Key bed

Abstract: Based on biostratigraphic position, chemical fingerprinting, and lithic characteristics such as relative thickness, several of the numerous K‐bentonite beds, or complexes of beds, in the Middle Ordovician of Baltoscandia are shown to be traceable over large areas. Because the type of volcanic eruptions that produced such widespread ash beds lasts only a short time (a couple of weeks, or less), the individual ash layers represent as close equivalents of time planes as one is likely to find in the Lower Paleozoic stratigraphic record. Although these clay beds were recorded in section descriptions as far back as in the 1880's, their volcanic nature was recognized in Baltoscandia only in the mid‐1940's. Subsequent research has added important data on the occurrence and chemical composition of the Middle Ordovician K‐bentonites, but the present study is the first detailed regional investigation of these beds across their entire distribution area in Baltoscandia. Four beds, or complexes of beds (the Gr...

Abstract: In central Kansas the Hartland Shale and Jetmore Chalk Members of the Greenhorn Limestone contain 16 burrow-mottled, ledge-forming, chalky limestone beds that can be traced across the entire outcrop. The same beds are recognized in equivalent strata of the Bridge Creek Limestone Member of westernmost Kansas, and all but one or two can be recognized in a Bridge Creek exposure in southwestern Pueblo County, Colorado. For a distance of nearly 450 mi some of these beds lie on, below, or close to equally persistent bentonite seams. The parallelism of bentonite seams and adjacent or nearly adjacent beds of limestone, and the uniform relative spacing of all the limestone beds prove that some of the latter are time parallel and suggest most convincingly that each of these widespr ad beds is time parallel. In contrast to intervening beds of laminated shaly chalk, the limestone beds lack internal stratification because of activity of a highly mobile infauna. The shaly chalk beds contain higher percentages of terrigenous detritus, organic carbon, and pyrite than the limestone beds, and are interpreted as reflecting greater rates of terrigenous detrital influx and an interstitial reducing environment that was inimical to development of a burrowing infauna. Smaller organic carbon and pyrite content of the limestones is believed to reflect a lower clay content of original sediments, and at least a partly oxidizing interstitial environment, as suggested by the high degree of bioturbation. Despite substrate differences, the aqueous environment directly above the sediment-water interface did not change significantly as sediment type alternated from detritus-rich to detritus-poor, because the shelly epifauna is essentially the same in adjacent be s of shaly chalk and chalky limestone. The widespread, apparently time-parallel limestone beds are believed to be the result of regionally manifested changes in volume of terrigenous detrital influx, coming principally from the west, that were superimposed on a more or less continuous accumulation of carbonate sediments. The noncrushed condition of macrofossils and fecal pellets suggests that limestone beds suffered early lithification; this process was probably influenced by initially greater purity of limestone-forming muds, slower sedimentation, and interstitial circulation resulting from bioturbation.

Abstract: This report describes volcanic ash beds that occur north of Otorohanga and west of Tirau in the central part of the North Island of New Zealand. The beds lie above one another as a mantle on the land surface, and form the parent material for many modern soils. Many of the beds, before burial by younger ash, were exposed to processes of soil formation and, as a result, the deposits include many fossil soils. Although their significance to agriculture is very well appreciated, the value of the beds for correlating terrestrial Pleistocene deposits and erosion surfaces is not widely recognised. A prominent break occurs within the ash deposits and marks a lengthy time interval. The name Kauroa Ash Formation is proposed for the beds below this break, and for the younger beds above it the name Hamilton Ash Formation is used. The beds of Hamilton Ash Formation post-date the early Pleistocene 220-250 ft terrace but pre-date the middle Pleistocene 110–130 ft terrace.