FROM SHELF TO BASIN, ANATOMY OF A GLACIATION RECORD (UPPER ORDOVICIAN, MOROCCO; ANR SEQSTRAT-ICE PROJECT 2013-2017)
Résumé
We investigated the Hirnantian glaciation record based on sedimentology, 3D architecture, palynology,
carbon isotopes and provenance study. Four settings of the Moroccan Gondwana platform are documented:
proximal and distal shelves, shelf canyons and “deep-water” fans; the three former experiencing subglacial
erosion during the glaciation acme. Contrasted successions at the 100 km scale allow inspection of sediment
partitioning through the glacial event.
The proximal shelf shows storm-dominated highstand wedges eroded by poorly differentiated glacial
surfaces or deeply truncated by sand-filled tunnel valleys. The main part of the record however consists of a
single glacial retreat succession comprised of a glaciomarine wedge and an overlying forced regressive delta,
both coeval with the final glacio-isostatic rebound (cf. presentation by P. Dietrich et al.).
In the distal shelf, glacio-eustatic lowstand wedges bounded by flooding surfaces recorded the glaciation
onset occurring more inland. Later, the glacial record s.s. is accommodated in shallow glacial incisions floored
by glaciofluvials. Prior to the glacial maximum, glaciomarine and tidal deposits constitute fingerprints of
subordinate retreat phases. The glacial maximum-ice-sheet front at > 200 km beyond the study area-is
restricted to a single glacial surface and an ensuing glacio-isostatically forced progradation; the latter being
interrupted by an ultimate glacial advance (an ice lobe?), which built a fold-and-thrust belt deforming outwash
sands. Shelf canyons, < 500 m deep, 2-10 km wide, were cut by retrogressive erosion in preglaciation deposits.
Their re-incision likely occurred during the early lowstands, as they did also before, and after, the Hirnantian.
In-canyon active sedimentation and accumulation characterize periods when, and places where a grounding
line occupied the canyon. Glaciomarine deposits are tied to rapid ice retreat phases. During ice minima, tidallyinfluenced
sands were deposited at canyon heads whilst, downslope, starved systems developed.
The deep-water turbiditic succession (> 350 m) includes a single glaciomarine interval and two coarsergrained
sandsheets (50-75 m) representing oversupplied conditions. A 13C isotopic signal allows its correlation
with basins of intermediate latitudes. Whether the turbidites were issuing from a canyon or from a shelf apron
is unknown but most of the succession relates to the glaciation acme. The poor development of glaciomarine
deposits suggests a buffer-in-canyon grounding-line fans or subaerial sandar, respectively between the ice front
and the deep-water depocenter. Detrital zircon geochronology suggests a significant contribution of reworked
preglacial deposits remobilized at lowstand conditions. Actual glaciogenics tied to maximum glacial advances
represent less than 50% of the succession. Those are mature sandstones remobilized via subglacial erosion
from the inner glaciated platform. None of the four settings offers a comprehensive glacial record, yet separately, each fairly documents
one particular stage within the glaciation development (e.g., glacial advances preceding the glaciation acme,
early glacial retreat). Waiting for a refined Hirnantian chitinozoan biozonation, high-resolution correlations
from one setting to the other essentially rely on sequence stratigraphic inferences. A provenance study is
currently in progress to corroborate the shelf-scale correlation framework. Correlations with glacial cycles
preserved over the inner platform (Mauritania, Algeria, Libya) remain controversial.