Depositional depth of laminated carbonate deposits: Insights from the Lower Cretaceous Valdeprado Formation (Cameros Basin, northern Spain)

摘要

The Lower Cretaceous (Berriasian) Valdeprado Formation (Cameros Basin, northern Spain) contains more than 900 m of laminated carbonates and pseudomorphs after sulfates. Traditionally, many sedimentary packages of different ages and lithologies have been interpreted as deep-water deposits based essentially on the abundance of laminations and the absence of subaerial exposure features. In contrast, the Valdeprado Formation provides an example of a shallow-water deposit dominated by laminations with scarce evidence of subaerial exposure, and gives criteria to solve the challenge of distinguishing shallow-water and deep-water, ancient laminated deposits. The two most abundant facies all along the Valdeprado Formation are: a) parallel-laminated limestone, formed by alternating carbonate mudstone and calcite and quartz pseudomorphs after displacive gypsum, and b) graded-laminated limestone, consisting of quartz, mica, ostracodes, and pseudomorphs after detrital gypsum grains at the base, which changes gradually upwards to carbonate mudstone. Parallel-laminated limestone and graded-laminated limestone could have been deposited in either deep or shallow environments as a result of salinity fluctuations driven by alternation of flooding and evaporation and by sediment resuspension processes, respectively. Subaerial exposure features, such as desiccation mudcracks, are scarce in most of the succession, except in a few meter-scale stratigraphic intervals where they are very abundant. Interestingly, in these intervals desiccation cracks are present at the tops of several successive laminae (up to 25 mudcracked laminae per meter of deposit), indicating that, at least during those periods of time, deposition occurred in shallow water bodies that were desiccated frequently. In the upper part of the stratigraphic section, parallel-laminated and graded-laminated limestones are associated with current-ripple and wave-ripple cross-laminated arenites, and ostracode mudstone to wackestone with centimeter-size pseudomorphs after lenticular gypsum, and abundant desiccation mudcracks and tepees, which also suggest sedimentation in shallow-water environments. Moreover, the laminated carbonates display continuous, parallel layering, and the same facies along the 40-km-long outcropping area. These deposits are directly interbedded with, and pass laterally to, siliciclastic sandy-muddy flat deposits in the western area of the basin, without clinoforms, slump structures, or slide masses in between. All of these features suggest deposition in shallow, perennial carbonate-sulfate water bodies and their peripheral mudflats, developed in a flat-bottomed basin with no marked gradients.