Stratigraphic Expression of the Transition From Basin Plain to Slope Sedimentation in Outcropping Strata of the Magallanes Basin, Chilean Patagonia

Abstract

Conceptual models of the transition from deep-marine slope strata to overlying shallow-marine deposits are well established and can aid prediction in data-limited scenarios. In contrast, the stratigraphic transition from distal, basin plain to overlying slope deposits is not as well understood. As a consequence, the facies and architecture of sandstone-rich deposits that are characteristic of the initiation of slope sedimentation have not been well documented. Cretaceous strata of the Magallanes Basin in Chilean Patagonia are characterized by a significant transition from basinwide mudstone-dominated strata of the Cerro Toro Formation, to the prevalence of southward propagating, kilometer-scale slope clinoforms of the Tres Pasos Formation. The laterally continuous, seismic-scale outcrops of the Tres Pasos and Cerro Toro Formations have been studied for decades, however, much of the focus has been on channelized lower to middle slope strata. The interval of interest, demarcated by the Chingue surface at its base, offers the opportunity to study the basin plain to slope transition. The strata underlying the Chingue surface are characterized by a ~1 km thick succession of concordant mudstone and interbedded, laterally continuous, thin-bedded (<30 cm) siltstone to very fine-grained sandstone beds that are interpreted as distal turbidite deposits. The Chingue surface is a highly irregular contact between these distal basin plain strata and overlying, sandstone-rich, turbiditic deposits 20–60 m thick. Strata dominantly consist of a non-amalgamated sandstone package at its base with individual beds that can be correlated 100s m laterally. Normally graded, fine- to medium-grained sandstone beds range from 10 cm to >1 m thick and commonly include abundant plane lamination. Locally, beds thicken upwards, and up-dip and down-dip stratal terminations have led previous workers to interpret a ponded origin for these units. Overlying these sandstone-prone deposits is a ~300 m thick succession of discordant mudstone and chaotic, discontinuous sandstone bodies interpreted as mass transport deposits (MTDs). The stratigraphic interval thus reveals an evolution from: 1) relatively quiescent basin plain sedimentation; 2) potential erosion across the basin floor and ponding of sand in irregular topography; 3) basin-wide mass wasting and accumulation of thick MTDs; and 4) establishment of a high-relief clinoform system that filled the basin through southward progradation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015