EOLIAN DEPOSITS IN THE BIGOUDINE FORMATION (LATE TRIASSIC, ARGANA VALLEY, MOROCCO): CLIMATIC SIGNAL VS GEOGRAPHIC FACIES

The Bigoudine Fm. (Argana Valley, Morocco) is Late Triassic-earliest Jurassic in age and consists of up to 1300 m of predominantly cyclical continental strata. The lowest Bigoudine (T6) consists of basal conglomerates and fluvial sandstones overlain by eolian sandstones. They are implicated as reservoir sandstones in oil and gas production in Morocco.

The basin-wide (in outcrop) eolian sequence of T6 is a few meters thick and is composed of sets of scalloped cross strata representing straight-crested complex dunes. Dune migration was generally to the southwest while well-preserved superimposed wind-ripples (small dunes) migrated to the northwest, parallel to the crestline of the main bedform. This demonstrates that the subaqueous interpretation of these units given by various authors is incorrect.

Conformably overlying T6, T7 has very well developed cyclicity with some well-developed gray and black lacustrine shales. There is a significant eolian-playa sand-patch component to the drier phases of the cycles. The uppermost Bigoudine (T8) conformably overlies T7 and continues the cyclical pattern of T7. The Bigoudine otherwise consists almost entirely of sand-patch cycles and several eolian sandstone marker beds. The uppermost few meters of T8 have very thin black and gray shales containing the Triassic-Jurassic boundary.

The laterally persistant eolian T7 and T8 sandstones are each about 1 m thick and show superimposed sets of tabular-planar cross strata bounded by second-order surfaces and probably representing small transverse dunes based on the presence of grainfall laminae. They represent the accumulation and a general migration to the southwest of dune-fields over dry flat playa surfaces. They are the driest phase of the sand-patch cycle. Their preservation can be explained by an early cementation by evaporites precipitated in the sediments as water evaporates near the sand-air interface.

Eolian sandstones of the Bigoudine represent genetic packages, tracable over dozens of kilometers, more discrete in T8 and T7 than in T6, caped by lacustrine supersurfaces. Because these eolian units retain their stratigraphic position relative to the distinctive adjacent cycles, they represent times that favored eolian sand accumulation and preservation rather than time-transgressive geographic facies.