CYCLCITY, TIME SCALE, AND CORRELATION
OF THE TRIASSIC AND EARLY
JURASSIC RIFT STRATA OF THE ARGANA
BASIN, MOROCCO
OLSEN Paul E.1, KENT
Dennis V. 1,2, ET-TOUHAMI Mohammed1,3, FOWELL Sarah
J. 4, and
WITTE Bill4
1Lamont Doherty Earth Observatory, Palisades, NY 10964, USA
Predominately continental strata of the Argana basin are divided into four
at least partially
unconformity bounded tectonostratigraphic
sequences (TSI to TSIV), each with a characteristic set of
acies and its own style of lacustrine
to evaporite cyclicity. The oldest tectonostratigraphic sequence
(TS-I) consists of the Ikakern Formation
(T1 and T2 of Tixeront, 1973) which is Permian in age. It is
predominantly fluvial, although locally
(e.g. near Timesgadouine), laterally continuous carbonate beds
mark out a prominent cyclicity the specific
nature of which is poorly known but under investigation. In
Canada the Honeycomb Point Formation of
New Brunswick and the Cap aux Meules Formation of the
Magdalen basin (Gulf of St. Lawrence)
appear to broadly correlate with the Ikakern, based on our
preliminary magnetostratigraphic in conjunction
with published data from the Cap aux Meules
Formation. The duration of time of deposition
of the Ikakern is presently unknown.
The Ikakern Formation is separated from the overlying TS II by a locally
strong angular
unconformity; this sequence is comprised
of the lacustrine, deltaic, and fluvial Timesgadouine
Formation (T3-T5 of Tixeront, 1973). At
least the middle part of the formation (lower T5) is strikingly
cyclical with a strong hierarchical pattern
with well developed ~20, 100, 400, and 1750 ky lake-level
cycles. This is strongly reminiscent of
the marginal lacustrine strata of the Triassic Lockatong
Formation of the Newark rift basin (Eastern
United States), consistent with its paleolatitude. Based on
accumulation rates derived from preliminary
Fourier analysis of the lower T5 section, and extrapolated
to the entire section, the preserved portion
of the Timesgadouine Formation was deposited over about
7 million years. Based on vertebrate biostratigraphy,
the Timesgadouine Formation correlates with the
Stockton Formation of the Newark basin;
the newly recovered magnetostratigraphy allows a more
detailed correlation. Adding the 7 my
from the Timesgadouine Formation to the younger
astronomically-calibrated portion of the
Newark basin section yields a date of 232 Ma for the oldest
Timesgadouine Formation.
Overlying the Timesgadouine Formation is the Bigoudine Formation (T6-T8
of Tixeront), which is
separated from the former by an at least
local gentle angular unconformity; the Bigoudine makes up
TS-III and the base of TS-IV. Nearly the
entire formation is cyclical, but the mode of cyclicity changes
up section. The lowest Bigoudine (T6)
consists of a basal conglomerate and fluvial sandstone overlain
by laterally-persistent cycles of red
mudstone and eolian sandstones. Locally there is some bedded
halite, although its relationship to the
cyclicity is unknown. Conformably overlying T6, T7 has very well
developed cyclicity resembling that of
the middle and upper Passaic Formation of the Newark Basin;
some cycles have well-developed black
lacustrine shales. There is a significant eolian-playa sand-
patch component to the drier phases of
the cycles, which becomes progressively more important
upward. T8 conformably overlies T7 and,
except for its uppermost few meters, continues the cyclical
pattern of T7, although with only one
well-developed black shale.
Interestingly, from north (Timesgadouine) to south (Argana), this black
shale changes color. The
transition can be seen in ravines just
northeast of Argana; the color changes without an obvious
change in facies. The Bigoudine consists
otherwise almost entirely of sand-patch cycles, with large
variations in the amounts of red shale.
The uppermost few meters of T8 have a well-developed
cyclicity with very thin black and gray
shales. This portion contains the Triassic-Jurassic boundary,
based on palynology, and represents the
lower part of the succeeding TS-IV. Preliminary
magnetostratigraphy of the lower half
of the Bigoudine allows the accumulation rates of the formation
to be determined by correlation with the
astronomically-tuned Newark basin time scale. Based on this
correlation, there are significant accumulation
rate changes through the formation. However, when the
accumulation rate is adjusted to that
seen in the Newark basin, Fourier analysis indicates the
pervasive presence of well developed ~20,
100, 400, and 1750 ky lake-level cycles that correlate
remarkably well to the Passaic Formation.
The correlation is fully supported by the available
palynology from the Bigoudine, and indicates
that the deposition of the Bigoudine began at about 218
Ma and ended at about 201 Ma, with the
conformable outpouring of the Argana Basalt.
Resting conformably on the Argana Basalt is the Amescroud Formation (uppermost
part of T8 of
Tixeront, 1973). The basal part of this
formation consists of at least one black shale and carbonate
cycle, followed by a mostly red-cyclical
sequence; it appears to have been deposited under
significantly wetter conditions than most
of the underlying Bigoudine Formation. Based solely on
cyclostratigraphic correlation, the outcropping
Amescroud spans only about 450 ky of the early
Hettangian.
From: Essamoud, R. and Ekamel, F. (eds.),
2002, Troisème Réunion du Groupe Marocain du
Permien et du Trias, Casablanca, 18-20
Avril 2002, Résumés, p. 29-30.