MORPHOLOGY AND EMPLACEMENT OF THE TRIASSIC-JURASSIC BASALTIC LAVA FLOWS OF THE CENTRAL ATLANTIC MAGMATIC PROVINCE (CAMP). EXAMPLE FROM THE ARGANA BASIN VOLCANIC SEQUENCE (WESTERN HIGH ATLAS, MOROCCO)

The Argana Basin volcanic pile comprises two lava flow fields produced by two successive eruptions. The lower is a succession of 3 to 13 flows, 60 to 135 m thick, while the upper, composed of 2 to 6 flows, is 60 m thick. A sediment layer or a paleosol separates the two formations. The lavas are compound pahoehoe flows and simple flows. The first type is almost exclusive of the Lower Formation, while the second type dominates the Upper Formation. Most lava lobes in the compound flows present: (i) a thin (0.2 - 0.4 m) basal lava crust, less vesicular, sometimes with pipe vesicles; (ii) a dense lava core, corresponding to 40 to 60 % of the flow thickness (< 20 m), with vesicle cylinders and platy joints at the base and top; and (iii) an upper lava crust, 1 - 13.5 m thick, formed by alternating dense and vesicular levels, that presents tumuli and squeeze up structures. The lobes are S-type (spongy), vesicular, without pipe vesicles or, more frequently, P-type (pipe) with pipe vesicles, vesicular base and a less vesicular or dense core. Simple flows are 3.5 to 30 m thick. Each flow maintains a constant thickness over considerable distances. The simple flows appear as simple cooling units, without multiple lobes. Some flows present flow-top breccia. The compound pahoehoe flows of the Lower Formation show excellent and unambiguous evidence of endogenous growth by inflation, suggesting slow emplacement during sustained eruptive episodes.