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Modeling the Stratigraphic and Facies Architecture of a Flow-Expansion Zone Constrained with Outcrop Data


The transition from confined to unconfined turbidity flow can result in the deposition of both high (∼100's:1) and low (∼10's:1) aspect ratio sedimentary bodies, which are not simply described as either channelform or lobate. The stratigraphic architecture and facies variability preserved from deposition in this transitional zone is often difficult to resolve in most subsurface datasets, whether deposited at a channel-lobe transition zone, or in a proximal channel-overbank setting. In order to constrain the geometry of sedimentary bodies that result from flow expansion, high-resolution analysis of a 1500 m long and ∼100 m thick outcrop is analyzed from the Tres Pasos Formation, Magallanes Basin, southern Chile. The stratigraphic complexity and facies variability is constrained through the construction of a three-dimensional (3-D) interpretation and a detailed geocellular model. There is a wide range of sedimentary body geometry observed, ranging from channelized (20 m thick over ∼350 m), highly lenticular units (1 to 12 m thick over 900 m), broad and tabular beds (up to 1 km long), and isolated scour-fill bedsets (aspect ratio: ∼10's:1). Forty-two stratigraphic sections were measured between 10 and 25 m apart, a distance appropriate to capture fine-scale lateral changes in facies and stratal architecture. Confidence in the documented stratigraphic architecture is high as major sedimentary packages and mappable surfaces were walked out and surveyed using a differential global positioning system (dGPS) in order to constrain their variation and lateral continuity. DGPS data (∼10 cm resolution) constrains key stratigraphic surfaces, measured section lines, and topographic intricacies in order to complement and refine a digital elevation model (DEM) of the outcrop belt. The measured section pathways were converted to pseudo-well pathways and populated with grain-size data and facies interpretations. Surveyed stratigraphic surfaces are modeled and tied to depositional unit tops in the pseudo-wells. Using this framework, facies codes and grain size information is used to populate the model in 3-D space, and the vertical and lateral continuity of beds and sedimentary bodies are quantified. The overall connectivity of the outcrop is highly variable, controlled by the mapped distribution of the various high to low aspect ratio sandstone bodies typical of the flow-expansion zone.