Foreset laminae: the "basic" set
Foreset laminae: the "basic" set
In vertical cuts, bed forms appear as cross-bedding or cross-lamination, as we have seen in some large-scale examples (sand waves, dunes). There is no unequivocal rule for using the term lamination instead of bedding or stratification. The important thing is to specify the scale, in terms of length (in outcrops of sufficient extent) and thickness of the structure. Bed or laminaset thickness indicates the form height, coinciding with it if the whole form has been preserved; otherwise, thickness gives only a minimum estimate of the original height. Local erosion of the bottom is an inherent aspect of the traction mechanism, and preferentially affects bed form crests.
Tractive features are formed under two conditions:
In the former case, illustrated by the picture, only one set of cross laminae is formed; in the latter, several laminasets are superposed with intervening discontinuities (local erosion surfaces: see plates 8, 54, and color photo 16). The delicate point is the identification of individual sets, representing single, continuous events of bed form migration. Every set is comprised between unconformable, or discordant surfaces: the coin staying for scale, for instance, is leaning against the middle of a laminaset made of sand, sandwiched between lenses of fine gravel. All the sand laminae dip to the left, but not at a uniform angle: from right to left, 4 packets can be distinguished. They are separated by angular contacts due to erosional truncations and representing stop-and-go of the bed form migration (if you remember, such surfaces are called reactivation surfaces ). When measuring the set thickness, attention must be paid not to cross one or more such unconformities: for example, if one measures the thickness of the whole sand bed near the left side of the picture, one will obtain a value that is almost double that of the individual set.
If the coset, or multiple set of laminae, was produced by the same type of bed form, it practically consists of a repetition of the same basic unit, or basic laminaset (see, for example, plates 6 36, 54).
Recognizing the vertical expression of sedimentary structures is important, especially when dealing with samples of limited lateral extent, such as hand specimens and cores. In the case of cross-bedding, care must be taken in estimating not only height of the structure (through identification of the basic set, see above), but also the inclination of foreset laminae.
This angle is referred to the base of the bed and reaches its maximum value in sections cut parallel to the paleocurrent. If it roughly corresponds to the angle of repose (which is known for various types of natural grains), the structure is a dune or a ripple. If, on the other hand, it is smaller, other bed forms are implied (see plates 37-41). The two cases are termed, respectively, high-angle and low-angle cross-lamination, and indicate different hydraulic regimes (combinations of current or wave velocity and water depth).
In doing this important test, one must realize that a random section of the sediment can show either the real or apparent inclination of the dipping laminae. A section making an angle of 45-90° with the paleocurrent direction will be strongly oblique or normal to it, and will show foreset laminae inclined at angles smaller than 15° even if the actual inclination is 30°. It would be convenient to get several sections, with different orientations, of the same bed, and compare them. The highest recorded angle can be taken as an approximation of the true dip angle, and that section will be assumed as about parallel to the flow.
To recognize the proper orientation of sections with respect to paleocurrents, see plate 34.