Abstract
A basin-margin depositional system is the stratigraphic product of terrigenous sediment delivery to the ocean, comprising a flat to low-gradient shelf, or topset, which transitions to a steeper deep-water slope, and, ultimately, a relatively flat basin floor. Erosional and depositional processes across these physiographic domains approximate a clinoform in the stratigraphic record. The shelf margin is a critical environment for terrigenous sediment dispersal because it links the shelf to deep water, and the coarse-grained deposits of associated strata are important subsurface reservoirs. Here, we characterize the shelf-margin and upper slope stratigraphy of the outcropping Upper Cretaceous Tres Pasos and Dorotea formations, Magallanes Basin, southern Chile. The Late Cretaceous Magallanes retroarc foreland basin was an elongate trough oriented parallel to the southern Andean arc and fold-and-thrust belt. The Tres Pasos and Dorotea formations record southward (basin axial) progradation of a high-relief shelf and slope system (>1000 m paleo-water depth) represented by a stratigraphic succession up to 3 km thick that is exposed for tens of kilometers along depositional dip. The character and distribution of deposits that define shelf margins contain evidence for a variety of processes related to deposition, erosion, sediment bypass, and mass wasting. The overall architecture of the Magallanes Basin strata is indicative of a graded basin-margin system interrupted by periods of slope oversteepening and development of out-of-grade conditions. These punctuated periods are recognized by sedimentological evidence for enhanced sediment bypass of the upper slope, and thick submarine fan successions in more distal segments. Development of oversteepened depositional topography is particularly significant as it instigated the only two major periods of coarse-grained sediment delivery to deep water over ~8 Myr during the Campanian. The controls on sediment dispersal beyond the shelf margin are commonly discussed in terms of allogenic forcings, such as tectonics, climate, and receiving-basin geometry, as well as autogenic behavior, such as delta-lobe switching. However, inherited depositional topography does not clearly fit within an allogenic/autogenic dichotomy. Depositional topography inherited from basin-margin evolution influences the position of subsequent shelf margins, which promote coarse-grained sediment delivery to deep water.
Highlights
A shelf-margin is characterized in depositional-dip profile from shallow-dipping topset, across steeper foreset, to shallow-dipping bottomset (Figure 1)
This study focuses on the characterization of outcropping shelf-margin stratigraphy, with an emphasis on the record of punctuated out-of-grade conditions related to inherited depositional topography in the Upper Cretaceous Tres Pasos and Dorotea formations, southern Chile
Ross et al (1994) proposed a series of mechanisms associated with shelf-margin readjustment that lead to enhanced coarse-grained sediment delivery beyond the shelf margin (Figure 1); the predicted stratigraphic architecture has yet to be widely linked to an outcrop record
Summary
A shelf-margin is characterized in depositional-dip profile from shallow-dipping topset, across steeper foreset (clinoform), to shallow-dipping bottomset (Figure 1). Hedberg (1970) described out-of-grade slopes, wherein the shelf margin and upper slope are zones of net erosion, sediment bypass, and mass wasting, whereas the lower slope and basin are associated with substantial deposition. The shelf margin is a gateway for the transport of coarse-grained sediment into deep water, and it is commonly attributed to either external forcings (i.e., allogenic controls) promoting sediment supply (Carvajal and Steel, 2009; Kertznus and Kneller, 2009) or internal dynamics of the system (i.e., autogenic controls), such as compensational stacking as a result of delta-lobe switching (Olariu and Bhattacharya, 2006; Muto et al, 2007; Straub et al, 2009; Hajek and Straub, 2017). Ross et al (1994) noted that inherited depositional topography can control the evolution of later shelf margins, potentially fostering development of oversteepened upper slopes and promoting mass wasting and/or coarse-grained sediment delivery to deep water (Figure 1). Inherited depositional topography does not clearly fit into the either/or, allogenic/autogenic dichotomy of controls on shelfmargin depositional evolution because underlying deposits might have been the product of a wholly distinct set of conditions
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