Authigenic, perigenic, and allogenic glauconites from the Castle Hill Basin, North Canterbury, New Zealand

Abstract

Two major vanenes of glauconite are distinguished within the Upper Cretaceous and Tertiary formations of the Castle Hill Basin. Type A is the only variety present in the Upper Cretaceous Broken River Coal Measures, is absent in the Upper Cretaceous to Eocene Iron Creek Greensand which overlies the coal measures, and is sparsely present in the younger (Oligocene to Miocene and possibly early Pliocene) detrital formations. This type is dusky yellow-green, crystallographically disordered with 40-50% expandable layers. It shows a characteristic "cauliflower" texture under the scanning electron microscope. Type A occurs mainly as a matrix/cement in the coal measures, but outlines of rounded grains can be discerned within this interstitial filling; it occurs only as partly rounded fragments in younger formations. Type B is the only variety present in the Iron Creek Greensand and is the most abundant glauconite variety in the younger formations. It is greenish black, crystallographically disordered with 15-25% expandable layers, and has more iron, potassium, magnesium, calcium, and strontium than Type A. Type B always occurs as pellets with spheroidal, ovoidal, or lobate morphology, and resembles the common glauconite de-scribed in most previous literature.Glauconite of Type B is formed originally at the sediment-water interface in a normal marine environment, i.e., it is authigenic and perigenic (transported locally). Glauconite of Type A developed after burial, under conditions of reduced marine salinity and/or influenced by organic decomposition products. In the coal measures it is authigenic, in part being the result of chemical modification of parent glauconite grains which themselves probably had formed at the sedimentwater interface. The vast majority of the glauconite in the younger formations studied has been recycled from the Broken River Coal Measures and the Iron Creek Greensand; i.e., in these occurrences the grains are allogenic.Different varieties of glauconite can be distinguished and used to decipher the stratigraphic record. Also, glauconite can be recycled, hence care must be exercised when using this mineral to provide radiometric dates for deposition of the enclosing sediments.

Oligocene-Miocene rocks of the Brechin Burn outlier, Waimakariri valley, Canterbury

“…However, the preservation of volcanic topography and the freshness of the basalt surface below the Oligocene sediments argues against this. Runangan-Whaingaroan volcanism is quite widespread (Oamaru, Clarence Valley, Reay 1980) and a discrete unconformity bounded late Eocene-Oligocene sedimentary wedge in northwest Canterbury is consistent with data from Gage (1970) and McConchie & Lewis (1978).…”

A Mössbauer Investigation of Glauconite and its Geological Significance

“…Occurrences have been reported in nonmarine sediments (e.g., Keller, 1956;Parry and Reeves, 1966;McConchie and Lewis, 1978), but where glauconite occurs as discrete grains it may have been reworked from marine deposits (e.g., McConchie and Lewis, 1978), or the chemistry of the nonmarine solutions may have been very similar to that of marine waters (Parry and Reeves, 1966). A very low (to negative) sedimentation rate in the genetic environment is considered essential (e.g., Van Andel and Postma, 1954;Cloud, 1955;Burst, 1958b;McRae, 1972), and water depths equivalent to the neritic environment (immediately sublittoral to the upper part of the continental slope) are generally inferred (see McRae, 1972).…”

“…A very low (to negative) sedimentation rate in the genetic environment is considered essential (e.g., Van Andel and Postma, 1954;Cloud, 1955;Burst, 1958b;McRae, 1972), and water depths equivalent to the neritic environment (immediately sublittoral to the upper part of the continental slope) are generally inferred (see McRae, 1972). A periodically agitated low energy environment appears to be necessary (Van Andel and Postma, 1954;McRae, 1972;Odin, 1975), but in many occurrences, current generated structures probably reflect reworking after the glauconite was formed (e.g., Ward and Lewis, 1975;McConchie, 1978;McConchie and Lewis, 1978). A pH of 7 to 8 (Fairbridge, 1967) and slightly reducing conditions (Eh < 0) are generally considered to be the most favorable (Cloud, 1955;Burst, 1958a;Fairbridge, 1967), although the reducing conditions may be confined to the immediate vicinity of the proto-glauconite (Burst, 1958a;Norris, 1964), while the overall conditions remain slightly oxidizing (Van Andel and Postma, 1954).…”

“…Differential thermal analysis curves for glauconite (Grim, 1968;McConchie and Lewis, 1978) show that essentially all dehydration and dehydroxylation ceased by this temperature and that structural changes took place at higher temperatures. The Fe2+/Fe z+ ratio was determined from the M6ss-bauer spectra of the glauconites using relative peak areas; wet chemical methods were not used because if charge transfers occur in glauconites at room temperature, as suggested by the Mrssbauer spectra taken in this study, then changes in the Fe2+/Fe 3+ ratio may occur during wet chemical analyses.…”

Paleoenvironmental controls on mineral assemblages in a shelf sequence: Te Kuiti Group, South Auckland, New Zealand

“…The variably abraded, rounded, polished, and sizesorted characteristics of glauconite pellets having spheroidal, ovoidal, vermicular, and free-form morphologies indicate that they have been transported from nearby sites of active glauconite formation on the sea floor before final incorporation into deposits (i.e., they are perigenic pellets of McConchie & Lewis 1978). The glauconite filling skeletal chambers is probably mainly authigenic, although even here it is possible that glauconite development began before final deposition of host grains.…”