Some Observations on the Nature of Depositional and Erosional Surfaces

Abstract

Abstract The stratal architechture of the Gulf of Suez can be discriminated on several different orders. There are the sequences that can be defined paleontologically: however, since biostratigraphically detectable lacunae can be caused by several different processes, graphic correlation time gaps do not always correlate to true depositional sequence boundaries. Lacunae can occur at regressive surfaces of erosion, where part of the sedimentary record is removed during sea level fall, at ravinements and related transgressive surfaces of erosion, and at condensed intervals where sedimentation rates are so slow that several fossil zones are compressed. Depositional sequences are bounded by regional erosional unconformities generated by a relative sea level fall. The bounding surfaces are regressive surfaces of subaerial or submarine erosion. Therefore, of the three catagories of lacunae listed above, only one, the first, constitutes a depositional sequence boundary. The number of depositional sequences related to long term relative sea level changes in the Suez Rift should, therefore, be expected to be smaller than the number of biostratigraphically defined sequences. There are also numerous small-scale, high order stratigraphic surfaces that can be put into a sequence framework that reflect autocyclic and eustatic sea level shifts. The major events controlling the sequence stratigraphy of the Suez Rift, however, were the tectonic processes responsible for extension, uplift, and subsidence. Consequently, through the entire Lower Miocene (Aquitanian, Burdigalian), and Lower Middle Miocene (Langhian) there are only two major depostional sequences. Introduction Modern quantitative techniques allow biostratigraphers to develop precise geochronologies, interpret paleobathymetry, recognize depositional environments and key stratal surfaces (condensed intervals, maximum flooding surfaces, and unconformities), calculate sedimentation rates, and recog-nize climate changes. The integration of paleontological data and their interpretation with other geological and geophysical data sets result in more precise stratigraphic correlations and paleoenvironmental interpretations within a sequence stratigraphic framework (Wescott and others, 1998). One important tool available to stratigraphers is graphic correlation, which is a method for deriving precise, consistent, more accurate correlations. The graphic cor-relation process involves either crossplotting two stratigraphic sections of similar age on x,y coordinates and projecting the observed fossil ranges of both sections to a line of correlation or crossplotting the observed ranges of a sin-gle stratigraphic section against a database of composited fossil ranges scaled in composite standard units (Carney and Pierce, 1995). This database is referrred to as a composite standard. It is contructed by combining fossil ranges from several stratigraphic sections. As more fossil groups are added, the composite standard becomes more robust and accurate. Non-fossil data, such as radiometric dates, can also be incorporated into the database to to further refine and calibrate the composite standard. A worldwide composite standard can be constructed from widely spaced sedimentary basins, or provincial standards can be built for individual basins where some fossil ranges may be restricted because of local environmental conditions during deposition