Friday, July 22, 2011

Foreland basins in a nutshell: Part II - Basic definition of a foreland basin

Part 1

Prior to the development of plate tectonic theory, sedimentary basins were discussed in terms of the geosynclinal system, in which basins were described and classified according to their geometry and fill. This was not a simple classification system. For example, in just a two-page Science paper in 1944, Kay described miogeosynclines, eugeosynclines, deltageosynclines and autogeosynclines. He recognized that the original definition, credited to Dana in 1873, had been misused and misinterpreted in the intervening seventy-odd years (Kay, 1951). In its simplest form, Kay defined a geosyncline as "the surface developed at the base of extensive surficial rocks that subsided deeply during their deposition or accumulation," (Kay, 1951, p3).

Kay used the terms eugeosyncline and miogeosyncline to distinguish between geosynclines that either have or lack active volcanism respectively. When discussing orogenies and foreland basins, these are the two historic terms that are most relevant (miogeosynclines are associated with the stable platform and eugeosynclines are associated with volcanic arcs). All of the other named geosynclines are really variations of these two types. For a good overview of geosynclines, and the problems with the theory, I recommend The Story of Geosynclines.

An idealized cross-section through a geosyncline.
Image source:
The mechanisms behind the formation of geosynclines were not always understood, and I think this is a good example of just how unifying plate tectonic theory really is. This was recognized at least as early as 1974, when Dickinson wrote that “the geosynclinal terminology used prior to the advent of plate tectonics is inadequate to describe fully the plate-tectonic settings of sedimentary basins” (p 1).

In fact, it was Dickinson who wrote that despite its emphasis on horizontal movements, plate tectonic theory provided the best mechanism for the large vertical displacements required in order for the thick accumulations of sediment found in some parts of the continent to occur (1974).

Foredeep trough is a term used to describe "an isostatically induced peripheral depression that developed in response to the load imposed on the lithosphere by the ... flow of supracrustal rocks up on to the flank of the craton" (Price, 1973, p 498). Price referred to supracrustal rock flow because he was describing deformation in the southern Canadian Rockies at a Rocky-sized scale of observation.

Even before plate tectonics, the term foreland was being used to describe regions adjacent to orogenic belts (Kay, 1954), but Dickinson (1974) is often credited with first defining foreland basins as such. He redefined eugeosynclines and miogeosynclines to fit within a plate tectonic framework, but, in his own rather eloquent words (p8):
It does not allow for the considerable sophistication of geosynclinal theory in full flower, and results in the unnecessary lumping of things that the full geosynclinal terminology accords different status. Nor does it meet the need to relate various types of sedimentary basins to different kinds of plate interactions, rather than just to the two main kinds of substratum.
Dickinson (1974) designated foreland basins as the retroarc basins on the cratonal or platformal interior of the continent in arc-trench systems, while recognizing that foreland basins can form during continental collisions as well. (This publication, Tectonics and Sedimentation, is also a good read).

The problem I personally have with these definitions is that they tie the formation of a foreland basin specifically to a developing fold-thrust belt. In Western Canada, the Cordilleran Orogeny involved more than just the fold-thrust belt, and I suspect the terrane collisions and subsequent deformation would have caused a flexural response prior to initiation of the thrust faults.

Leckie and Smith (1992) defined a foreland basin as follows: "A succession of sedimentary rocks deposited in a cratonic region adjacent to an active orogenic belt. Sediments are derived mainly from the orogenic belt and thicken toward it." I like this one because it doesn't limit the orogenic belt to the thrust-faults.

Regardless of how it is defined, what a foreland basin is fundamentally the same. It is the basin adjacent and parallel to an active orogeny. The subtle variations in the definitions, do however, impact how the timing of the initiation of the foreland basin is determined; this is one of the major components of my thesis.

The difference between the two types of foreland basins, retroarc and peripheral, will be the subject of part III.

Dickinson, W. (1974), Plate Tectonics and Sedimentation, in Tectonics and Sedimentation, edited by W. Dickinson, pp. 1-27, Society of Economic Paleontologists and Mineralogists.
Kay, M. (1944), Geosynclines in continental development, Science, 99(2580), 461. 
Kay, M. (1954), North American geosynclines, The Geological Society of America, New York.
Leckie, D. A., and D. G. Smith (1992), Regional setting, evolution, and depositional cycles of the Western Canada Foreland Basin, American Association of Petroleum Geologists Memoir, 55, 9–46.
Price, R. (1973), Large-scale gravitational flow of supracrustal rocks, southern Canadian Rockies, Gravity and tectonics, 491–502. 

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