Encyclopedia of Planetary Landforms DOI 10.1007/978-1-4614-9213-9_558-1 # Springer Science+Business Media New York 2014
Synaeresis Crack Polygons Henrik Hargitaia*, P. G. Erikssonb, E. L. Simpsonc and O. J. Okaforb a Planetary Science Research Group, Institute of Geography and Earth Sciences, Eötvös Loránd University, Budapest, Hungary b Department of Geology, University of Pretoria, Pretoria, South Africa c Department of Physical Sciences, Kutztown University, Kutztown, PA, USA
Definition Commonly discontinuous crack patterns, sometimes polygonal in plan view with V- to U-shaped cross-sectional profiles, formed in mud laminae and thin beds commonly within successions of interbedded sandstones and mudstones, due to dewatering in a subaqueous setting (and not due to desiccation under subaerial exposure) (Fig. 1).
Synonyms Manchuriophycus; Microbial mat-related cracks and diastasis cracks (Cowan and James 1992; some examples only); Subaqueous shrinkage crack polygon
Description Crack patterns on laminae and thin mud bed surfaces (within interbedded mudstone-sandstone successions of subaqueous – marine and nonmarine – deposition) that display a variety of planforms varying from polygonal through long and sinuous cracks to short, spindle-shaped cracks can occur as either negative-relief crack features or, if filled, as positive features on the soles of overlying sandstone beds; normally only a single generation of cracks on a particular mud bed surface, in cross-section, may be either v-shaped or u-shaped; mud curls at polygon margins are absent (Reineck and Singh 1975, p. 51; Plummer and Gostin 1981; Boggs 1987, pp. 183–184; Collinson and Thompson 1989, pp. 148–150; Ricci Lucchi 1995, p. 142).
Morphometry Polygonal crack patterns seldom exceed a few cm in maximum dimensions and tend to form relatively equant geometries which seldom entirely cover an exposed surface of mud beds in the field – partial preservation (or even limited scale primary formation) contrasts with the more pervasive subaerial desiccation cracks.
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Encyclopedia of Planetary Landforms DOI 10.1007/978-1-4614-9213-9_558-1 # Springer Science+Business Media New York 2014
Fig. 1 Synaeresis cracks within interbedded muddy and sandy sedimentary rock interbeds, seen in plan (left) and profile (right) views. Note the limited polygons formed, common spindle shapes, and more irregular patterns overall compared to desiccation cracks (Photos by Ed Simpson)
Fig. 2 Manchuriophycus: sinuous cracks within ripple troughs; coin (2 cm across) for scale (Photo courtesy Pieter Bosch)
Subtypes (1) Polygonal patterns: display no orientation, are relatively uncommon in rock record, and cracks mostly narrow (mm-scale width, cm-scale depth) with small polygons (several cm across) (Plummer and Gostin 1981; Boggs 1987, pp. 183–184). Often form rather indistinct polygonal crack patterns compared to subaerial desiccation cracks, which are better defined, commonly multigenerational (up to three), and characterized by concave-upward flexure and folding of laminae or beds. (2) Spindle-shaped cracks: may be orientated (rather weakly), can cross each other but do not close off to form polygonal shapes (Plummer and Gostin 1981; Boggs 1987, pp. 183–184), generally wider in spindle centers compared to terminations with widths on mm scale and lengths of cm-dm scale. (3) Sinuous to serpentine cracks in ripple troughs: often known by the pseudo-trace fossil appellation of Manchuriophycus (Fig. 2), occur as multiple (small numbers of 1–3), elongated (several dm scale) cracks of similar width-depth dimensions to other subtypes, confined to
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Encyclopedia of Planetary Landforms DOI 10.1007/978-1-4614-9213-9_558-1 # Springer Science+Business Media New York 2014
ripple troughs (Plummer and Gostin 1981) and may locally impinge on crests but generally not cross them.
Formation The subaqueous equivalent to desiccation crack polygons. It forms either at the sediment-water interface or substratally within saturated sandstone-mudstone successions (Plummer and Gostin 1981). They either form due to dewatering of clay following rapid flocculation of particles within a water body or reflect shrinkage of swelling clay minerals due to salinity changes in the water (Reineck and Singh 1975, p. 51; Plummer and Gostin 1981; Boggs 1987, pp. 183–184). Plummer and Gostin (1981) stress the likelihood that many synaeresis cracks form substratally within saturated mud-sand successions rather than exclusively or largely at the sediment-water surface. It is important to emphasize that both the spindle and sinuous (Manchuriophycus) varieties are easily confused with microbial mat-induced features within sandy sediments, a confusion that applies equally to polygonal crack patterns of both subaerial and subaqueous origin as well (see Schieber et al. 2007 for a very detailed discussion of microbial mat-related features in clastic sediments). In many such cases, it may prove impossible to discriminate specific origin as either nonbiogenic or mat related.
Planetary Analog It was proposed for crack-forming mechanism in Burns formation, Meridiani Planum, Mars (McLennan et al. 2005).
History of Investigation Plummer and Gostin (1981) admirably summarize significant historical literature including a large body of experimental work, and their paper is of significance as it details discrimination between desiccation crack polygons and synaeresis crack polygons. The most important distinguishing criteria are associated sedimentary structures substantiating subaerial exposure for desiccation cracks and their obvious absence for synaeresis cracks.
Origin of Term Derived from the spontaneous loss of water from a gel; first geological report by J€ ungst (1934).
See Also ▶ Desiccation Crack Polygon
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Encyclopedia of Planetary Landforms DOI 10.1007/978-1-4614-9213-9_558-1 # Springer Science+Business Media New York 2014
References Boggs S Jr (1987) Principles of sedimentology and stratigraphy. Merrill, Columbus Collinson JD, Thompson DB (1989) Sedimentary structures, 2nd edn. Unwin Hyman, London Cowan CA, James NP (1992) Diastasis cracks: mechanically generated synaeresis-like cracks in Upper Cambrian shallow water oolite and ribbon carbonates. Sedimentology 39(6):1101–1118 J€ ungst H (1934) Zur geologischen Bedeutung der Sÿnarese. Geolog Rundsch 15:312–325 McLennan SM et al (2005) Provenance and digenesis of the evaporite-bearing Burns formation, Meridiani Planum, Mars. Earth Planet Sci Lett 240:95–121 Plummer PS, Gostin VA (1981) Shrinkage cracks; desiccation or synaeresis? J Sediment Res 51(4):1147–1156 Reineck H-E, Singh IB (1975) Depositional sedimentary environments. Springer, Berlin Ricci Lucchi F (1995) Sedimentographica, photographic atlas of sedimentary structures, 2nd edn. Columbia University Press, New York Schieber J, Bose PK, Eriksson PG, Banerjee S, Sarkar S, Altermann W, Catuneanu O (eds) (2007) Atlas of microbial mat features preserved within the siliciclastic rock record, vol 2, Atlases in Geoscience. Elsevier, Amsterdam
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