Assessment of a Reef Community From Lower Jurassic (Pliensbachian) Strata in the Central High Atlas Mountains of Morocco

Abstract

During the Early Jurassic, reefs in the shallow seas of the Atlas Rift experienced substantial changes as they recovered from the end-Triassic mass extinction. Excellent Lower Jurassic reef deposits documenting this change occur in the Central High Atlas region of Morocco, and herein we describe Owl Olistolith, a micro-olistolith found in lower Pliensbachian-aged (∼ 188.7 million years ago) Moroccan strata. The olistolith records the composition of a reef that grew within the Atlas rift zone and represents a snapshot of reef recovery ∼ 10 million years after the end-Triassic mass extinction. Owl Olistolith is derived from a reef that was originally situated on an outer platform within fair weather wave base; it broke loose and was transported to deeper water and deposited amongst marls. Corals and microbialites formed the primary framework of the reef; microproblematica, foraminifera, and other minor components were also present. The reef can be divided into two dominant facies: a microbialite facies that contains no corals (54%–94% microbialites), and a coral-microbialite facies with substantial proportions of both microbialite (23%–50%) and corals (14%–72%). The micro-olistolith contains at least 15 distinct coral types. In this study, seven coral genera were identified, three of which represent taxa that span the Triassic/Jurassic boundary, including Coryphyllia, Stylophyllopsis, and Margarosmilia. These results indicate that, although surviving taxa played a significant role, newly evolved corals were the most important taxa in the reestablishment of reef ecosystems in the Early Jurassic of Morocco.