Late Jurassic Paleogeography and Paleoclimate in the Northern Iberian Basin of Spain: Constraints from Diagenetic Records in Reefal and Continental Carbonates

Abstract

Abstract Sedimentation in the northern part of the Iberian Basin during the early Kimmeridgian was characterized by the development of coral reef complexes of the Torrecilla en Cameros Formation, which were situated along a seaway that connected the Boreal and Tethys domains. Along the northern portion of that seaway, close to the Boreal Sea, early diagenesis of the Torrecilla fringing reef complex was controlled by rising sea level and by local tectonism, leading to alternations of submergence and reefal exposure. While exposed, reef corals were neomorphosed and dissolved. When the corals were submerged, secondary porosity was filled by calcite cements precipitated from marine-derived waters. During the latest stages of reef development, rate of sea-level rise could no longer keep pace with tectonic uplift and, while younger accretionary units continued to be deposited, older units were exposed subaerially, leading to the precipitation of meteoric calcite cement and to the development of a paleosol. This stage of diagenesis also affected the reef complex during late Kimmeridgian times, as the Boreal and Tethys coastlines progressively retreated to the north and southeast, respectively. Progressive retreat of the Boreal Sea and subsequent Late Jurassic to Early Cretaceous rifting that formed the Cameros Basin resulted in the deposition of Tithonian continental carbonates directly on these reefal units. Oxygen isotope compositions of these continental carbonates are 3% more negative than Kimmeridgian meteoric cement, suggesting an evolution in the isotopic composition of meteoric water in response to an increasing continental effect on meteoric precipitation. In contrast, early Kimmeridgian reef complexes of the Torrecilla en Cameros Formation in the southern sectors of the seaway, closer to the Tethys Sea, were exposed during the late Kimmeridgian as the Tethys coastline retreated, leading to dissolution of corals and subsequent precipitation of meteoric calcite cement. Oxygen isotope compositions of these meteoric cements are 2% more negative than time-equivalent meteoric calcites precipitated in the northern Torrecilla Reef Complex. Given the regional paleogeography, this difference suggests that sources of meteoric water must have been different for the northern and southern sectors, and probably reflects southward transport of air masses and water vapor from the Boreal Sea to the northern Torrecilla sector, and northwest transport of water vapor from the Tethys Sea to the southern sectors. Tithonian–Berriasian continental carbonates, which unconformably overlie reef complexes in the southern and northern sectors, exhibit an opposite trend, with the δ18O of continental carbonates in southern sectors being 1.6% more positive than time-equivalent carbonates that overlie the Torrecilla Reef Complex to the north. This shift in the trend of the isotopes between northern and southern sectors is coeval with paleogeographic changes that occurred in response to Late Jurassic–Early Cretaceous rifting of the Iberian Plate and the progressive retreat of Boreal and Tethys coastlines. During this time, continental carbonates in both the southern and northern sectors of the seaway precipitated from meteoric waters that were sourced from the Tethys Sea to the southeast. More negative δ18O values in the northern Torrecilla sector reflect enhanced continental fractionation effects as the influence of the Boreal Sea-derived meteoric waters diminished.