Fjord sedimentation in the Late Carboniferous of northwestern Argentina

LIMARINO, C.O.; MARENSSI, S.A.; TRIPALDI, A.; CASELLI, A.T.

Florencia, Italia

Congreso; 32nd International Geological Congress; 2004

International Union of Geological Sciences

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-fareast-language:ES-AR;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} -->      Fjords are relatively narrow and steep-sided glacier valleys flooded by the sea. The interplay of glacial (including fresh water runoff) and marine processes control sedimentation and the ecology of such settings. During glacial advances the ice excavate down into the valley floor and during glacial retreat rapid glacioeustatic sea-level rise produce relatively deep marine conditions even at short distances from the ice-front.      Late Paleozoic glacigenic deposits related to the extensive Gondwana glaciation have been described from many places in southern South America. In northwester Argentina they have received much attention and recent papers focused on the detailed sedimentology, stratigraphy and correlation with other gondwanian glacial deposits. However, the main geologic framework were sedimentation took place remains speculative.      Based on detailed facies description, measurements and mapping, along with accepted paleogeographic interpretations we propose that the lower part of the Guandacol Formation (Namurian) in the Huaco anticline locality, San Juan Province, NW Argentina, accumulated in a fjord-like setting.      Rapid vertical facies changes from true tillites to resedimented diamictites, shales with dropstones and finally mudstones with thin marl beds attest not only the glacial to post-glacial transition but also the presence of tens of meters water depth even at short distance from the ice-front. Basal coarse-grained diamictites are located in the deepest part of the valley or against the valley margins. Thinly-bedded, fine-grained diamictites form strongly prograding clinoforms and shales with mudstones onlap the valley margins. Lateral facies changes and the relatively narrow outcrop belt, oriented in a roughly NW-SE direction, also point out to steep-sided valley confined sedimentation. Although scarce, fossil invertebrates and palynomorphs attest to a marine setting. Trace fossils and low faunal diversity suggest a stressed marine habitat most probably related to an important fresh water runoff and high sediment supply delivered from the glacier front.