Palaeokarst within the Lower to Middle Ordovician Goodwin Formation, Pogonip Group (upper Ibexian-lower Whiterockian) was examined in detail at Meiklejohn Peak, Nevada USA in order to determine its origin, evolution, ...Palaeokarst within the Lower to Middle Ordovician Goodwin Formation, Pogonip Group (upper Ibexian-lower Whiterockian) was examined in detail at Meiklejohn Peak, Nevada USA in order to determine its origin, evolution, and relationship to sea level change. Detailed outcrop and petrographic examination of dolostone breccias and host rock reveals that palaeokarst was formed and affected by two distinct cycles of sea level change. A relative transgression resulted in deposition of lagoonal, ooid shoal, and shallow subtidal facies as sea level rose. Exposure of the carbonate platform led to the formation of multiple phreatic caves below the water table, as well as the development of numerous vadose conduits from the downward percolation of meteoric waters. Vadose water flow through early cave-wall and cave-roof collapse breccias resulted in rounding of smaller breccias clasts via physical transport and corrosion, while subsidence of subsurface karst led to the formation of a palaeodoline at the exposure surface. A second relative transgression deposited lagoonal sediments over the older karst; subsequent re-exposure of the carbonate platform resulted in the development of small breccia pockets as well as grikes within the youngest lagoonal sediments, and may have led to further corrosion of the older, deeper subsurface karst. The distal location of the study area within the carbonate platform suggests karst formation was the result of a substantial drop in relative sea level; the presence of multiple generations of palaeokarst imply that at least two higher-frequency cycles of sea-level change overprint the larger regression.展开更多
Abstract The taphonomy of fishes living in lacustrine environments has been extensively studied in both the laboratory and the fossil record;the taphonomy of marine fishes,however,is poorly known.Triassic marine fishe...Abstract The taphonomy of fishes living in lacustrine environments has been extensively studied in both the laboratory and the fossil record;the taphonomy of marine fishes,however,is poorly known.Triassic marine fishes with heavy ganoid and cosmoid scales,which provided protection from rapid taphonomic loss,offer a means to examine marine fish taphonomy in the fossil record.Four genera of Early Triassic fishes(the ray-finned actinopterygians Albertonia,Bobasatrania,Boreosomus,and the lobe-finned coelacanth(sarcopterygian),Whiteia)from the Wapiti Lake,British Columbia locality of the Lower Triassic Sulphur Mountain Formation were examined in order to gain a better understanding of the taphonomy of fish in marine environments,determine ambient environmental conditions in the region during the Early Triassic,and ascertain the habitat and mode of life of the fish.Results indicate that environmental conditions that contributed to the preservation of the fossil fishes of the current study included deposition in deep,quiet waters,which reduced the odds of disarticulation,colder waters under higher pressure,which slowed decay and limited postmortem floatation,and waters that were anoxic,which discouraged predators and scavengers.In addition,the thickness of the primitive ganoid and cosmoid scales of the fossil fishes also increased their preservation potential.Taphonomic,physiological and environmental indicators suggest that Whiteia,Albertonia,and possibly Bobasatrania lived in deep,cold waters near the oxygen minimum zone,while Boreosomus lived higher in the water column.While the anatomical and physiological characteristics of modern fishes will likely continue to inhibit marine taphonomy studies,examination of ancient fish,particularly those with ganoid or cosmoid scales,may provide future avenues of research to gain a better understanding of marine fish taphonomy and provide a powerful tool to examine ancient fish behavior and their environment.展开更多
基金AAPG Grants-In-Aid funding provided financial support for this research (Kervin)
文摘Palaeokarst within the Lower to Middle Ordovician Goodwin Formation, Pogonip Group (upper Ibexian-lower Whiterockian) was examined in detail at Meiklejohn Peak, Nevada USA in order to determine its origin, evolution, and relationship to sea level change. Detailed outcrop and petrographic examination of dolostone breccias and host rock reveals that palaeokarst was formed and affected by two distinct cycles of sea level change. A relative transgression resulted in deposition of lagoonal, ooid shoal, and shallow subtidal facies as sea level rose. Exposure of the carbonate platform led to the formation of multiple phreatic caves below the water table, as well as the development of numerous vadose conduits from the downward percolation of meteoric waters. Vadose water flow through early cave-wall and cave-roof collapse breccias resulted in rounding of smaller breccias clasts via physical transport and corrosion, while subsidence of subsurface karst led to the formation of a palaeodoline at the exposure surface. A second relative transgression deposited lagoonal sediments over the older karst; subsequent re-exposure of the carbonate platform resulted in the development of small breccia pockets as well as grikes within the youngest lagoonal sediments, and may have led to further corrosion of the older, deeper subsurface karst. The distal location of the study area within the carbonate platform suggests karst formation was the result of a substantial drop in relative sea level; the presence of multiple generations of palaeokarst imply that at least two higher-frequency cycles of sea-level change overprint the larger regression.
文摘Abstract The taphonomy of fishes living in lacustrine environments has been extensively studied in both the laboratory and the fossil record;the taphonomy of marine fishes,however,is poorly known.Triassic marine fishes with heavy ganoid and cosmoid scales,which provided protection from rapid taphonomic loss,offer a means to examine marine fish taphonomy in the fossil record.Four genera of Early Triassic fishes(the ray-finned actinopterygians Albertonia,Bobasatrania,Boreosomus,and the lobe-finned coelacanth(sarcopterygian),Whiteia)from the Wapiti Lake,British Columbia locality of the Lower Triassic Sulphur Mountain Formation were examined in order to gain a better understanding of the taphonomy of fish in marine environments,determine ambient environmental conditions in the region during the Early Triassic,and ascertain the habitat and mode of life of the fish.Results indicate that environmental conditions that contributed to the preservation of the fossil fishes of the current study included deposition in deep,quiet waters,which reduced the odds of disarticulation,colder waters under higher pressure,which slowed decay and limited postmortem floatation,and waters that were anoxic,which discouraged predators and scavengers.In addition,the thickness of the primitive ganoid and cosmoid scales of the fossil fishes also increased their preservation potential.Taphonomic,physiological and environmental indicators suggest that Whiteia,Albertonia,and possibly Bobasatrania lived in deep,cold waters near the oxygen minimum zone,while Boreosomus lived higher in the water column.While the anatomical and physiological characteristics of modern fishes will likely continue to inhibit marine taphonomy studies,examination of ancient fish,particularly those with ganoid or cosmoid scales,may provide future avenues of research to gain a better understanding of marine fish taphonomy and provide a powerful tool to examine ancient fish behavior and their environment.
