Phymatoderma-bearing turbidites (Oligocene, Tierra del Fuego): Ichnologic implications for discrimination of sustained and episodic gravity flow deposits.

J. J. PONCE; E. B. OLIVERO; D.R. MARTINIONI

Trelew

Congreso; First international Congress on Ichnology - Ichnia 2004; 2004

Museo Paleontológico Egidio Feruglio

ABSTRACT.   Distinguishing sustained, long-lived, from episodic, short-lived, gravity flow deposits only through physical sedimentary features could be problematic. Careful ichnologic analysis is an important tool for discrimination of sustained and episodic gravity flow deposits, as shown from two stacked first order cycles in deep-water Oligocene turbidites at eastern Tierra del Fuego. A 91m thick, mudstone-sandstone lower cycle (a) consists of an upward coarsening-thickening succession, followed by an 81m thick cycle (b) with a reverse trend and an erosive base. Cycle (a) consists of episodic flow deposits (a1, classical turbidites, distal lobe facies); and sustained, flow deposits (a2, channel fill with lateral accretion structures), arranged in minor (3m thick) upward coarsening-thickening (a1) and fining-thinning (a2) cycles. Phymatoderma, Zoophycos, Chondrites, Tasselia, and a vertical spreite structure are preserved at the top of cycle (a), typically in episodic flow deposits. Bioturbation is absent within the channelized deposits of long duration flows (a2) and only the topmost layer of these deposits is bioturbated. Cycle (b) consists of fine conglomerates and medium sandstones, covered by sharp based rhythmically bedded fine sandstones and mudstones. Only the basal 4m bear burrowed levels, mainly with Phymatoderma and subordinated Zoophycos, Chondrites and Tasellia. The rest of cycle (b) lacks bioturbation, recording abundant interstitial fluid escape structures (convolute bedding) indicating rapid deposition. Overall, cycle (b) most likely reflects the main back-stepping of long-lived flows, whose deposits agradationally filled a major channel. Continuous, high sedimentation rates avoid or reduce suitable conditions for colonization of the substrate by organisms.