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 described 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.