Stratigraphy, palaeoenvironment and mollusks (gastropods and bivalves) of the Toarcian succession of the Central Unit of the South Riffian Ridges (northern Morocco)

Highlights

• Toarcian marine macrofauna from northern Gondwana margin.

Abstract

In the Central Unit of the South Riffian Ridges, the Toarcian succession is represented by two thick terrigenous detrital formations, Sned and Lmachkha formations, each one is divided into two members. The lower member of the Sned Formation, often rich in belemnite rostra and ammonites of the Tenuicostatum Zone, consists of grey marlstones, with rare marly limestone intercalations. The upper member consists of silty marlstones, with frequent thin limestone beds and contains rare ammonites of the Serpentinus Zone. Its upper part is locally rich in terebratulids. The lower member of the Lmachkha Formation consists of a silty marlstone and silty limestone alternation, and is the richest in benthic macrofauna, composed of rhynchonellids, gastropods, common bivalves, and rare ammonites of the Bifrons Zone. Seven taxa of gastropods and six taxa of bivalves were recorded from this member and described for the first time from the study area. The upper member of the Lmachkha Formation consists of silty marlstone and silty limestone alternation, and contains rare bivalves and rare ammonites of late Toarcian age. The lithofacies and pelagic macrofauna of the lower member of the Sned Formation characterise a relative deep marine environment unfavourable to benthic macrofauna. The second member indicates a gradual decrease in sea-level allowing from its upper part the recurrence of benthic macrofauna, especially terebratulids. The lithofacies and macrofauna of the lower member of the Lmachkha Formation indicate a shallow terrigenous shelf environment. The second member, containing coral reefs in the western and southern peripheries of the Central Unit, characterises a still shallow marine environment. This significant but gradual decrease in sea-level of the Central Unit from the latest early Toarcian to late Toarcian is likely related to the acceleration of sea-floor spreading in the Central Atlantic Ocean.

Introduction

The South Riffian Ridges (southwestern foreland of the Rif Belt) is a small palaeogeographic domain of northwestern Morocco, located between the Gharb and Saiss plains composed of Neogene post-orogenic deposits and the External Prerif Zone (Rif Belt) of Triassic to middle Miocene deposits (Fig. 1). This domain corresponds to several small to medium belt-shaped mountains located north of the cities of Meknes and Fez, and is organised in two outcrop belts, the Western and Eastern Ridges (Fig. 2) with an arched shape, separated by a large depression filled by upper Miocene post-orogenic marlstones. The first outcrop consists of several more-or-less isolated mountains, located between Wadi Beht and Wadi Rdom; the second one corresponds to the Moulay Idriss Zerhoun Massif and its northern extension (the Tselfat Massif). The Meso-to Cenozoic successions of the South Riffian Ridges are composed of three superimposed stratigraphic sequences (Faugères, 1978, 1982; Benzaggagh et al., 2016) bounded by major unconformities (Fig. 2). The lower sequence corresponds to Triassic to Bajocian strata. The middle sequence consists of Albian to Paleocene deposits (Faugères, 1978; Benzaggagh et al., 2017) unconformably resting on lower to middle Bajocian deposits, and limited to the eastern (Talerhza Basin) and northeastern (Tselfat Massif) edges of the South Riffian Ridges. The upper sequence is transgressive over the most part of the South Riffian Ridges domain and consists of molasse-type rocks and marlstones of middle? to late Miocene in age, unconformably resting on various Jurassic and Cretaceous formations. Within the Jurassic basement, Faugères (1978, 1982) distinguished three lithostratigraphic units, equivalent to three palaeogeographic zones (Fig. 2, Fig. 3), which are, from the outer to the inner parts of the Jurassic basin, as follows:

• the Peripheral Unit, corresponding to the outer part of the Jurassic South Riffian basin, and displaying a reduced and incomplete stratigraphic sequence, with two thick formations; the lower one is composed of red clays and sandstones with evaporates and basaltic lava flows of Triassic-earliest Jurassic age, whereas the upper formation is composed of dolostones and neritic massive limestones of Early Jurassic (early to “middle” Liassic) age;

• the Intermediate Unit, more complete, showing, in addition of the two previous formations, a thick series of marlstones and sandstones with bioclastic and oolitic limestones of late Pliensbachian (middle Domerian) to middle Bajocian age, often poor in macrofauna;

• and the Central Unit, corresponding to the inner part of the Jurassic basin of the South Riffian Ridges, and forming all Jurassic mountains located north of the Moulay Idriss Zerhoun Fault to the northern end of the Tselfat Massif (Fig. 2). This unit is characterised by pelagic deposits from Lower Pliensbachian (middle Carixian) to middle Bajocian. Faugères (1978) distinguished in that stratigraphic interval ten formations (Fig. 3). Within this unit, the Lower Jurassic (Pliensbachian) is dominated by bedded limestones and marly limestone alternation (Fig. 3, Fig. 4); the uppermost Lower Jurassic (Toarcian) and Middle Jurassic (Aalenian to middle Bajocian) is widely dominated by marlstones and silty marlstones (Fig. 3, Fig. 4, Fig. 5). The Toarcian succession of this unit is the richest in marine macrofauna, mainly ammonites, belemnites, gastropods, bivalves, brachiopods, and corals.

The aim of this paper is to specify the Toarcian stratigraphic succession of the Central Unit of the South Riffian Ridges, its marine macrofauna and palaeoenvironmental conditions.

The causes of the early Toarcian transgression and the mass extinction of benthic macrofauna at the Pliensbachian-Toarcian boundary are discussed. The taxonomic study is limited to the middle Toarcian gastropods and bivalves, and is the first one dealing with these benthic groups from the study area.