Global case studies of soft-sediment deformation structures (SSDS): Definitions, classifications, advances, origins, and problems

Abstract

Soft-sediment deformation structures (SSDS) have been the focus of attention for over 150 years. Existing unconstrained definitions allow one to classify a wide range of features under the umbrella phrase “SSDS”. As a consequence, a plethora of at least 120 different types of SSDS (e.g., convolute bedding, slump folds, load casts, dish-and-pillar structures, pockmarks, raindrop imprints, explosive sand–gravel craters, clastic injections, crushed and deformed stromatolites, etc.) have been recognized in strata ranging in age from Paleoproterozoic to the present time. The two factors that control the origin of SSDS are prelithification deformation and liquidization. A sedimentological compendium of 140 case studies of SSDS worldwide, which include 30 case studies of scientific drilling at sea (DSDP/ODP/IODP), published during a period between 1863 and 2017, has yielded at least 31 different origins. Earthquakes have remained the single most dominant cause of SSDS because of the prevailing “seismite” mindset. Selected advances on SSDS research are: (1) an experimental study that revealed a quantitative similarity between raindrop-impact cratering and asteroid-impact cratering; (2) IODP Expedition 308 in the Gulf of Mexico that documented extensive lateral extent (>12 km) of mass-transport deposits (MTD) with SSDS that are unrelated to earthquakes; (3) contributions on documentation of pockmarks, on recognition of new structures, and on large-scale sediment deformation on Mars.Problems that hinder our understanding of SSDS still remain. They are: (1) vague definitions of the phrase “soft-sediment deformation”; (2) complex factors that govern the origin of SSDS; (3) omission of vital empirical data in documenting vertical changes in facies using measured sedimentological logs; (4) difficulties in distinguishing depositional processes from tectonic events; (5) a model-driven interpretation of SSDS (i.e., earthquake being the singular cause); (6) routine application of the genetic term “seismites” to the “SSDS”, thus undermining the basic tenet of process sedimentology (i.e., separation of interpretation from observation); (7) the absence of objective criteria to differentiate 21 triggering mechanisms of liquefaction and related SSDS; (8) application of the process concept “high-density turbidity currents”, a process that has never been documented in modern oceans; (9) application of the process concept “sediment creep” with a velocity connotation that cannot be inferred from the ancient record; (10) classification of pockmarks, which are hollow spaces (i.e., without sediments) as SSDS, with their problematic origins by fluid expulsion, sediment degassing, fish activity, etc.; (11) application of the Earth's climate-change model; and most importantly, (12) an arbitrary distinction between depositional process and sediment deformation. Despite a profusion of literature on SSDS, our understanding of their origin remains muddled. A solution to the chronic SSDS problem is to utilize the robust core dataset from scientific drilling at sea (DSDP/ODP/IODP) with a constrained definition of SSDS.