Lower Devonian Brachiopods and Stratigraphy of North Palencia (Cantabrian Mountains, Spain)

A continuous sequence of Devonian sediments is exposed in the northern part of the province of Palencia (NW-Spain), on the southern slope of the Cantabrian Mountains. This study concerns the stratigraphy and paleontology of the Lower Devonian formations. At the base of the sequence a clastic formation is found, called the Carazo Formation. This unit can be sub-divided into three members, from top to bottom: (c) alternating shales and sandstones, (b) quartzitic sandstones, partly strongly ferruginous, (a) shales and micaceous sandstones. Members a and b did not yield fossils. The upper part of the formation is richly fossiliferous. The main element of the fauna consists of brachiopods, which occur together with tentaculites, trilobites, ostracods, and pelecypods. The well-preserved and characteristic brachiopods are illustrated. The fauna indicates a Lower Gedinnian age for these sediments. The next lithologic unit, called the Lebanza Formation, consists of some 100 metres of well-bedded limestones with shale intercalations at the base and top of the formation. This formation is extremely fossiliferous. Brachiopods dominate; tentaculites, trilobites, pelecypods, corals, stromatoporoids, bryozoans, and crinoid stems also occur. The diagnosis and systematic position of 18 of the brachiopods are discussed, viz. 3 dalmanellids; 7 rhynchonellids, including 3 new species; 4 terebratuloids; and 4 rostrospirids. Special attention is paid to the internal structure which was studied in serial sections by means of the peel technique. Drawings of the more important sections are given with each diagnosis. In total, 25 species were determined in the brachiopod assemblage. Different assemblages occur in the upper and lower parts of the formation. The association in the lower part gives no definite indications about the age of these rocks. With their stratigraphical position taken into account, an Upper Gedinnian to Lower Siegenian age is concluded. The upper part of the formation shows only typical Siegenian species. On account of the correspondence with faunas of the Middle Siegenian formations of the Massif Armoricain (Brittany, France), the age of this part of the formation can be established as Middle Siegenian. The correlations with sequences in adjacent areas are discussed. There is a marked divergence from the brachiopod succession in other parts of the Cantabrian Mountains, due to different oecological factors.