SEDIMENTARY STRUCTURES AND GEOMETRY OF SEDIMENTARY DEPOSITS
5.7 Geometry of Sedimentary Deposits and Lateral Facies Changes
water, common today, evolved in the Tertiary so that their rootlets would not indicate subaerial exposure.
Where plants have been growing in a semi-arid environment, calcretes may develop within the soil profile and the plant roots may then become calcified to formrhizocretions(see Fig. 5.63, Section 5.5.6.2).
5.7 Geometry of Sedimentary Deposits and Lateral Facies
sediments may form fans, wedges and aprons where deposited at the toe of a slope. Examples include alluvial fan, fan-delta, toe-of-slope and submarine fan deposits.
Facies, defined by the lithological, textural, structural and palaeontolog- ical features of the sedimentary rock, commonly change laterally as well as vertically in a sedimentary succession. This can involve a change in one or all of the parameters defining the facies. Lateral changes can be very rapid, over several or tens of metres, or more gradational, when the change takes place over several kilometres. Facies changes reflect changes in the environmental conditions of sedimentation (Chapter 8).
Observations of the geometry of individual beds and rock units normally present no problem. In a quarry or cliff exposure, follow beds laterally to determine their shape and then make notes and sketches (or take photographs).
With the larger-scale geometry of sediment bodies, if exposures are very good as in some mountainous and vegetation-free regions, it may be possible to see the lateral changes and sediment-body shape directly, and to see the relationships between sedimentary units. Where exposure is limited, make detailed logs across the same part of the rock succession at several or many localities. To be sure that sections are equivalent, it is necessary to have either a laterally continuous horizon in the succession (a marker bed), or the presence of the same zonal fossils. If lateral facies changes are suspected in an area of poor exposure then detailed mapping and logging of all available exposures may be required to demonstrate such changes.
5.7.2 Stratigraphic relationships:
offlap, downlap, onlap and truncation
truncation onlap
downlap
Figure 5.82 Relationships between sedimentary units: truncation, onlap and downlap.
From the study of seismic sections, several types of large-scale relation- ship between sediment bodies have been noted: truncation, onlap, down- lap, offlap and toplap (Fig. 5.82). It is important to be able to document such relationships in a succession, since they relate to changes in rel-
ative sea level andaccommodation space(concepts used in sequence stratig- raphy: see Sections 2.10.2 and 8.4). However, in the field, they are rarely seen unless large coastal or mountainside cliff exposures are available; they may be revealed by regional stratigraphic studies and mapping.
Downlapmay be seen as gently to steeply dipping surfaces (the clinoforms described in Section 5.3.3.14, for example, are downlapping beds: Figs 5.33
Figure 5.83 Massive reef limestone (left) with offlapping reef-debris beds downlapping to the right onto condensed deeper-water limestone. Cliff is 40 m high. Triassic, Catalonia, Spain.
and 5.83), and the downlap surface may be visible. Downlap – offlap repre- sents progradation of a sedimentary unit and the downlap surface itself is usually a thin, condensed bed or sediment-starved horizon with much bio- turbation, perhaps with glauconite or phosphorite. In some instances with clinoforms, toplap may be visible.
Onlapof one sedimentary unit onto or against another is a common large- scale arrangement but again it is rarely observed in one exposure. It can be seen where strata gradually bury a unit with topographic expression, such as a carbonate platform margin or reef (Fig. 5.84), or where strata fill a large, broad channel structure and lap up onto its margin, or where strata onlap a tilted surface (Fig. 5.85). However, onlap on a larger scale can be demonstrated by determining the age of the base of a stratigraphic unit over a large area, when it may then be apparent that it youngs in a particular direction, i.e., it is onlapping the underlying strata that way. The nature of the onlap surface itself (which may be anunconformity) may also change laterally; for example, it may become a more prominent palaeokarst in the direction of onlap. Onlap of this type in marine strata reflects a relative rise of sea level and the onlap surface may be a sequence boundary and/or transgressive surface (see Section 8.4).
A truncation surface is where strata are cut off beneath a prominent bedding plane. Again this may be visible in good outcrops but commonly it is on a larger scale, and examining exposures over a large area may be necessary to identify this type of stratigraphic boundary (anunconformity).
Truncation surfaces are the result of uplift, perhaps folding, and erosion of strata. Truncated surfaces may also be onlap surfaces, with the sediments
Figure 5.84 Massive reef limestone (left) being onlapped by younger fine-grained limestones. Later compaction has accentuated the dip of the onlapping strata. Height of reef 30 m. Triassic, Catalonia, Spain.
Figure 5.85 Onlap of sedimentary strata. Notice the subtle discontinuity within this package of shallow-water limestones and mudrocks. Height of cliff 80 m. Jurassic, Mareb, Yemen.
immediately beneath the surface getting older in the direction of younging of the overlying unit. This would reflect the increased time for erosion of the rocks below the onlapping unit.
For these larger-scale relationships between stratigraphic units, look care- fully at the bedding planes in a good outcrop and follow them along.
• Is there any gentle dip of beds down to a surface? This would bedown- lap (downlap may involve more steeply dipping beds but this should be obvious).
• Is there any truncation of underlying strata up to a prominent bedding plane? This would indicate anunconformity.
• Laterally, does a bed overlap the one below onto a prominent surface?
This would beonlap.
These may all be very subtle arrangements so that you may need to study the exposure carefully, perhaps by standing back or by using binoculars on cliff-faces, to see these features. Bear in mind, however, that these angular relationships between stratigraphic units depend on the plane of the section, and with some outcrops there is the problem of perspective, such as when looking up to high cliffs, for example.