Three upper carboniferous limestone-rich, high-destructive, delta systems with submarine fan deposits, Cantabrian mountains, Spain

A synthesis of the development of the Variscan orogene, of which the present Cantabrian Mountains form only a part, was given by Matte (1968). Wagner (1970) presented a general outline of Carboniferous stratigraphy in this area. For purposes of a general background it is sufficient to state that from the Lower Cambrian until the end of the Devonian, sedimentation in this area was typical of a stable shelf environment. Formations can be traced over tens to hundreds of kilometres and most sediments are evidently shallow water deposits. This entire sedimentary pile has a thickness in the order of 2 to 3 km. Towards the end of the Devonian, uplift and erosion (de Sitter, 1965; van Adrichem Boogaert, 1967), resulted in an important hiatus in the central Asturian area and along part of the Leon line. This uplift was followed by another period of very uniform sedimentation during which limestones were deposited in most of the Cantabrian Mountains. This relatively uniform picture changed drastically during the lower and middle Westphalian, when the important Curavacas folding phase (Palentian phase of Wagner & Wagner-Gentis, 1963) occurred. The sediments deposited after these tectonic movements are partly preserved in a number of so-called "basins" (de Sitter, 1965). These "basins" (Fig. 2) are mainly tectonic units which are merely fragments of the originally much more extensive sedimentary basins. These Upper Carboniferous sediments contrast strongly with the earlier ones in that they are typified by great lateral and vertical variability and by their great thickness. The Westphalian alone, for instance, is over 4000 m thick in many places. SUBJECT OF STUDY In these Westphalian deposits marine and non-marine strata show rapid lateral and vertical transitions into one another. In the marine parts of the sequences limestones are common. Some of these limestones can be traced for several kilometres; others, however, abruptly wedge out or interfinger with siliciclastic deposits. Isolated lenses, sometimes forming a distinct band, may also occur. The cause(s) of these irregularities in the limestone deposits was (were) the original subject of this study. It was soon realized that these causes could not be understood unless the limestones were studied together with the surrounding siliciclastics on a regional scale. The Pisuerga area, from which published paleontological data were available, was selected for this study.