The distribution of major and minor elements in sedimentary carbonate rocks and the mechanisms responsible for this distribution are considered on the basis of published information contained in geologic studies, and in studies of present-day environments of carbonate deposition, isotopic composition of carbonates, and experimental aqueous and nonaqueous carbonate systems. There are five parts in the series,and an extensive bibliography appears in Part IV.

Geochemistry of carbonate sediments and sedimentary carbonate rocks : pt. III, Minor element distribution.

In the British Pennines, from the middle of the lenisulcata Zone to the lower part of the communis Zone (Upper Carboniferous, lower Westphalian A), nine horizons, of which six are new or newly described, yield elongate specimens of the bivalve Carbonicola with evidence of former steep, near vertical burrowing in sediments of low organic carbon content. The faunas, of the groups of C. bellula (Bolton) sensu lato and of elongate C. aff. communis Davies & Trueman, were evidently invaders of the Pennine prodelta. They grade with established faunas of prolific highly variable Carbonicola found typically in richly carbonaceous shales and characterised by shallow burrowing, greater size, increased prcportionate shell height and length of anterior end, and probably increased obesity. Parallels exist with work on Anthraconaia. Anthraconauta. and the ecology of recent Unionacea. Results also bear on the palaeoecological ranges of Curvirimula and Naiadites.

Shape of shell of Carbonicola in relation to burrowing

Nodules containing siderite and calcite in the Yorkshire Lias have been studied quantitatively using petrographic, x-ray and chemical methods. Ankerite is recorded from the Lias for the first time. It is shown how the relative abundance of early diagenetic siderite and calcite correlates with sandiness of the sediment and provides an index of environmental change.

Siderite- and Calcite-bearing Concretionary Nodules in the Lias of Yorkshire

A combined petrographical and geochemical study has been made on a per- sistent carbonate horizon from the Mansfield Marine Band, an important marker horizon in the Westphalian Coal Measures of the east Pennines. The composition of this carbonate is unusual for it normally consists of ankerite, but in some localities the carbonate is siderite and not ankerite. The position of the carbonate in the sedimentary cycle is discussed and the conclusion drawn that physicochemical conditions were suitable for the precipitation of CaC03 to occur. Furthermore the detrital material in the carbonate band continued to accumulate at a similar rate to the shales above and below the carbonate. The alteration of the CaC03 is thought to have been a diagenetic effect and the result of a change in the physicochemical environment. Variations in the concentration of dissolved sulphur species and their effect on the solubilities of iron and manganese provide an adequate mechanism to explain how CaC03 may alter to siderite on the one hand or ankerite on the other.

An Unusual Carbonate Band in the East Pennine Coalfield (england)

"The shell substance of the non-marine lamellibranchs from a band in the lower similis-pulchra zone of Lancashire is shown to be ankerite. The same mineral also forms pockets inside certain of the shells, the pockets being developed most frequently in the stratigraphically uppermost part of the spaces between the valves. It is considered that the ankerite in the pockets has occupied spaces left by shrinkage of the original mud-filling of the shells. Chemical analyses, specific gravity, and optical properties of the ankerite are given; its composition closely approximates to (Mg 2 Fe) Ca 3 (CO 3 ) 6 ."

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0016756800062968

R. A. Howie

Papers

On the occurrence of ankerite in a band of non-marine lamellibranchs from the Lancashire Coal Measures