Bioherms in Middle Devonian of Northeastern Spanish Sahara, Northwest Africa: ABSTRACT

Abstract

Two groups of Middle Devonian (Eifelian) bioherms, here defined as the Gor Loutad and the Gor Morehba reef areas, have been recognized east and southeast of Semara, a village about 160 km. east-southeast of El Aaiun. The Gor Loutad reef area (Lat. 26°45^prime N.; Long. 10°45^prime W.) which was seen only from the air, comprises about 20 biohermal mounds and ridges distributed in a narrow northeast-southwest trending belt. To the southwest is the Gor Morehba reef area (Lat. 26°30^prime N.; Long. 11°25^prime W.) which was visited on the ground. The Gor Morehba reefs are expressed topographically as an elongate area of low hills trending northeast-southwest for approximately 52 km.; the width of the belt ranges from 10 to 15 km. This area comprises more than 15 elongate reef ridges and elliptical to circular reef mounds. The ridges are as much as 5-6 km. in length and 1 km. in width; the mounds have an average maximum diameter of 1-2 km. Topographically, these reefs stand as much as 100 meters above present drainage levels. The reef-forming limestone averages less than 30 meters in thickness and the total thickness of section affected in any way by the reefing is about 100 meters. Flank dips on the ridges are as much as 20° while dips on the flanks of the mounds are as great as 50°; such dips are in contrast with the average 2° regional dip of the Devonian section. The core of the reef mounds and ridges is a massive light gray limestone made up of broken fragments of coral, calcareous algae, and clastic limestone. It is overlain by, and grades down the flanks into, well bedded dark gray limestone which in turn passes into the shaly limestones and calcareous shales of the inter-reef areas. Well exposed sections of the ridges and mounds suggest that the pattern of reef development was partly controlled by submarine channeling prior to deposition of the reefs. The inter-channel ridges and more isolated highs became the loci for growth of carbonate-secreting organisms. Such limestone (or reef) growth continued for a relatively short time until the influx of clastic material filled the inter-reef areas and eventually passed over the centers of reef development. Locally, initial dips exposed in the overlying clastic section reflect the underlying reef pattern. End_of_Article - Last_Page 129------------