rift-associated precipitate from seawater-derived epithermal fluids that migrated along a fault associated with the tilted block and were injected into the porous turbidite beds. These warm fluids may have cooled rapidly and precipitated silica at the boundaries of the turbidite beds as a result of contact with cooler pore waters. The color pattern in the quartz cement, observed by cathodoluminescence and fluorescence techniques, and changes in the trace element geochemistry mimic the textural change of the different quartz layers and indicates growth synchronism of the different quartz phases. Fluorescence petrography of neomorphosed low-Mg-calcite bioclasts in the silicified turbidites shows extensive zonation and details of replacive crystal growth in the bioclasts that are not observed by cathodoluminescence. Fluorescence microscopy also reveals a competitive growth history during neomorphism of the adjacent crystals in an altered carbonate bioclast. Barremian-Aptian background pelagic sediments from Hole 641C have characteristics similar to pelagic sediments from the Blake-Bahama Formation described by Jansa et al. (1979) from the western North Atlantic. Sediments at this site differ from the Blake-Bahama Formation type locality in that the Barremian-Aptian pelagic sediments have a higher percentage of dark calcareous claystone and some turbidites are silicified at Site 641. The stable isotopic composition of the pelagic marlstones from Site 641 is similar to those of other Berriasian-Aptian pelagic sediments from the Atlantic.

The drowning of a siliciclastic shelf: insights into oceanographic reconstructions of the northern Arabian Platform during the Early Cretaceous

“…Both of these mechanisms would be more effective earlier during the transgression than during later phases, as both shore distance increases through time and the initial erodible material will be hence consumed. Interestingly, a similar concurrent increase in MTDs was reported from offshore Galicia (ODP core 641C) (Dunham et al ., ; Haggerty & Germann, ). In this case, phases of thick MTDs were identified as punctuating a finely laminated, fine grained organic‐rich pelagic sediments.…”

Mass-Transfer and Fluid Flow along Extensional Detachment Faults in Hyperextended Rift Systems: The Examples of Tasna in the Alps, Mauléon in the Pyrenees, and Hobby High Offshore Iberia

“…At the Iberia margin, in addition to the Mg and Mn, silicified levels (Figure 10) also occur in the Aptian section, which is syn-mantle exhumation [81]. This occurrence is unlikely to be related to saussuritization/albitization in the continental crust, since deformation and fault activity in the continental crust have ceased at that stage, as observed by the sealing of faults in the distal continental margin by the Aptian section [85].…”

“…The observed increase in Mg and Mn contents is not related to facies variation, since the Aptian sedimentary sequence comprises carbonate marlstones and thin limestones. At the same interval, and as a single level (e.g., ~10 m-thick section 641C-8R-CC to 9R-2), silicified carbonates and authigenic quartz pore-filling cements are described to be enriched in Fe and Mn [81]. Therefore, it is necessary to evoke an external source, different from the detrital one, in order to explain such enrichments and silicification in this specific sedimentary section.…”