In Geological Society of America, Abstracts with Programs, vol. 35, no. 3,p 16.
EOLIAN DEPOSITS IN THE BIGOUDINE FORMATION (LATE TRIASSIC, ARGANA VALLEY,
MOROCCO): CLIMATIC SIGNAL VS GEOGRAPHIC FACIES
ET-TOUHAMI, Mohammed, LGVBS, Département des Sciences de
la Terre, Université Mohamed Premier, Oujda, Oujda, 60 000, Morocco,
email@example.com and OLSEN, Paul E., Lamont-Doherty Earth
Observatory, Columbia Univ, 61 RT 9W, Palisades, NY 10964-1000
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