Los niveles arrecifales del Neógeno de Purchena (S.E. Cordilleras Béticas)

Abstract

Neogenic detrital materials around Purchena show three interbedded laycers of reef deposits which have been studied from four stratigraphic sections. The lower layer, 5 m. thick, shows biogenic structures as well as detrituos derived from them. Corals and algae build up reef skeletons by the growing of young colonies on the old ones. Lagoonal sediments have also been recognized and they are build of micritic limestones with Alveolinidae, Miliolidae, Pelecipods and algae. The middle layer comprises two units: the lower one set up by coral gravels which venishes out in a northern and northwerterly direction. The upper one is similar to the former. Fossils in a living position are more frequent especially at the top of the reef sections. Lagoonal limestones are also very well represented. The upper bioclastic layer is a conglomerate with very abundant algae. The growth of the reef system was conditioned by the detrital sedimentary influence and the reef deposits laterally intertongue with detrital ones. Growing started when the corals and algae had a solid substratum provided by algal detritus or pelecipod shells mecanically accumulated. Then, massive forms are developed which are broken in several ways, especially by organisms and wave action. Detrital sediments finally buried the reefs and there were no further recurrencies of them during the Tierras Blancas Formation sedimentation. The study of neighboring areas leads us to think of an intermediate reef complex between the fringing and barries ones, located in a region under strong detrital sedimentary influence, where reefs grew especially as patch reefs. The prevalent warm climate conditions originated evaporite deposits, some time before the formation of the middle layer.