Limestone pseudoconglomerates in the Late Cambrian Gushan and Chaomidian Formations (Shandong Province, China): soft‐sediment deformation induced by storm‐wave loading

Abstract

This paper focuses on the formative processes of limestone pseudoconglomerates in the Gushan and Chaomidian Formations (Late Cambrian) of the North China Platform, Shandong Province, China. The Gushan and Chaomidian Formations consist mainly of limestone and shale (marlstone) interlayers, wackestone to packstone, grainstone and microbialite as well as numerous limestone conglomerates. Seventy-three beds of limestone pseudoconglomerate in the Gushan and Chaomidian Formations were analysed based on clast and matrix compositions, internal fabric, sedimentary structures and bed geometry. These pseudoconglomerates are characterized by oligomictic to polymictic limestone clasts of various shapes (i.e. flat to undulatory disc, blade and sheet), marlstone and/or grainstone matrix and various internal fabrics (i.e. intact, thrusted, edgewise and disorganized), as well as transitional boundaries. Limestone pseudoconglomerates formed as a result of soft-sediment deformation of carbonate and argillaceous interlayers at a shallow burial depth. Differential early cementation of carbonate and argillaceous sediments provided the requisite conditions for the formation of pseudoconglomerates. Initial deformation (i.e. burial fragmentation, liquefaction and injection) and subsequent mobilization and disruption of fragmented clasts are two important processes for the formation of pseudoconglomerates. Burial fragmentation resulted from mechanical rupture of cohesive carbonate mud, whereas subsequent mobilization of fragmented clasts was due to the injection of fluid materials (liquefied carbonate sand and water-saturated argillaceous mud) under increased stress. Storm-wave loading was the most probable deformation mechanism, as an external triggering force. Subsequent re-orientation and rounding of clasts were probably prolonged under normal compactional stress. Eventually, disrupted clasts, along with matrix materials, were transformed into pseudoconglomerates by progressive lithification. Soft-sediment deformation is prevalent in alternate layers of limestone and mud(marl)stone and/or grainstone, regardless of their depositional environments.

Flat pebbles and their edge‐wise fabric in relation to 2‐D microbial mat

“…Gradual extension of activity of sulfur‐reducing bacteria as the sediment cover gradually built up perhaps enhanced the rate of undersurface carbonate cementation (cf., Schieber, ; Vasconcelos et al, ). The clasts are thus intrastratal in origin, as discussed in Pratt () and Chen et al (). In hindsight, the carbonate composition of all the examples of flat‐clast conglomerate cited herein (Appendix ) does not seem surprising, microbial stimulation for carbonate precipitation being long known, especially in the Precambrian context (Grotzinger, ; Grotzinger & Knoll, ; Sarkar et al, ).…”

“…Intraclast generation selectively from the dark layers tell‐tales different degrees of consolidation between the two repeatedly alternating layers and generation of a two‐layer system with contrasting responses to stress. It is not surprising that the dark layers of purported microbial mat origin show some degree of consolidation while the lighter layers still remained unconsolidated (cf., Chen et al, ; Chen & Lee, ). Deformation of subvertical clasts in the breccia type (ii) and (iv) attests to flexibility of the clasts (Chen et al, , ).…”

“…It is not surprising that the dark layers of purported microbial mat origin show some degree of consolidation while the lighter layers still remained unconsolidated (cf., Chen et al, ; Chen & Lee, ). Deformation of subvertical clasts in the breccia type (ii) and (iv) attests to flexibility of the clasts (Chen et al, , ). Despite this, the lamina set underlying the breccia (ii) body, as a whole, has undergone brittle fracture to thrust upon its own undisturbed counterpart.…”

“…Nonetheless, clasts that surround the fan structure show enough flexibility. Pore‐fluid escape that caused the upward fanning of clasts might have led to rapid cementation of the sediment (Chen et al, ). In contrast, the clasts in breccia type (iv), whether within the tepee structure or without, hardly bear any cracks.…”

“…Many clasts in these examples are, nevertheless, found to be bent, implying their formation before complete cementation (Chen, Chough, Chun, & Han, ; Chen, Han, Zhang, Fan, & Yang, ). Many of the deposits elicit edge‐wise fabric (Chen et al, ; Chen, Chough, Chun, Han, & Lee, ; Mount & Kidder, ; Myrow et al, ; Van Loon, Han, & Han, ), even where evidence for clast reworking is lacking (Bertrand‐Sarfati & Moussine Pouchkine, ). It is not surprising that flat clast conglomerates in their shallow marine depositional settings are, in general, attributed to storm erosion (Appendix ); only one known suite is inferred to have been deposited below the storm wave‐base (Bertrand‐Sarfati & Moussine Pouchkine, ).…”