Microbial mats implicated in the generation of intrastratal shrinkage (“synaeresis”) cracks

Dario Harazim, Richard H. T. Callow, Duncan McIlroy

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60 Citations (Scopus)


Intrastratal shrinkage (often termed ‘synaeresis’) cracks are commonly employed as diagnostic environmental indicators for ancient salinity-stressed, transitional fluvial-marine or marginal-marine depositional environments. Despite their abundance and use in facies interpretations, the mechanism of synaeresis crack formation remains controversial, and widely accepted explanations for their formation have hitherto been lacking. Sedimentological, ichnological, petrographic and geochemical study of shallow marine mudstone beds from the Ordovician Beach Formation of Bell Island, Newfoundland, has revealed that crack development (cf. synaeresis cracks) on the upper surface of mudstone beds is correlated with specific organic, geochemical and sedimentological parameters. Contorted, sinuous, sand-filled cracks are common at contacts between unbioturbated mudstone and overlying sandstone beds. Cracks are absent in highly bioturbated mudstone, and are considered to pre-date firmground assemblages of trace fossils that include Planolites and Trichophycus. The tops of cracked mudstone beds contain up to 2·1 wt% total organic carbon, relative to underlying mudstone beds that contain around 0·5 wt% total organic carbon. High-resolution carbon isotope analyses reveal low δ13Corg values (−27·6‰) on bed tops compared with sandy intervals lacking cracks (−24·4 to −24·9‰). Cracked mudstone facies show evidence for microbial matgrounds, including microbially induced sedimentary structures on bedding planes and carbonaceous laminae and tubular carbonaceous microfossils in thin section. Non-cracked mudstone lacks evidence for development of microbial mats. Microbial mat development is proposed as an important prerequisite for intrastratal shrinkage crack formation. Both microbial mats and intrastratal shrinkage cracks have broad palaeoenvironmental distributions in the Precambrian and early Phanerozoic. In later Phanerozoic strata, matgrounds are restricted to depositional environments that are inhospitable to burrowing and surface-grazing macrofauna. Unless evidence of synaeresis (i.e. contraction of clay mineral lattices in response to salinity change) can be independently demonstrated, the general term ‘intrastratal shrinkage crack’ is proposed to describe sinuous and tapering cracks in mudstone beds.
Original languageEnglish
Pages (from-to)1621-1638
Number of pages18
Issue number7
Early online date6 Jun 2013
Publication statusPublished - Dec 2013

Bibliographical note

DH acknowledges financial support from the Grant‐in‐Aid scheme of AAPG and student grants sponsored by IAS and GSA. RC acknowledges the support of a postdoctoral fellowship at Memorial University from the Slopes 2 Consortium funded by BG Group, BP, ConocoPhillips, DONG, GDF Suez, Hess, Petrobras, RWE Dea, Statoil and Total. DMc acknowledges the financial support of a Canada Research Chair and an NSERC discovery grant. Alex Liu is thanked for helpful discussion and suggestions on a previous version of this manuscript. The suggestions of Paul Myrow and one anonymous reviewer helped to improve the manuscript. Editors Stephen Lokier and Tracy Frank are gratefully acknowledged for editorial assistance and helpful suggestions.


  • bioturbation
  • microbial mat
  • organic matter
  • shrinkage crack
  • synaeresis


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