Late Jurassic Carbonate Platform of the Galicia Margin

Abstract

The carbonate platform of Tithonian-?Berriasian age encountered on the western flank of the Galicia Bank at Site 639 is part of an extensive Late Jurassic carbonate shelf extending from Algarve to the Lusitanian Basin, Galicia Bank, and eastern Grand Banks of Newfoundland. The platform, which locally could be up to 400 m thick, probably overlies older sedimentary or low-grade metamorphic strata at Site 639. The 287 m of carbonates penetrated at this site was deposited mainly in shelf lagoon and shelf edge shoals, with localized low-relief biohermal mounds, in response to eustatic sea-level fluctuations and regional subsidence during the Tithonian-Berriasian. Minor block faulting influenced lithofacies distribution at the shelf edge. Shallow burial cementation was the most significant diagenetic event. After burial by several hundred meters of younger sediments, the top of the carbonate platform underwent extensive dolomitization during a period of tectonic fragmentation, followed by subaerial exposure and erosion of the platform. Such processes resulted in a complex carbonate diagenetic history with several periods of dolomitization. The major dolomitization event is the result of the replacement or mixing of formation fluids with evaporitic brines in a shallow subsurface setting. The emerged carbonate platform drowned during the middle early Valanginian as a result of eustatic sea-level rise and tectonic subsidence and was covered by Valanginian turbidites. During a second, more intensive period of tectonic deformation, probably in the late Valanginian, the development of large rotational faults resulted in tilting of the carbonate platform blocks toward the east. After tilting, the outer margin rapidly subsided to abyssal depths during the Late Cretaceous-early Tertiary. The development of the carbonate platform helps in dating the major tensional episodes on the western flank of Galicia Bank as late Berriasian-earliest Valanginian and ?late Valanginian in age.

Site 1065

“…8; Jansa et al, 1988). It is likely, therefore, that a Tithonian carbonate platform, similar to that described by Jansa et al (1988), developed over what is now Vasco da Gama Seamount and over the ridge to the east of it (Fig. 4).…”

“…The presence of slate and meta-arenite lithoclasts in the conglomerates indicates exposure and erosion of Variscan basement rocks, presumably to the north in the Vasco da Gama Seamount area (see previous discussion of "Depositional Processes" subsection for Unit II, this section, this chapter). The lithoclasts of shallow-water limestones exhibiting lithologies similar to those recovered during Leg 103, described by Jansa et al (1988), suggest that limestones slightly older than Unit V were being eroded, or that contemporaneous carbonate buildups located on tectonic highs were shedding debris into the area around Site 1065. The latter scenario is similar to that described by Leinfelder and Wilson (1989, in press) for Kimmeridgian sediments filling the Arruda Sub-basin 30 km to the north of Lisbon.…”

Site 1069

“…"The presence of slate and meta-arenite lithoclasts in the conglomerates indicates exposure and erosion of Variscan basement rocks, presumably to the north in the Vasco da Gama Seamount area. The lithoclasts of shallow-water limestones exhibiting lithologies similar to those recovered during Leg 103, described by Jansa et al (1988), suggest that limestones slightly older than Unit V were being eroded, or that contemporaneous carbonate buildups located on tectonic highs were shedding debris into the area around Site 1065. The latter scenario is similar to that described by Leinfelder and Wilson (1989, in press) for Kimmeridgian sediments filling the Arruda Sub-basin 30 km to the north of Lisbon.…”