Friday Field Foto #59: Patagonian deep-marine conglomerate deposits
This week’s Friday Field Foto heads back down to southern South America.
While doing my own PhD work down there, I had the opportunity to help out another student with his project on the underlying formation in the basin filling succession.
The Upper Cretaceous Cerro Toro Formation is well-known among sedimentologists for the spectacularly-exposed conglomerate member. These conglomerates were deposited in a marine basin in water depths of at least 1500 m (5000 ft) (based on foraminifera in overlying shale).
The conglomerates are turbidites and fill a ~5 km-wide channel belt that ran along the axis of the elongate foreland basin. Picture the submarine channel system running along the axis of Chile trench (althought that is in the subduction zone trench, whereas this would’ve been in the foredeep on the other side of the arc … but you get the idea).
In the photo above, note the two nearly-equal thickness packages – a lower one dominated by traction structures (the nearly-horizontal stratification) and some sandstone lenses, and an upper, more massive, package. The clasts are generally pebble to cobble sized and mostly Paleozoic metasedimentary rocks.
I’m second author on a recent paper in Sedimentology about this system. I am absolutely swamped right now and haven’t found the time to put together a good post about the study … stay tuned.
In the meantime, here’s the abstract:
Deep-water foreland basin deposits of the Cerro Toro Formation, Magallanes basin, Chile: architectural elements of a sinuous basin axial channel belt (Sedimentology, v. 55, doi: 10.1111/j/1365-3091.2007.00948 )
Coarse-grained deep-water strata of the Cerro Toro Formation in the Cordillera Manuel Señoret, southern Chile, represent the deposits of a major channel belt (4 to 8 km wide by >100 km long) that occupied the foredeep of the Magallanes basin during the Late Cretaceous. Channel belt deposits comprise a ca 400 m thick conglomeratic interval (informally named the ‘Lago Sofia Member’) encased in bathyal fine-grained units. Facies of the Lago Sofia Member include sandy matrix conglomerate (that show evidence of traction-dominated deposition and sedimentation from turbulent gravity flows), muddy matrix conglomerate (graded units interpreted as coarse-grained slurry-flow deposits) and massive sandstone beds (high-density turbidity current deposits). Interbedded sandstone and mudstone intervals are present locally, interpreted as inner levée deposits. The channel belt was characterized by a low sinuousity planform architecture, as inferred from outcrop mapping and extensive palaeocurrent measurements. Laterally adjacent to the Lago Sofia Member are interbedded mudstone and sandstone facies derived from gravity flows that spilled over the channel belt margin. A levée interpretation for these fine-grained units is based on several observations, which include: (i) palaeocurrent measurements that indicate flows diverged (50° to 100°) once they spilled over the confining channel margin; (ii) sandstone beds progressively thin, away from the channel belt margin; (iii) evidence that the eroded channel base was not very well indurated, including a stepped margin and injection of coarse-grained channel material into surrounding fine-grained units; and (iv) the presence of sedimentary features common to levées, including slumped units inferring depositional slopes dipping away from the channel margin, lenticular sandstone beds thinning distally from the channel margin, soft sediment deformation and climbing ripples. The tectonic setting and foredeep architecture influenced deposition in the axial channel belt. A significant downstream constriction of the channel belt is reflected by a transition from more tabular units to an internal architecture dominated by lenticular beds associated with a substantially increased degree of scour. Differential propagation of the fold-thrust belt from the west is speculated to have had a major control on basin, and subsequently channel, width. The confining influence of the basin slopes that paralleled the channel belt, as well as the likelihood that numerous conduits fed into the basin along the length of the active fold-thrust belt to the west, suggest that proximal–distal relationships observed from large channels in passive margin settings are not necessarily applicable to axial channels in elongate basins.