Synthesis of observations of the reconnaissance field trip, November-December 2015
By prof. Antonio Teixell, prof. David Gómez-Gras and prof. Francisco Martínez with the collaboration of prof. M.L. Arboleya Dept. de Geologia, Universitat Autònoma de Barcelona, Bellaterra, Spain
The Zat Valley is a deeply incised river valley in the northern slopes of the Marrakech High Atlas of Morocco, an orogenic belt that formed during the Cenozoic times in the northern African plate. A reconnaissance field survey was carried out by members of the dept. of Geology at UAB with the objective to identify the main lithologic units and tectonic structures. 30 samples were collected along a N-S profile following the valley.
As for the Geology, the Zat Valley can be subdivided in two parts: 1) a northern part consisting of the Ceno- zoic Haouz Plain and the foothills belt, dominated by folded Cretaceous and Tertiary rocks, and 2) a southern part upstream, displaying the highest topographic altitudes (over 3500 m), and characterized by Precam- brian-Paleozoic and Triassic rocks, affected by steep thrust and normal faults. Observations have been made and samples have been collected in each of the zones, which offer a diverse and interesting geology.
The foothills belt is dominated by Cretaceous and Tertiary rocks folded into a system of ENE- to NE-trending folds. No major thrusts are observed between the folds, although the abrupt boundary between the Haouz basin and the foothills suggest the existence of a frontal thrust fault by which the High Atlas foothills overrode the Cenozoic basin, although this fault is not exposed at the surface, covered by recent alluvial and colluvial deposits. Fig. 1 shows a preliminary cross-section along the Zat Valley illustrating the structure of the foothills belt. Synclines are wide, and occupied by a succession that spans from the Infracenomanian red beds to the Eocene limestone. Anticlines are narrow, and in outcrop they are characterized by uppermost Triassic red shale and basalt. Among the main synclines are the Ait Ourir and Toukribine synclines; these synclines dis- play a box-fold geometry, with flat bottom and steep to overturned limbs. Deformation corresponds to upper crustal levels; no tectonic cleavage has been observed.
The Toukribine syncline contains the most complete and representative stratigraphic succession of the foo- thills belt, which from base to top include: 1) basalts of late Triassic-early age, 2) terrestrial red sandstones, conglomerates and shales which are poorly dated but traditionally attributed to the upper Jurassic-lower Cretaceous (the so-called Infracenomanian unit), 3) a maker bed of marine, fossil-rich (oysters and bivalves) limestone of Cenomanian-Turonian age, 4) terrestrial red sandstones and shales of upper Cretaceous age, and 5) fossil-rich, silicified limestones indicating a new marine transgression of Paleocene-mid Eocene age.
Triassic and basement rocks dominate the rest of the Zat Valley upstream, from the Tighdouine area to until the High Atlas drainage divide in the South. The structure of the Triassic rocks is the result of the Triassic extension plus the superimposition of the Cenozoic compression (the Atlas orogeny), whereas the Precam- brian and Paleozoic rocks, more intensely deformed, have experienced older orogenic events (Pan-African and Variscan orogenies). In the northern part, near Tighdouine, Triassic red beds display a structure of tilted blocks, limited by steep faults oriented NE-SW. An antiformal structure just west of Tighdouine, shows Paleo- zoic (Carboniferous) rocks in its core. Carboniferous rocks show an intense folding structure (Fig.2) that is fossilized by the Triassic, and hence we attribute it to the Variscan orogeny. Beautiful outcrop-scale folds and thrust faults are exposed along the road cuts of the road to Trafawt, near the mineralized source of Zawyat Sidi Ahmed al Wafi, offering very didactic examples
Most of the faults of this upstream, southern part, as the main Trafawt fault itself near Tighdouine (to which the mineralized source is associated), keep a normal fault attitude, and hence we interpret them as Triassic syn-depositional faults of the rifting stage that have been little or not reactivated during the Atlas mountain building. In fact, in the Zat Valley we have observed relatively minor evidence of compressional deformation throughout, suggesting uplift of rigid, coherent blocks with little internal strain. This does not mean that thrust faults or thrust-related folds are inexistent. A major thrust fault near the Azgour locality is observed bringing in contact Triassic and Precambrian rocks. This fault, called the Meltsene thrust, is also very steeply dipping in the Zat Valley (dipping ca. 70 SSE) and uplifts the Precambrian crystalline rocks of the southern side (Fig. 2). The remainder of the Zat valley south of the Meltsene thrust is characterized by Precambrian exposure, being younger rocks inexistent. This Precambrian massif appears to be the continuation of the Toubkal massif well known to the west. It is largely composed by igneous rocks which appear largely undeformed, although this
19deserves further investigation. A prominent brittle fault is observed 200 m south of the village of Tizirt This fault dips, and bears a cataclastic fabric associated, providing an illustrative example of a brittle fault rock in crystalline protolith. The sense of slip of this fault cannot be determined directly from the exposure in the Zat Valley, but the fault is in continuity with the so-called Ourika fault that in the Ourika Valley involves Triassic rocks and downthrows the northern side. Hence, it can be interpreted as a Triassic extensional fault, which has probably rotated and oversteepened during the Atlas compression.
The stratigraphy of the Triassic red beds of the middle part of the Zat Valley is characterized by the following units: 1) a basal conglomerate and sandstone unit, 2) a siltite unit (called the Ramuntcho formation), 3) a thick fluvial channelized sandstone unit (the Oukaimeden sandstone formation), and 4) an upper shale formation, capped by volcanic basalts. Previous studies have reported Permian red beds in localized parts of the High Atlas, including the Zat Valley, but these remain to be identified in the study area.
The Precambrian basement crops out extensively in the upper part of the Zat Valley. They are Neoprote- rozoic Pan-African rocks, making part of the so-called Ouzellarh “promontory” or “salient” . The oldest Pre- cambrian rocks are para- and orthogneisses, migmatites and amphibolites belonging to the Bleida Group. The orthogneiss protoliths of this group have yielded in previous studies a radiometric age of 743 ± 14 Ma. On top of the Bleida Group lies the Sarhro Group, only represented by small outcrop in the upper Zat valley.
This group consists of a thick flysch-like synorogenic volcano-sedimentary sequence dated around 630 Ma. The Bleida and Sarhro Group rocks have undergone metamorphism and deformation during the Pan-African Orogeny, dated at 660-600 Ma.
Late to post-orogenic Pan-African magmatism is represented by 615-580 Ma granitoids of the Ouarzazate Group intrusive in the previously described rocks. In the Zat valley, these granitoids are represented by coarse-grained pink granites. Postdating these rocks a gabbro-diorite-granodiorite-granite association of around 580 Ma also occurs. Later than these association, grey andesites, andesitic tuffs and breccias, felsic tuffs, rhyolitic breccias with interbedded cinerite, and finally two levels of violet and white rhyolite together with coeval granite plutons (575-560 Ma) complete the Ouarzazate Group that appear in the Toubkal area.
The Paleozoic in the Zat valley is only represented by a flysch formation, made of sandstone and siltstone al- ternance with calcareous pebbles, occurring just south of the Tafrawt fault. This flysch of Visean age belongs to the Souktana member of the Al Haouz Group.
Further research in the Zat Valley will be aimed to investigate the constitution, to delimit the extent, and to remark the singularities of each geologic unit in terms of natural heritage.