Facies and internal architecture of deep-water channel fill in the Cerro Toro Formation, Sarmiento Vista, Chile, SM Hubbard, BW Romans, T Erohina

Tags: Cerro Toro Formation, Sierra del Toro, Tzvetina Erohina, channel deposits, Stanford, California, southern Chile, Internal Architecture, Deep-water Channel Fill, Chile Stephen M. Hubbard, Ultima Esperanza District Country Chile, Formation Name, Outcrop, conduits, Ultima Esperanza District, Basin Setting Foreland, Brian W. Romans, Summary Outcropping, C. J. O'Byrne, R. D. Shew, T. H. Nilsen, Facies architecture, G. S. Steffens, J. R. J. Studlick, Deep-water channel, Cerro Mocho, Sarmiento Vista, AAPG Studies, Stanford University, B. W. Romans, Condor West channel, References Barton, Donald R. Lowe Department of Geological and Environmental Sciences
Content: 37
Facies and Internal Architecture of Deep-water Channel Fill in the Cerro Toro Formation, Sarmiento Vista, Chile Stephen M. Hubbard, Brian W. Romans, Tzvetina Erohina, and Donald R. LowE Department of Geological and Environmental Sciences, Stanford University, Stanford, CAlifornia, USA
EXECUTIVE SUMMARY Outcropping strata of the Cerro Toro Formation at Sierra del Toro (Sarmiento Vista outcrop), southern Chile, highlight the internal complexity associated with submarine channel deposits. The outcrop exposures consist of facies deposited in an elongate fan-channel complex, greater than 100 km (62 mi) long, that occupied the axis of the Magallanes foreland basin during the Late Cretaceous (Campanian). Paleocurrent data indicate that flow was dominated by southward-directed axial currents. Based on the distribution of multiple, large-scale, channelized depositional elements in the outcrop belt, the deposits at Sarmiento Vista are interpreted to represent one of numerous feeder channels that debouched into the channel complex (Figure 1). The entire channelform depositional element studied is <1 km (<0.6 mi) wide through the bottom part of the succession. However, a marked broadening of the channel is apparent at the top of the interval where the topmost beds extend over a lateral distance of at least 2.5 km (1.6 mi). The outcrop is oriented perpendicular to the paleochannel axis and exhibits 180 m (590 ft) of total relief over a lateral distance of approximately 250 m (820 ft). Fine-grained, out-of-channel deposits are mostly covered by vegetation and scree, limiting detailed interpretation of the channel margins. The channelform depositional element is composed of sandstone and sandy-matrix conglomerate (reservoir facies), as well as muddy-matrix conglomerate (nonreservoir facies). Alternating intervals of the sandy and muddy deposits are roughly correlatable across the outcrop, partitioning the sedimentary body into numerous reservoir compartments. Deposits are generally coarsest grained in the axis of the channelform sedimentary body and fine laterally towards the margins. Evidence of sediment bypass, including traction-structured sandstones and conglomerates, is common throughout the outcrop. Nonamalgamated sandy turbidites (thinand thick-bedded) within the channelform body are potentially derived from overbank sedimentation or from smaller, more dilute gravity flows that passed through the channel. They are particularly notable at the edges of the channelform body, and are associated with the stratigraphic surface that demarcates the significant broadening of the channel.
Outcrop Summary
Sarmiento Vista
Specifics Sierra del Toro 51°5.179S; 72°39.312W
Region/province/state Ultima Esperanza District
Country Chile
Formation Name
Cerro Toro Formation
Late Cretaceous (Campanian)
Basin Setting
Foreland (Magallanes basin) related to the rising Andean Cordillera to the west. Axial flow dominant with lateral conduits from the western thrust front.
Basin Size
>6000 km2 (>3750 mi2)
General Outcrop Description and Stacking Pattern
Channelform body with steep margins. Out-of-channel deposits largely covered by scree and vegetation.
Depositional Setting Interpretation
Deposition of coarse-grained sediment took place within a large-scale channel complex that flowed southward along the axis of the deep, narrow basin.
Overall Outcrop Dimensions in Panel
Length 1250 m (4100 ft)
Thickness 250 m (820 ft)
Average Net-to-gross 40­60%
Grain-size range of sand/conglomeratic elements Fine-grained sand to clasts 21 cm (8 in.) in diameter in channel bodies
Minor faulting and folding associated with uplift of Andes
Figure 1. Landsat image of the Ultima Esperanza District showing the location of the Sarmiento Vista outcrop on Sierra del Toro, as well as other localities where the Cerro Toro Formation conglomerate is exposed. The shaded yellow area represents a paleogeographic interpretation of the channel location. Based on outcrop localities of conglomerate and paleocurrent data, it is interpreted that coarse-grained sediment was fed into the north­south-trending channel complex via multiple conduits from the west. Stratigraphic relationships indicate that the conduits were not all active at the same time. Note that paleoflow was generally southward. Inset map shows location of study area (denoted by orange circle) in southern South America.
References Barton, M. D., G. S. Steffens, and C. J. O'Byrne, 2007, Facies architecture of a submarine-slope channel complex, Condor West channel, Cerro Toro Formation, Chile, in T. H. Nilsen, R. D. Shew, G. S. Steffens, and J. R. J. Studlick, eds., Atlas of deep-water outcrops: AAPG Studies in Geology 56, p. 149­153. Hubbard, S. M., B. W. Romans, and S. A. Graham, 2007, Deep-water channel margin architecture, Cerro Toro Formation, Cerro Mocho, Chile, in T. H. Nilsen, R. D. Shew, G. S. Steffens, and J. R. J. Studlick, eds., Atlas of deep-water outcrops: AAPG Studies in Geology 56, p. 128­131. Studies in Geology 56 Copyright ©2007 by The American Association of Petroleum Geologists. DOI: 10.1306/1240924St563265
Figure 2A. The Sarmiento Vista outcrop on the northeastern face of Sierra del Toro. Paleocurrent indicators through most of the stratigraphy suggest that the channel flowed southwards (this is a strike section). The lower part of the channelform deposit is <1000 m (<3280 ft) wide, and the relief on the margin is 180 m (590 ft); this relief is accomplished over a lateral distance of 250 m (820 ft). Notably, the uppermost conglomerate layer (highlighted at the top and bottom by white arrows) extends over a lateral extent of several kilometers (>2 mi). Rectangle represents the area shown in detail in the line drawing in Figure 2B. Red lines correspond to the locations of the measured sections in Figure 3. Figure 2B. Sedimentary lithologies and stratigraphic surfaces as traced from the area defined within the rectangle in Figure 2A. A master surface defines the margin between coarse-grained channel deposits and finer grained out-of-channel units (defined by the heavy black line). The coarsest material within the channelform body is present in the channel axis, fining laterally towards the margins of the deposit. It is possible that the non-amalgamated turbidite facies present within the channelform element represent overbank deposits. Mapping of facies relationships at the edge of the channel (just east of section Sarmiento Vista #1) is significantly impeded by scree cover. Figure 3. The stratigraphic succession at Sarmiento Vista is characterized by alternating intervals of sandy-matrix conglomerate/sandstone and muddy-matrix conglomerate that can be correlated across the outcrop. Paleocurrent measurements indicate that flow within the lower, confined part of the channel was predominantly to the south. The uppermost beds, characterized by an abrupt, eastward shift in paleocurrent direction, may be linked to a change in channel orientation that was associated with a significant broadening of the channelform body from <1 km (0.6 mi) to at least 2.5 km (1.6 mi) wide. The cause of the channel broadening is not known, although it may have been related to an increase in sediment supply into the basin. Note that the stratigraphic thicknesses of the sections are in meters (100 m = 328 ft), and that their locations are presented in Figure 2. 141 Facies and Internal Architecture of Deep-water Channel Fill in the Cerro Toro Formation, Sarmiento Vista, Chile
Figure 4. Characteristics of channel fill. A) Overview of the Sarmiento Vista outcrop (same as in Figure 2A) with the locations of each of the features highlighted in the following photos identified. B) Plane laminations and climbing ripples deposited from gravity flows overspilling the channel margin (increments on measuring staff are 0.1 m [0.3 ft] each). C) Close-up of erosive channel margin (dashed line), highlighting a thin, onlapping shale bed (large arrow) and abundant shale rip-up clasts (small arrows). D) Thick succession of high-density turbidites. E) Interbedded traction-structured conglomerate and sandstone typical of the channel fill. F) Internal incision into muddy conglomerate within the greater channelform body. White arrow points to a local reverse fault. G) Thick succession of traction-structured conglomerate at the top of the outcrop, characterized by well-developed clast imbrication. H) Large raft block within a thick, sandy turbidity-current deposit (person for scale approximately 1.8 m [5.9 ft] tall). I) Cross-stratified sandstone. J) Gravel-filled incision cut into fine-grained slump deposits (unit person standing on) at the base of the channel. 142
Studies in Geology 56

SM Hubbard, BW Romans, T Erohina

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