Silurian reefal buildups: Spiti-Kinnaur, Himachal Himalaya, India

Summary

The Silurian sequence in Spiti and Kinnaur was studied at Takche, Gechang, Muth-Shian, Leo and Manchap. At all these localities the sequence commences with an intertidal—near-shore argillo-arenaceous succession. The overlying part at Takche comprises arenaceous dolomite, calcareous sandstone and dolomite. It sporadically contains rugose and tabulate corals deposited in a shallow undathem near the shore. The succession at Gechang is more calcareous and in some parts rich in corals; stromatoporoids occur locally. Here bioclastic wacke/packstones and framestones formed a buildup which was possibly formed in a lagoon. The Muth-Shian area is characterized by bioclastic mudstones, bioclastic wacke/packstones and framestones. Reef builders are corals, stromatoporoids and solenoporoids. These sediments seem to be a lateral equivalent of the coastal arenaceous sequence exposed at Baldar and may represent a fringe reef.

The diversity of the fauna and microfacies increases in the Leo area, where mud/wackestones, bioclastic pack/grainstones, bioclastic floatstones, bindstones and framestones occur. The main reef-builders are tabulate corals (Halysites predominates), laminar to domal stromatoporoids, algae and bryozoa which grew on a protected reef flat near the back of a reef.

The Manchap reef is built by tabulate corals, bryozoa, stromatoporoids and algae.Vermiporella inhabited open spaces ofHalysites chains, forming a typical community. Tabulate corals grew on stromatoporoids, rugose coralsare encrusted by stromatoporoids.

Wacke/pack/grainstones, float/rudstones and frame/baffle/bindstones occur. The facies and the biocommunities indicate deposition on a reef flat and flank. Upward growth of the reef into a turbulent zone, its later destruction and regrowth possibly account for intimate interlayering and intertonguing of these diverse facies types. All the buildups were formed in protected areas with moderate wave energy.

The cements are siliceous, ferruginous micritic and sparitic and rarely show ferruginous subequant calcite. The reefal sequences are overlain by arenaceous rocks enclosing a few tabulates andPsilophyton. The reef growth was depth-controlled, caused by transgressive and regressive cycles. Reef growth started slowly in muddy water and reached its maximum in clear water.