Paleogene conglomerates in Tierra del Fuego: New evidence of an Uppermost Cretaceous - Paleocene major break in the marine sedimentation of the Austral basin.

D. R. MARTINIONI; E. B. OLIVERO; S. PALAMARCZUK

Santa Rosa

Congreso; Congreso “Paleógeno de América del Sur”; 1996

Universidad Nacional de La Pampa

ABSTRACT:   A Paleogene marine succession of coarse to medium conglomerate and pebbly sandstone, with minor fine sandstone and mudstone, is first described just north of Hito XIX. A faulted block of Aptian-Albian strata rests on a north vergent reverse fault above this succession which in turn is thrusted against younger rocks on its northern front. The main exposure is 40 m thick. Coarse to medium polymictic orthoconglomerate makes up the 54% of the measured section. The mean thickness of the beds is 0.90 m ranging between 0.05 and 2.80 meters. They are poorly sorted, massive or crudely stratified, with sharp, erosive lower contacts. Matrix supported and well sorted rocks are rare. Clasts are well rounded and their maximum diameters fall within 2 to 40 centimeters. The larger clasts are dominantly prolate or oblate and the smaller are mainly equant. Clast imbrication is common and indicates a north-west directed paleocurrent. Prevalent clasts are indurated black shale and acidic volcanic rock, much less common are bluish-gray sandstone, calcareous concretion, vein quartz and granite. Pebbly sandstone represents the 8% with an average bed thickness of 0.27 m (range 0.05-1.20 m). Some beds are lenticular. Strata have sharp, erosive lower contacts, and some are normally graded and/or faintly laminated. Fine sandstone comprises 24% and average bed thickness is 0.96 m ranging between 0.15 and 1.60 meters. Beds have sharp contacts and some are parallel laminated. Mudstone, with millimeter- to centimeter-scale interbedding of very fine sand, makes up 14%. The sandy laminae of these heterolithic packages commonly show ripple cross- lamination. Average thickness of mudstone intervals is 0.44 m (range 0.05-0.85 m). Micropaleontological studies of the fine-grained facies revea1 the presence of Paleogene, probably Upper Paleocene - Eocene, dinoflagellates. Reworked Upper Cretaceous dinocysts are frequent including a mixing of species of three different associations known to be restricted in Antarctica to the Santonian; Lower Campanian; and Upper Campanian. Similar rocks, currently studied by one of the authors (DRM) as part of his doctoral thesis, were observed to lay on basal Tertiary and/or Upper Cretaceous rocks, suggesting an important unconforrnity. To the east, at Cerro de las Bandurrias and Lago Yehuin, polymictic orthoconglomerates, previously assigned to the Beauvoir Formation (Lower Cretaceous), are ascribed herein to the Paleogene after a reexamination of the outcrops. Lithofacies and fossil evidence suggest that these conglomerates can be correlated with the thick subsurface Ballena Group (Eocene) located north of Lago Blanco, Chile. On the basis of well and seismic data this sequence is interpreted as a possible fan-delta fed from an uplifted area located to the south. Stratigraphic relationship, clast composition, paleocurrent direction and inferred correlation of these conglomerates with subsurface possible fan-delta deposits are all consistent and indicate that they represent a major break in the sedimentation of the Austral Basin following a marked tectonic uplift of the Upper Mesozoic marine deposits.