It is well known among geologists that mud cracks can form under water, especially in situations where the sediment is immature and rapidly accumulated. But even in mature sandstone such as the Coconino Sandstone, we are finding "mud cracks" that cannot be related to dessication in any normal sense of the word.
For example Reineck and Singh state:
"Mud cracks (shrinkage cracks) can also originate subaqueously as a result of synaeresis (Jüngst 1934). A rapidly flocculated clay layer develops shrinkage cracks due to compaction (White 1961). Similarly, an increase in salinity can also generate shrinkage cracks in the mud layers (Burst 1965). This process can be important in coastal lagoons and inland sebkhas where salinity of water increases markedly during certain periods. Kuehen (1963, 1965) and Dangeard et al. (1964) also produced underwater shrinkage cracks in the laboratory.
"Such subaqueous shrinkage cracks differ from subaerial desiccation cracks in that they are not so well-developed, the cracks are rather narrow, and they do not possess well-developed V-shapes in transverse sections. In general, subaqueous shrinkage cracks are less regular in form and often incomplete. Sometimes, cracks are developed as open, straight to curved cracks occurring singly or in sets, having a preferred orientation. The cracks are 2-8 cm in length and known as linear-shrinkage cracks. According to Picard and High (1973) linear shrinkage cracks develop when relatively thick water-saturated thixotropic muds dehydrate usually under standing water. " (Depositional Sedimentary Environments. Second Edition. Springer-Verlag, New York. 1980 p.60).
Does this mean all mudcracks can be explained as being underwater? No. Each occurrence must be restudied carefully with this possibility in mind before a decision is reached. What other evidences can be brought to bear? What other consequences should subaerial exposure have for the deposit? Where did the depositing water go without evidence for erosion (if this is the case)? How long was the cracked surface exposed prior to being buried? Why were the cracks filled in with the sediment of the overlying bed (if this is the case), rather than being infilled with materials from the exposed surface, if significant time lapsed before deposition of the overlying layer. How was the overlying material deposited without disrupting the mudcracks, or if they were disrupted, what can you learn from that? What explanation other than dessication can account for halite pseudomorphs sometimes associated with mudcracks? These are examples of some of the questions that could be asked and should be asked but rarely are, before one ascribes to either explanation for mud cracks found in the fossil record.
Burst, J. F. (1965) Subaqueously formed shrinkage cracks in clay. J. Sediment. Petrol. 35,348-353
Dangeard, L., Migniot, C., Larsonneur, C., Baudet, P. (1964) Figures et structures observées au cours du tassement des vases sous l'eau. Compt. Rend. 258, 5935-5938
Jüngst, H. (1934) Zur geologischen Bedeutung der Synärese. Geol. Rundschau 15, 312-325
Kuenen, Ph. H. (1963) Experimentele sedimentstructuren. Koninkl. Ned. Akad. Wetenschap Amsterdam, Versl. Gw. Verg. Ard. Nat. 72, 65-66
Kuenen, Ph. H. (1965) Value of experiments in geology. Geol. Mijnbouw 44, 22-36
Picard, M. D., High Jr., L. R. (1973) Sedimentary structures of ephemeral strems, 223 p. Amsterdam: Elsevier
White, W. A. ( 1961 ) Colloid phenomena in sedimentation of argillaceous rocks. J. Sediment. Petrol. 31,560-570
2010 Arthur V. Chadwick, Ph.D.