Cambrian Reefs in the Western North China Platform, Wuhai, Inner Mongolia

Abstract

Mid to late Cambrian thrombolites and maze‐like maceriate reefs from the western North China Platform, Wuhai, Inner Mongolia, northwestern China, occur in the middle of a succession dominated by thin‐bedded lime mudstone‐shale/marlstone alternations, and are laterally surrounded by limestone conglomerate and/or grainstone. Thrombolite, characterized by meter‐scale lenticular mounds composed of millimeter‐ to centimeter‐scale mesoclots and wackestone matrix, occurs in the lower middle part of the sequence. Thrombolite mesoclots are composed of microstromatolites with alternating dark gray and light gray micritic laminae. The maze‐like maceriate reefs occur in the middle to the upper part of the sequence, commonly forming lenticular mounds up to 1 m thick. They are characterized by centimeter‐ to decimeter‐scale branched maze‐like structures, whose biogenic portions (maceria) are selectively dolomitized. The maceriae are composed of poorly preserved microstromatolites and siliceous sponges. Inter‐macerial sediments consist of lime mud and scattered bioclasts. These Wuhai reefs are generally similar to but older than various other Cambrian reefs previously reported from the Shandong region, northeastern China.

Stromatolite‐rimmed thrombolite columns and domes constructed by microstromatolites, calcimicrobes and sponges in late Cambrian biostromes, Texas, USA

“…14; Khanna et al ., 2020a; Lehrmann et al ., 2020). These distinctive and well‐preserved fabrics shed light on the origins of structures of similar age and appearance that are widely distributed around North America (Shapiro & Awramik, 2006, and references therein) and are also well‐known in Argentina (Armella, 1994; Raviolo et al ., 2010) and China (Lee et al ., 2010, 2014, 2016; Chen et al ., 2014).…”

“…4e) noted upper Cambrian ‘mazelike structures’ in column interiors in Utah. Favosamaceria has also been described from Argentina (Raviolo et al ., 2010), and similar fabrics have been described from various localities in North China (Lee et al ., 2010, 2014, 2016) where Chen et al . (2014, p. 250) noted that “The deposition rate of inter‐macerial sediment was most likely balanced with the growth rate of maceriae, sustaining a low synoptic relief”.…”

Depositional history, tectonics, and provenance of the Cambrian-Ordovician boundary interval in the western margin of the North China block: COMMENT

“…Myrow et al (2015) focused on the upper part of this succession, consisting of fine-grained sedimentary facies such as lime mudstone and marlstone alternations, and they reported that trilobites belonging to the Black welderia zone, indicating the middle Guzhangian Stage of Cambrian Series 3 (Park et al, 2013), occur 18.51 m below the Cambrian-Ordo vician boundary. Lee et al (2016) reported trilobite Jiulongshania rotundata from strata 16.4 m below the boundary, which also occurs within the Blackwelderia zone of Myrow et al (2015). Sedimentary facies as well as trilobite occurrences collectively suggest a relatively slow sedimentation rate, indicating that the uppermost part of the Cambrian succession in Wuhai area would not exceed the Guzhangian Stage (Myrow et al, 2015;Lee et al, 2016).…”

“…At the Subaiyingou (or Subeigou) section where the study was conducted, the Cambrian-Ordovician boundary occurs ~360 m above the Precambrian-Cambrian unconformity (Lee et al, 2016). Myrow et al (2015) focused on the upper part of this succession, consisting of fine-grained sedimentary facies such as lime mudstone and marlstone alternations, and they reported that trilobites belonging to the Black welderia zone, indicating the middle Guzhangian Stage of Cambrian Series 3 (Park et al, 2013), occur 18.51 m below the Cambrian-Ordo vician boundary.…”

“…Lee et al (2016) reported trilobite Jiulongshania rotundata from strata 16.4 m below the boundary, which also occurs within the Blackwelderia zone of Myrow et al (2015). Sedimentary facies as well as trilobite occurrences collectively suggest a relatively slow sedimentation rate, indicating that the uppermost part of the Cambrian succession in Wuhai area would not exceed the Guzhangian Stage (Myrow et al, 2015;Lee et al, 2016). The lower part of the Sandaokan Formation unconformably overlying the Cambrian succession was correlated to the conodont biozone Histiodella holodentata of the middle to upper middle Darriwilian Stage, Middle Ordovician (Myrow et al, 2015).…”