Textural Trends in a Conglomeratic Slurry Flow Deposit, Cretaceous Cerro Toro Formation, Chile: Implications for Debris Flow Transformation

Crane, William H.¹ (1) ChevronTexaco, San Ramon, CA

The Upper Cretaceous Cerro Toro Formation of the Magallanes Basin, Chile is dominated by fine-grained sandstone and mudstone punctuated by thick sections of interbedded conglomerates and sandstones. These coarse-grained deposits have been interpreted as the fill of a series of deep-water channels. The conglomerates in the Cerro Toro Formation vary from clast-supported units with a coarse-grained sandy matrix interpreted to represent bedload deposition from high-density turbulent flows to muddy matrix-supported units interpreted as debris flow deposits. Intermediate between these types of deposits are a series of sedimentation units showing a basal, clast-supported division overlain by a muddy, matrix-supported division. These deposits occur widely in the Cerro Toro outcrop belt and have been described as conglomeratic slurry flows and called “Cerro Toro Style” slurry flows. A single well-exposed, 15 m thick deposit selected for study shows three distinct subunits: a basal, clast-supported division; a clast-rich but matrix-supported zone; and an upper clast-poor matrix-supported division. Grain size distribution, vertical grading trends, sand:mud ratios in the matrix material, and maximum clast size data strongly indicate that this unit was deposited by a flow that was turbulent at times during transport and sedimentation. Although conglomerate clasts in the upper division are matrix-supported, this division is grain-supported sandstone and does not appear to have been deposited by a fully cohesive debris flow. The Cerro Toro style slurry bed characterized indicates that comprehensive textural analysis can be critical to assessing flow rheology and may provide guidance towards predicting down-dip flow evolution.