beds

depositional surfaces

The analysis of complex sedimentary systems involves their description, classification and eventual interpretation. To this end sediment depositional settings are characterized and subdivided into hierarchies of genetically related discrete stratigraphic "elements", "bodies", or "units". The geometric shapes of these depositional units are defined by their boundaries and collectively they form the building blocks of the sedimentary architecture.

• Unit: general geological term that refers to a distinct geologic entity with a lower and upper confining boundary. This unit can be a bed within a larger group, forming a bedset or, as in this specific case, an element within a larger group of elements.
• Body: general geological term that refers to a mass of sediment with a lower and upper confining boundary that is distinct from other masses of sediment. This may be a group of beds or elements.
• Element: specific geological term that refers to a distinct body or assemblage of bodies of sediment with lower and upper confining boundaries that are genetically related to each other and were generated in a common depositional milieu. Architectural elements have been defined by Pickering et al (1998) as an interpretive characterization of a sedimentary feature distinguished on the basis of its geometry, scale and facies.

Unfortunately confusion may result if "elements", "bodies", or "units" are applied synonymously in the same description of the stratigraphy of the sedimentary rocks. This is because of the more general meanings of "bodies", and/or "units" and the fact they tend to be used regardless of the lithofacies assemblage, vertical profile, internal and external organization, geometry and/or the depositional system in which they occur. Each of these terms may be prefixed by "depositional", "stratigraphic" or "architectural".

As we see above some stratigraphers making a sequence stratigraphic analysis and wishing to avoid this confusion define the genetically related sedimentary "architectural elements" as the basic descriptive building block of sedimentary basin architecture.

A top-down hierarchical classification of "architectural elements" is used that often starts at a sedimentary basin scale. This is sub-divided downward into a series of broad elements, each in turn subdivided ever downward to the ultimate subdivision of "architectural elements", the laminae or even the individual sand grain. This top down classification is used to provide a framework of the basin to its interrelated broader larger scale "architectural elements" and their tie to the smaller scale "architectural elements". sequence stratigraphic analysis tends to iteratively use a mix and match of the top down classification with a bottom-up classification. This interactive approach uses the general to guide an understanding of the specific and vice versa.

The architectural elements of a prograding barrier island shoreline. Note the sheets of the barrier island beaches, the lobes of the ebb tidal deltas, the stacked and amalgamated channel fill and en-echelon shingling of the storm washover beach materials.

A hierarchy of “architectural elements" has been related by Sprague et al (2002) directly to the hierarchy of "stratal units" of sequence stratigraphy. Indeed when “architectural elements" are integrated with biostratigraphic data they provide part of the framework from which cycles of base level rise and fall can be interpreted. This approach eventually enables the prediction of the three-dimensional architecture of the sedimentary rocks across the basin.

The concept of “architectural elements" has been built from Brookfield's (1977) hierarchical order of the boundaries of aeolian sediment; Allen's (1983) fluvial bounding sufaces and his concept of "architectural elements"; Miall's (1985) apply a hierarchy of scales to fluvial depositional units; and Pickering et al's (1989) hierarchies of boundaries for the architectural elements of deepwater systems .

Sprague et al (2002 a & 7 b) have combined the use of both the boundaries and the enclosed sediments to describe a system of hierarchical frameworks for both deepwater and fluvial sediments that is based solely on the physical stratigraphy of the strata; a modification from Pickering et al (1995) illustrated above. The Sprague et al (2002 a & 7 b) architectural framework is thus comprised of both genetically related stratigraphic elements and their associated boundaries.

Cited References
Allen, J. R. L. 1983, Studies in fluviatile sedimentation: bars, bar complexes and sandstone sheets (low sinuosity braided streams) in the Brownstones (L. Devonian), Welsh Borders. Sedimentary Geology, 33, 237-293.
Brookfield M. E. 1977, The origin of boundaries in ancient aeolian sandstones: Sedimentology, v. 24, p. 303-332
Miall 1985, Architectural elements and boundaries: A new method of facies analysis applied to fluvial deposits: Earth-Science Reviews, v, 22, p. 261-308
Pickering, K.T., Hiscott, R., and Hein, F.J., 1989. Deep-marine Environments: Clastic Sedimentation and Tectonics: London (Unwin Hyman).
Sprague,A. R., P. E. Patterson, R.E. Hill, C.R. Jones, K. M. Campion, J.C. Van Wagoner, M. D. Sullivan, D.K. Larue, H.R. Feldman, T.M. Demko, R.W. Wellner, J.K. Geslin1 (2002), The Physical Stratigraphy of Fluvial strata: A Hierarchical Approach to the Analysis of Genetically Related Stratigraphic Elements for Improved Reservoir Prediction, (Abstract) AAPG Annual Meeting
Sprague, A. R., M. D. Sullivan, K. M. Campion, G. N. Jensen, F. J. Goulding, T. R. Garfield, D. K. Sickafoose, C. Rossen, D. C. Jennette, R. T. Beaubouef, V. Abreu, J. Ardill, M. L. Porter, and F. B. Zelt, (2003), The Physical Stratigraphy of Deep-Water strata: A Hierarchical Approach to the Analysis of Genetically Related Stratigraphic