13.3: Basins Caused by Crustal Loading and/or Flexure

Foreland Basins

Figure \(\PageIndex{1}\): Idealized cross section through a foreland basin (Page Quinton via Wikimedia Commons; CC BY-SA 4.0; which is modified from Miall and Catuneanu (2019) which was after Kauffman (1977, 1984).

Tectonics and Crustal Dynamics

Foreland basins develop in convergent plate tectonic settings where shortening causes the development of stacked thrust sheets which in turn cause flexural subsidence in adjacent areas.  These basins can be very large (thousands of km²) and contain thick sedimentary successions (hundreds to thousands of meters).

Foreland basins can develop in two positions relative to the rising mountain belt:

• Peripheral (or proforeland) basins develop on the subducting plate in front of the advancing thrust front. 
• Retroforeland basins develop on the overriding plate behind the core of the orogen.

In both cases, the basins develop in response to crustal shortening, the development of stacked thrust sheets and loading of the crust. 

Within individual foreland basin systems, individual subbasins or depocenters can develop.  Wedge-top (piggyback) basins form atop active thrust sheets.  The foredeep is the primary and deepest depocenter in most foreland basin systems; it forms ahead of the thrust front at the point of maximum downwarping of the crust.  Further basinward, flexure of the crust can cause the development of a low amplitude forebulge, which is a slightly uplifted area that can experience nondeposition and/or erosion.  The forebulge separates the main foreland basin from the gently downwarped back-bulge basin.

Morphology

In plan view, foreland basins are relatively large features that are elongate along the trend of the thrust front and orogenic core.  In cross-section they typically display a strongly asymmetric profile with the thicket basinfill developed in the foredeep immediately adjacent to the thrust front.  Some retroforeland basins (ex: Laramide basins of the American west) are structurally segmented into smaller subbasins separated by uplifted blocks.  Wedgetop basins developed atop the thrust sheets are discontinuous and have intense internal deformation.  Backbulge basins are typically smaller, thinner, and more symmetric than the main foreland basin. 

As deformation progresses, the thrust front can overrun older portions of the basinfill as it migrates outward away from the orogenic core.  This results in a complex and diachronous succession that might be more deformed (even syndepositionally) in proximal areas and youngs in more distal areas.      

Sedimentology

In simple, conventional foreland basins the fill is strongly asymmetric and thicknens toward the foredeep.  Early in the history of the basin, the generation of accommodation outpaces the ability of river systems to deliver sediment.  This early, “flysch” phase of sedimentation is characterized by deep-water (commonly marine) sedimentation characterized by black shales and mudrocks/sandstones associated with turbidite deposits.  Greywackes and immature (sometimes intraformational) conglomerates and breccias can be present locally.  Coarser-grained, syntectonic conglomerates might be present in areas adjacent to the thrust front.

As mountain building continues, subsidence rates decrease and more established river systems become capable of transporting large amounts of sediment into the basin.  During this later molasse phase of sedimentation, average water depths decrease and shallow water/nonmarine clastic sedimentation becomes the norm across much of the basin. 

Overall, the major trends in foreland basin sedimentation include:

• Thickening of the basin fill adjacent to the thrust front
• Overall shallowing via the transition from flysch to molasse sedimentation
• An overall decrease in grain size away from the mountain front

Accretionary Wedges and Forearc Basins

Figure \(\PageIndex{2}\): Cross section of an accretionary wedge and forearc basin formed at a convergent margin (Page Quinton via Wikimedia Commons; CC BY-SA 4.0;  which is after the Accretionary prisms and forearc basins entry in Geological Digressions webpage).

At subduction zones, where oceanic crust is subducted beneath an overriding plate (continental or oceanic), a linear volcanic arc develops on the overriding plate as a consequence of partial melting.  The arc forms a structural buttress or backstop for the compressive forces that exist in the area between the arc and the trench (an area commonly referred to as the arc-trench gap).  Within the arc-trench gap, sediment accumulation can take place in the accretionary wedge and the forearc basin. 

The accretionary wedge (or prism) is a wedge-shaped body of deformed sediment composed of marine sediment and fragments of oceanic crust scraped from the subducting plate.  Common sedimentary rocks include black shales, cherts, turbidites, and other litholgies formed in deepwater environments.  This material is intensely deformed and crosscut by imbricated, arc-dipping thrust faults and may form bathymetric highs.  Although the driving processes are different, the accretionary wedge and thrust faults broadly resemble the fold and thrust belts formed in other convergent settings. 

Unlike foreland basins which develop in areas that are adjacent to, and loaded by thrust faults, forearc basins develop on the overlying plate in the low areas between the volcanic arc and the bathymetric highs of the accretionary wedge.  They are filled by relatively immature sediment shed from the volcanic arc and/or the accretionary wedge.  Forearc basins are relatively small, dynamic, and internally deformed areas of sediment accumulation.      

References and Resources

• Classification of sedimentary basins
• Basins formed by lithospheric flexure
• Sedimentary basins: Regions of prolonged subsidence
• Accretionary prisms and forearc basins

• Allen, P.A., and Allen, J.R. Basin Analysis: Principles and Application to Petroleum Play Assessment: Wiley-Blackwell, 640 p.

• Kauffman, E.G., 1977, Evolutionary rates and biostratigraphy, in Kauffman, E.G. and Hazel, J.E. eds., Concepts and Methods of Biostratigraphy, Stroudsburg, Pennsylvania, Dowden, Hutchinson and Ross Inc., p. 109–142.

• Kauffman, E.G., 1984, Paleobiogeography and evolutionary response dynamic in the Cretaceous Western Interior Seaway of North America, in Westerman, G.E. ed., Jurassic-Cretaceous Biochronology and Paleogeography of North America, Geological Association of Canada, Special Paper 27, p. 273–306.

• Miall, A.D., and Catuneanu, O., 2019, Chapter 9 - The Western Interior Basin, in Miall, A.D. ed., The Sedimentary Basins of the United States and Canada (Second Edition), Elsevier, p. 401–443, doi:10.1016/B978-0-444-63895-3.00009-7.