The Stratigraphy of the Cambrian Lancara Formation between the Luna River and the Esla River in the Cantabrian Mountains, Spain

The Lancara Formation is a unit of carbonate sediments of Lower to Middle Cambrian age in the Cantabrian Mountains of northern Spain. The formation is divisible into a Dolomite Member, a Limestone Member and a Griotte Member. The Dolomite Member and the Limestone Member consist mainly of very shallow marine carbonate sediments, devoid of any fossils. Algal structures like stromatolites and oncolites are the only traces of Cambrian life found in them. It is likely that the Dolomite Member represents a sebkha-facies since it is mainly composed of finely to medium crystalline dolomites with intraformational breccias and ‘birdseye’ structures. The limestones are predominantly intrasparudites with stromatolites and oncolites. Locally the limestones have been subaerially exposed in Cambrian times. The Limestone Member is overlain by the Griotte Member. Locally the contact is disconformable. The Griotte Member is composed of red, argillaceous, nodular limestones and shales. These are very fossiliferous and contain glauconite-like pellets (muscovite-1M). The red color of the sediment is due to dispersed hematite. The nodular structure can have been caused by pressure solution, burrowing or brecciation. The formation as a whole represents a transgressive marine sequence. It starts with sebkha-like deposits and changes upward via algal limestones (algal reef?) into open marine biosparudites and biomicrudites and shales. The subaerial exposure and disconformable contact might indicate a local uplift and local regression of the sea prior to the deposition of the Griotte Member. A brief survey on trace elements (Cu, Co, Ni, Sr) was carried out with an atomic absorption spectrophotometer. In the ‘sebkha’ dolomites Cu values showed peaks where the dolomites contain argillaceous matter. Co and Ni were predominantly concentrated in the algal limestones and the Griotte Member. Sr values were high in the algal limestones and in a shale bed underlying the stromatolite bed. The dolomites had generally a low Sr. content. The amount of Sr in the Griotte Member was also lower than in the algal limestones.