. Bulletin of the British Museum (Natural History), Geology. Colorado 150 NIOBRARA FORMATION \ ' \ ' \ ' \ ' CODEIL SST BLUE HIIL SHALE FAIRPORT CALCAREOUS SHALES —z+z—r Z-+ —Z+-Z- BRIDGE g CREEK "- LIMESTONE 2 HARTLAND g CALCAREOUS X SHALES Z ;;; Lincoln g LIMESTONE 1 ' ' ' ! ' 'i ',> GRANEROS — z— SHALE •--- DAKOTA SANDSTONE Jl y o n D D Culver Cliff ( ) ? Zone Zone. PLENUS MARLS CHALK MARL UPPER GREENSAND Fig. 11 Microfaunal comparison of the successions in Colorado and the Isle of Wight (England). Grimsdale & van Morkhoven (1955) attempted to form
. Bulletin of the British Museum (Natural History), Geology. Colorado 150 NIOBRARA FORMATION \ ' \ ' \ ' \ ' CODEIL SST BLUE HIIL SHALE FAIRPORT CALCAREOUS SHALES —z+z—r Z-+ —Z+-Z- BRIDGE g CREEK "- LIMESTONE 2 HARTLAND g CALCAREOUS X SHALES Z ;;; Lincoln g LIMESTONE 1 ' ' ' ! ' 'i ',> GRANEROS — z— SHALE •--- DAKOTA SANDSTONE Jl y o n D D Culver Cliff ( ) ? Zone Zone. PLENUS MARLS CHALK MARL UPPER GREENSAND Fig. 11 Microfaunal comparison of the successions in Colorado and the Isle of Wight (England). Grimsdale & van Morkhoven (1955) attempted to formulate a method facilitating accurate estimation of the depth of deposition from analysis of faunal contents of samples. It had long been appreciated that a thanatocoenosis in an abyssal sediment from above the compensation depth would contain 90-100% planktonic foraminifera, while one from the neritic zone would be almost totally composed of benthonic individuals. However, no practical means of integrating this information into a method for determining the depth of deposition has been developed. Grimsdale & van Morkhoven's attempts used the faunal counts of samples collected on twelve traverses (largely neritic) made by Phleger & Parker (1951). Grimsdale & van Morkhoven discussed the lack of correspondence of their graphs and, after compiling a list of the limitations, concluded that while the theory was basically sound the depth of deposition could not be accurately estimated using this technique. More recently Eicher (1969a) has used population per- centages to demonstrate, in a general way, the deepening and subsequent shallowing of the Green- horn Sea in Colorado. Flexer & Starinsky (1970), however, based the bulk of their work on the assumption of a direct relationship between the planktonic-benthonic ratio and the depth of deposition. This was following Eicher (1969a), and many other workers had concluded that determinations based only on plank
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