A review on palaeogeographic implications and temporal variation in glaucony composition

摘要

Abstract This study presents a review on palaeogeographic implications and temporal variations of glaucony covering both modern and ancient records. Phanerozoic glaucony preferably forms in a shelf depositional setting. Deep marine conditions and low seawater temperature discourage formation of glaucony. Around 75% of glaucony is recorded from the Cretaceous to the Holocene sediments, which are related to the abundance of the most common substrates, faecal pellets and bioclasts. TFe₂O₃ (total), Al₂O₃, K₂O and MgO contents of glaucony vary appreciably through geological time. While TFe₂O₃ content of most Mesozoic and Cenozoic glaucony exceeds 20%, it is always less than 20% in Precambrian varieties. High K₂O, Al₂O₃, MgO and low TFe₂O₃ distinguish the Precambrian glaucony from its Phanerozoic counterpart. Precambrian glaucony, preferably formed within a K-feldspar substrate, is always rich in potassium irrespective of its degree of evolution, while high K-content in Phanerozoic evolved glaucony indicates significant stratigraphic condensation. K₂O vs. TFe₂O₃ relationship of glaucony exhibits three different evolutionary trends corresponding to three common modes of origin. Depositional conditions may influence the composition of glaucony as slightly reducing conditions favour Fe enrichment, whereas oxidising conditions cause Fe depletion in glaucony. Keywords Glaucony ; Glauconite ; Evolutionary trend ; Substrate ; Palaeogeography ; Faecal pellet prs.rt("abs_end"); 1. Introduction Glaucony is generally considered as a product of marine authigenesis, which is commonly associated with transgressive deposits and condensed sections ( Odin and Matter, 1981 , Amorosi, 1995 , Amorosi, 1997 , Amorosi and Centineo, 1997 , Kitamura, 1998 , Harris and Whiting, 2000 , Giresse and Wiewióra, 2001 and Hesselbo and Huggett, 2001 ; Meunier and El Albani, 2007 , Banerjee et?al., 2008 , Banerjee et?al., 2012a , Banerjee et?al., 2012b and Chattoraj et?al., 2009 ). Glaucony occurs typically in forms of 60?μm–1000?μm green clay aggregates in sedimentary rocks ranging in age from the Late Paleoproterozoic to the Holocene ( Table 1 ; Webb et?al., 1963 , Odin and Matter, 1981 , Dasgupta et?al., 1990 , Amorosi, 1994 , Amorosi, 2012 , Deb and Fukuoka, 1998 , Lee et?al., 2002 , El Albani et?al., 2005 , Amorosi et?al., 2007 , Bandopadhyay, 2007 , Banerjee et?al., 2008 , Banerjee et?al., 2012a , Banerjee et?al., 2012b , Banerjee et?al., 2015 and Chattoraj et?al., 2009 ). Glaucony forms in a wide variety of substrates including faecal pellets, bioclasts, feldspar, mica and quartz. Although the stratigraphic implications of glaucony are fairly well understood because of its common association in sedimentary sequences representing simple sedimentation breaks to mega-condensed sections ( Amorosi, 2012 ; and references therein), the records of glaucony occurrence through geological time are poorly known, and temporal variation in glaucony composition has never been addressed. The occurrence of modern glaucony in deeper shelf and slope regions is frequently extrapolated to interpret the depositional settings of ancient glaucony-bearing sedimentary sequences. However, the ancient varieties of glaucony occur in wide ranging palaeo-depositional conditions. Although a few researchers have attempted to relate the glaucony composition with the depositional environment, factors affecting the glaucony composition are poorly understood.