The coral reef of Nijar, Messinian (uppermost Miocene), Almeria Province, S. E. Spain

Abstract

The Nijar Reef is a thoroughly exposed and representative example of the Messinian coral reefs outcropping along the periphery of the western Mediterranean. Facies patterns in the Nijar Reef Complex are presented in this paper as a generalized guide or model for the study of other Messinian reefs. Messinian paleogeography of this area was characterized by an archipelago whose islands are present-day "Sierras." At the beginning of the Messinian and before development of the reefs, marine terrigenous sediments (Marginal Terrigenous Complex) were deposited on an irregular erosion surface on the metamorphic basement. The Reef Complex developed as a belt, fringing the positive areas (islands) when terrigenous deposition decreased. The Reef Complex consists of: 1) reef-core, constructed by a framework of vertical, branching colonies of Porites coated by laminated micrite (probably submarine cements), which form pinnacles as well as thickets; and 2) fore-reef, made of sloping beds of broken skeletons and debris. The fore-reef is subdivided into: i) reef-talus slope, ii) proximal slope, and iii) distal slope. The reef talus forms massive strata with dips of 20-30 degrees and abundant pebble-to-boulder size slumped blocks of reef-core and coral breccia. The proximal slope shows dips of 10-20 degrees of well-bedded, thick strata with internal parallel layering of granule to sand-size skeletal calcarenites. The distal slope is made of predominant bioturbated skeletal calcisiltites and fine-grained calcarenites with angles lower than 10 degrees . The distal slope grades into the basinal pelagic marls of the Nijar plains. The original reef morphology is well-preserved in present-day topography, with relicts of pinnacles, reef valleys, and slopes. Messinian volcanic highlands located to seaward complicated the local paleogeography with atoll-like features. Reef growth was concomitant with a gradual fall of sea level and occurred before the Messinian gypsum was deposited on top of the basinal marls in what is now the plains area. The Reef Complex is partially truncated by a major erosion surface, and it is overlain by oolitic shoals and stromatolites of the so-called Terminal Carbonate Complex. The Terminal Complex is interpreted as an upslope equivalent (in age) to the Messinian Upper Evaporite, and it implies a new cycle of sea level rise.