The formation of authigenic deposits during Paleogene warm climatic intervals: a review

Abstract

Although Paleogene warm climatic intervals have received considerable attention for atmospheric and oceanographic changes, the authigenic mineralization associated with these time spans remains overlooked. An extensive review of the literature reveals a close correspondence between the high abundance of glauconite and warm climatic intervals during the Paleogene period. The abundance of phosphorite, ironstone, lignite and black shale deposits reveals similar trends. Although investigated thoroughly, the origin of these authigenic deposits is never understood in the background of Paleogene warming climatic intervals. A combination of factors like warm seawater, hypoxic shelf, low rate of sedimentation, and enhanced rate of continental weathering facilitated the glauconitization. The last factor caused the excess supply of nutrients, including Fe, Si, K, Mg and Al through the rivers, the cations needed for the formation of glauconite. The excessive inflow of nutrient-rich freshwater into the shallow seas further ensured high organic productivity and stratification in shallow shelves, causing hypoxia. The consequent rapid rise in sea-level during the warm periods created extensive low-relief shallow marine shelves starved in sediments. Oxygen-deficiency in the shallow marine environment facilitated the fixation of Fe into the glauconite structure. The inflow of nutrient-rich water during the warm climatic intervals facilitated the formation of phosphorite, ironstone, and organic-matter-rich sedimentary deposits as well. Although global factors primarily controlled the formation of these authigenic deposits, local factors played significant roles in some of the deposits. Therefore, phosphorites formed in marine conditions with open circulation within the tropical zone. While lush growth of rainforest covers in the tropical belt facilitated the formation of coastal lignite.

Introduction

The Paleogene period witnessed several global hyperthermal events (Zachos et al. 2001). Out of them, the most significant had been that took place at the end of the late Paleocene and the beginning of early Eocene intervals when the seawater temperature rose by about 4 °C (Jenkyns 2003; Hessler et al. 2017). These hyperthermal events were triggered by an enhanced supply of greenhouse gases that ushered rapid evolutionary and/or environmental turnovers. These events are marked by records of sharp sea-level rise, ocean de-oxygenation (Sluijs et al. 2014 and references therein), shoaling of the calcite compensation depth (CCD), enhanced hydrological and weathering cycles (Nicolo et al. 2007) and increased supply of kaolinite to the marine realm (Gibson et al. 2000 and references therein). Several studies link the formation of authigenic minerals to sea-level changes in sequence stratigraphic context (Morad et al. 2012). On the contrary, the role of seawater temperature and composition on authigenic mineral formation representing the ‘greenhouse world’ is rarely investigated beyond carbonate sediments. This paper finds a correlation of authigenic mineralization with the fluctuations in global seawater temperature. It points out marked enhancement in authigenic mineralization in marine sediments during Paleogene warm climatic intervals.

Glauconite formed abundantly during the Paleogene, constituting up to 24% of the total record (Banerjee et al. 2016a). Recently Bansal et al. (2019) attributed the high abundance of glauconite in the Upper Cretaceous to a combination of factors like high sea-level, enhanced continental weathering in warm and humid climatic conditions and oxygen depletion on shelf seas. However, because of the lack of biostratigraphic control, these authors could not relate the abundance of the Upper Cretaceous glauconites to specific geological events. Therefore, it is unclear whether the glauconite is distributed evenly within the Late Cretaceous, or it is restricted to specific time intervals. Palaeo-oceanographic conditions of the Late Cretaceous time largely continued in the Paleogene (Jenkyns 2003). The biostratigraphically constrained sections in the Paleogene provide an opportunity to explore whether the occurrence of glauconite depended on subtle changes in palaeo-oceanographic conditions corresponding to warm climatic intervals. Phosphorite is a common associate of the Late Cretaceous glauconites, particularly Tethyan deposits (Banerjee et al. 2019). Lignite, phosphorite, and ironstone deposits of commercial importance are well known in the Paleogene sedimentary succession. However, the relationship between the abundance of these minerals and hyperthermal events is never investigated. This paper aims to present the commonalities of authigenic minerals formed during the Paleogene warm climatic intervals. Although the focus of this study is on glauconitization, phosphorite, ironstone, and lignite formation are also considered. To this effect, a thorough review has been presented.

Global record of hyperthermal events

Paleogene time represents a complex evolution of Earth’s climate bracketed within the overall warmer Cretaceous to colder Neogene transition (Zachos et al. 1993). Deep-sea benthic foraminiferal δ¹⁸O and δ¹³C values reveal extreme warming during the Paleogene (Zachos et al. 2001). Short-lived (~ 200 kyr) events of rapid climatic shifts characterize the Paleogene climate. The ‘hyperthermal’ events coincide with negative carbon isotopic excursions (CIEs) (Fig. 1; Cramer et al. 2003; Nicolo et al. 2007; Stap et al. 2009; Zachos et al. 2010). The negative CIE implies a rapid delivery of isotopically depleted carbon into marine shelves and the rise of pCO₂ in the atmosphere subsequently. The climatic transitions during the Paleogene had a severe impact on the biosphere and lithosphere. Microfossil records show severe decline and diversifications in pelagic and open marine ecosystems during these thermal events (e.g., Thomas 1998; Crouch et al. 2001; Kelly 2002; Khanolkar and Saraswati 2019).

Fig. 1

Temporal distribution of Paleogene glauconite, lignite, phosphorite and oolitic ironstone in different geographic zones (right). Green rectangles indicate glauconites with precise biostratigraphic information while the vertical black arrows indicate glauconite without precise age control. Yellow, brown and red rectangles indicate age range of lignite, phosphorite and ironstone, respectively. Note overlapping of data in a few cases. The number corresponds to those provided in Table 1. Further note that most glauconite deposits occur within three warm climatic intervals marked by grey bands, separated by cooling intervals (white bands). The grey bands also incorporate lignite, phosphorite and oolitic ironstone. Also note that the early Paleogene, i.e. from early late Paleocene (Selandian) to later part of early Eocene (Ypresian), contain the highest number of glauconite deposits. The ‘hyperthermal’ events within the grey bands are marked along with the carbon and oxygen isotopic curves. The sea-level cycle is adapted from Haq et al. (1987). Numbers correspond to those provided in Table 1. MECO: Middle Eocene climatic optimum; EECO: Early Eocene climatic optimum; ETM2: Eocene thermal maximum 2; ETM3: Eocene thermal maximum 3; PETM: Paleocene–Eocene thermal maximum; ELPE: Early late Paleocene event; MPBE: Mid Paleocene biotic event; LDE: Latest Danian event; MPBE: Mid Paleocene biotic event; DAN-C2 represents two short-lived carbon and oxygen isotope excursion at the early Danian (Gradstein et al. 2012); Black solid circless with ‘Oi’ represents Oligocene isotope excursion events (Miller et al. 2009)

Early Paleogene time records warming of Earth’s surface in the period from late Paleocene (ca. 59 Ma) to early Eocene (ca. 52 Ma). Most of the hyperthermal events viz. the Paleocene–Eocene thermal maximum (PETM) or H-1 (Cramer et al. 2003) and the Eocene thermal maximum 2 (ETM2) or Eocene layers of mysterious origin (ELMO) (Lourens et al. 2005) and the Eocene thermal maximum 3 (ETM3) or H2 or “X” event (Nicolo et al. 2007; Stap et al. 2010; Zachos et al. 2010), besides several short-lived climatic perturbations viz. I1 and I2 (Cramer et al. 2003; Nicolo et al. 2007), belong to this period. These hyperthermal events belong to three warm climatic intervals (Fig. 1). The early Paleogene warming interval includes the early late Paleocene event (ELPE, Bralower et al. 2002), also known as mid-Paleocene biotic event (MPBE, Bernaola et al. 2007), and the latest Danian event (LDE, Bornemann et al. 2009) in the Paleocene, and PETM, ETM2, ETM3 and EECO (early Eocene climatic optimum) in the early Eocene. A 17 Myr of cooling trend succeeds upwards and is interrupted by another warm climatic interval incorporating the middle Eocene climatic optimum (MECO) during the early Bartonian (Fig. 1). It is followed upwards by a long-term cooling trend that continues till the early Oligocene, as the arctic ice-sheets formed. A short-lived warming interval incorporates the late Oligocene warming event (LOWE), representing the last hyperthermal event during the Paleogene (Zachos et al. 2001). The Paleocene hyperthermal events viz. Dan C2-event, latest Danian event (LDE), and early late Paleocene event (ELPE) have received less attention compared to the hyperthermal events in the Eocene (Schulte et al. 2013).

Paleogene authigenic mineral formation

Occurrence of glauconite

Our study presents 124 Paleogene glauconite occurrences that formed principally in four major continents/zones, and these account for > 90% of the total global record of this time (Table 1; Fig. 2). These zones are: A) North American continental margin (eastern and western coastal plain deposits); B) Palaeo-Tethys, including northern Africa, parts of southern Europe, Middle East and India to the east; C) Palaeo-North Sea, extending from the United Kingdom to the west to northern Germany in the east; and D) High southern latitudes, including New Zealand eastern Tasman Plateau and Argentina (Figs. 2, 3, 4, 5, 6). Paleogene glauconite also occurs in places in Africa, including Ivory Coast, Nigeria and South Africa, and Asia, including Russia, China and Japan. The majority of the glauconite deposits formed on the well-developed continental margin on the northern hemisphere.

Table 1 Paleogene glauconites along with precise age, associated lithology, mineral and biostratigraphic assemblage (the serial no. of data correspond to those provided in Figs. 1, 2, 3, 4, 5, 6)

Fig. 2

Global distribution of Paleogene glauconite. Note that glauconite occurs mostly in the four zones marked by dashed rectangles, viz. a) North American continental margin (eastern and western Gulf coastal plain deposits), b) Palaeo-Tethys, including northern Africa, parts of southern Europe, Middle East and India to the east, c) Palaeo-North Sea deposits, extended from UK to the west to northern Germany in the east, and d) High southern latitudes, including New Zealand including a few deposits from eastern Tasman Plateau (numbers correspond to those provided in Table 1)

Fig. 3

Palaeogeographic map of Paleocene showing distribution of glauconite–phosphorite, glauconite–lignite and glauconite–oolitic ironstone associations (adapted from Boucot et al. 2013). Note that extensive glauconite–phosphorite association formed along the northern margin of African shelf. Further note that glauconite–lignite association remained confined to tropical climatic zones (numbers correspond to those provided in Table 1)

Fig. 4

Palaeogeographic map of early Eocene showing distribution of glauconite–phosphorite, glauconite–lignite and glauconite–oolitic ironstone associations (adapted from Boucot et al. 2013). Note that extensive lignite–glauconite association occurs at the eastern part of Tethys along the Indian subcontinent (numbers correspond to those provided in Table 1)

Fig. 5

Palaeogeographic map of middle and late Eocene showing the distribution of glauconite–phosphorite, glauconite–lignite and glauconite–oolitic ironstone associations (adapted from Boucot et al. 2013). Note that phosphorite–glauconite association is restricted to the eastern part of African continental shelf instead of the northern part. Scattered deposits with lignite–glauconite association occur within the tropical–boreotropical climatic belts (numbers correspond to those provided in Table 1)

Fig. 6

Palaeogeographic map of Oligocene showing the distribution of glauconite–phosphorite, glauconite–lignite and glauconite–oolitic ironstone associations (adapted from Boucot et al. 2013) (numbers correspond to those provided in Table 1)

Out of 124 occurrences, approximately ~ 55% have reliable age control, biostratigraphic or radiometric. Therefore, they are presented separately from those not having similar age constraints in Fig. 1. The stratigraphic distribution of glauconite during the Paleogene shows a non-uniform trend (Fig. 1). Eocene, Paleocene, and Oligocene record 49%, 35%, and 16%, respectively, of total glauconite occurrences (Table 1; Fig. 1). These time intervals also included the hyperthermal events of the Paleogene (Fig. 1). The occurrence of the age-constrained glauconites coincides with three bands of warm climatic intervals discussed earlier, while the cooler intervals alternating with them have negligible glauconite occurrences (Fig. 1).

Glauconite–phosphorite association

Phosphorite deposits of economic significance are associated with glauconitic sandstone, siltstone and shale (Banerjee et al. 2019 and references therein; Boukhalfa et al. 2020). Our study reveals a cluster of glauconite–phosphorite deposits along the palaeo-Tethyan margins (Figs. 2, 3, 4, 5; see also Soudry et al. 2006). The co-occurrence of glauconite and phosphorite is reported in 17 cases, all of which correspond to the Paleogene warm climatic intervals (Fig. 1).

Extensive Paleocene–early Eocene phosphorite deposits occur along the northern margin of the African continent (Lucas and Prévôt-Lucas 1995; Soudry et al. 2006; Kechiched et al. 2018) (Figs. 3, 4). These deposits represent the so-called ‘Tethyan phosphorites’ (Soudry et al. 2006). Broadly similar lithology defined by clays, marls, dolomite, and foraminiferal limestone hosts phosphorites in these basins. Phosphorites deposited in low palaeo-latitudes (< 30°) in Tunisia and Algeria (Kouwenhoven et al. 1997; Messadi et al. 2016; Garnit et al. 2017; Kechiched et al. 2018) (Fig. 3). During the early Eocene, phosphorite-rich sediments, hosting glauconite, extended towards the north in shallow marine deposits of Germany (Dill et al. 1996), and to the east in Dababiya Quarry Member in Egypt (Metwally and Mahfouz 2018) (Fig. 4). During the middle to late Eocene, the locus of phosphorite deposition shifted towards the eastern and northern parts of the Tethyan domain (Fig. 5). The deposition of phosphorite took place in Iran and in Oman (Beavington-Penney et al. 2006; Zarasvandi et al. 2019) and in Poland (Czuryłowicz et al. 2014). Phosphorite deposition was less common in the Oligocene. Besides the Tethyan margin, glauconite and phosphorite deposits formed within the Oligocene succession of Australia and South Africa (Kelly and Webb 1999; Wigley and Compton 2006; Tóth et al. 2010) (Fig. 6). Throughout the Paleogene, most of the phosphorite–glauconite association was restricted to arid paleoclimate, low-latitudinal passive margin settings (Figs. 3, 4, 5, 6).

Glauconite–lignite association

Lignite is a common deposit of the Paleogene time (Table 1; Fig. 1). The formation of Paleogene lignite overlaps with glauconite within the warm climatic intervals (Fig. 1). Paleogene glauconite occurs in the same stratigraphic succession with economically exploitable lignite in 15 cases. During the Paleocene, lignites formed within a short span of ~ 10 Ma from late Danian to late Lutetian (Mancini and Tew 1993; Liu et al. 1997; Steurbaut et al. 2003; Egger et al. 2009; Ferrow et al. 2011; Samanta et al. 2013a; Sluijs et al. 2014). The oldest record of the lignite–glauconite association from North American Gulf Coastal Plain deposits corresponds to the Danian–Selandian transition (Fig. 1). At the Paleocene–Eocene transition, lignite formed even at high palaeolatitudes in Svalbard, Norway (Dypvik et al. 2011) (Fig. 3). The late Paleocene and middle Eocene lignite–glauconite associations are best developed in the eastern margin of Tethys (Figs. 3, 4). Lignite formed in restricted marine conditions (Chattoraj et al. 2009; Saraswati et al. 2014, 2018). The glauconite–lignite association reduced abruptly during middle and late Eocene (Fig. 1). During the late Oligocene, lignite–glauconite association was restricted only to palaeo-North Sea basin (De Man and Van Simaeys 2004) (Fig. 5). Lignites formed in humid, tropical to boreotropical, and even warm temperate climatic conditions favoring lush growth of vegetation (Figs. 3, 4, 5, 6). Most lignite–glauconite occurrences of high northern latitudes coincide with the Paleocene–Eocene transition, but it is restricted to middle Eocene in Russia and late Oligocene in Belgium (Table 1; Figs. 5, 6). At high southern latitude lignite is devoid of glauconite during the Paleocene and early Eocene.

Glauconite–oolitic ironstone association

Oolitic ironstone deposits contain ~ 5% iron oolith/ooid and more than 15% iron, with goethite, siderite, chamosite, odinite, and berthierine as chief iron-bearing minerals (Rudmin et al. 2019). The majority of the oolitic ironstone of the Cenozoic time preferably formed in the late Paleocene to early–middle Eocene period in marginal marine environments (van Houten 1992; Rudmin et al. 2019). van Houten (1992) found that the majority of the oolitic ironstone deposits of the Cenozoic time are associated with glauconites and phosphorites, hardgrounds and coal measures. Our review reports 14 cases of glauconite–oolitic ironstone association, closely linked to the warming events of the Paleogene (Table 1; Fig. 1). In most of these deposits, glauconite and oolitic ironstone deposited in close proximity to lignite and phosphorite.

Glauconite–oolitic ironstone association occurs within the Paleocene Kressenberg Formation in Austria (Egger et al. 2009) (Fig. 3). Oolitic ironstone deposits proliferate in the London Basin during the late Paleocene and early Eocene (Huggett and Gale 1997). Glauconite–ironstone association declines during the onset of cold climatic conditions after EECO (ca. 50 Ma) (Figs. 1, 5, 6). This deposit in Egypt, Iran, and Oman coincides with middle Eocene climatic optimum (MECO) (Beavington-Penney et al. 2006; Baioumy 2007; El-Habaak et al. 2016; Zarasvandi et al. 2019). The late Oligocene glauconite–oolitic ironstone association occurs in the North American continental shelf deposit, from the palaeo-North Sea basin and high southern latitude deposits in Australia and New Zealand (Kelly and Webb 1999; Hesselbo and Huggett 2001). Oolitic ironstones tend to form in tropical/boreotropical and warm temperate climate during most of the Paleogene. During the middle Eocene, the locus of their formation shifted towards the northern margin of the African Shelf (Fig. 5).

Discussion

The formation of glauconite during warming intervals

Although the allogenic glauconite occasionally occurs in the ancient rock record (Amorosi 1997), the vast majority of glauconites form on the seafloor in situ with negligible sediment input (Odin and Matter 1981; El Albani et al. 2005; Amorosi et al. 2007, 2012; Banerjee et al. 2012a, 2012b, 2015, 2016a, 2016b; Baldermann et al. 2013, 2017). Prolonged chemical exchange between seawater and sediments is a prerequisite for the formation of glauconite (Odin and Matter 1981). Several case studies indicate that the composition of glauconite bears subtle evidence of seawater composition of the past (El Albani et al. 2005; Meunier and El Albani 2007; Banerjee et al. 2008, 2015, 2016a, 2016b; Bansal et al. 2017, 2018, 2019, 2020a, 2020b; Tang et al. 2017a, 2017b; Liu et al. 2020; Mandal et al. 2020)

The high global abundance of glauconite in a short time span encompassing the Paleogene warming intervals rules out the local diagenetic effects on the formation of glauconite. Our literature survey shows the distribution of glauconite and other authigenic minerals on the Paleogene ocean floor across the globe. Therefore, the driving factors behind the formation of these authigenic minerals must have acted globally. The close correspondence between the occurrence of glauconite and warming intervals in Paleogene suggests a genetic link between the two (Figs. 1, 7). Several factors determine the formation of glauconite, including depositional conditions, sedimentation rate, seawater temperature, availability of iron and potassium (Odin and Matter 1981; Amorosi 1995, 1997, 2011, 2012; Amorosi and Centineo 1997; El Albani et al. 2005; Amorosi et al. 2007, 2012; Meunier and El Albani 2007; Banerjee et al. 2016a, 2016b; Mandal et al. 2020). The following section discusses the influence of the controlling factors in the formation of authigenic glauconite during the Paleogene.

Fig. 7

Model explaining the formation of glauconite in shallow marine environment during the Paleogene time. Phosphorite and ironstone formed broadly in similar conditions. OM: Organic matter; FWWB: Fair-weather wave base; SWWB: Storm-weather wave base

The depositional environment has strong control over the formation of glauconite by regulating the rate of sedimentation, redox conditions as well as the supply of abundant ions. Although seawater contains abundant potassium, its iron content is very less, particularly in the deep marine environment. Iron is supplied into the shallow sea by the weathering of continental landmasses. However, the depositional environment remains oxic and sediment supply remains high in shallow marine environments, which discourages the growth of glauconite. In modern oceans, glauconite forms abundantly within the outer shelf and deeper environments (Odin and Matter 1981; Amorosi 2012; Banerjee et al. 2016a). However, the Paleogene glauconite formed primarily in shallow seas, possibly below the fair-weather wave base (Table 1; Fig. 7). Significantly, glauconites formed predominantly in shallow marine conditions during the Cretaceous period, which is also known for warm climatic conditions (Bansal et al. 2019). However, Bansal et al. (2019) could not establish the relationship between warming intervals and glauconite occurrence because of poor biostratigraphic controls of the Cretaceous successions.

Recent glauconite forms mostly along the eastern and western margins of Africa and North America, southern margin of Australia, and western margin of South America. The formation of glauconite always remained confined within 60° latitudes on both sides of the equator (Porrenga 1968; Odin and Matter 1981). Except for one report of glauconite from Norway, all the Paleogene glauconites also show a similar latitudinal distribution, i.e. within the confinement of 60° palaeo-latitudes. They are absent in the high latitudes (in the Arctic and Antarctic region) (Figs. 3, 4, 5, 6). The absence of glauconite in the polar region and its paucity in the extra-tropical region indicates that a high temperature of seawater facilitates the formation of this mineral. As carbonate deposition shifted to the shallow marine environment during the greenhouse climate, likewise glauconitization too shifted to shallow seas during the Paleogene hyperthermal events. The formation of glauconite is five times slower in the cold water at a depth of 2.5 km, compared to the shallow marine region (Baldermann et al. 2013). Microbiota plays a crucial role in the fixation of iron into the smectite structure, transforming it into glauconite in the modern deep marine environment (Baldermann et al. 2017). In the case of shallow marine glauconite, such a microbial role is not apparent. The chemical composition of deep marine glauconite differs from their shallow marine counterparts by having more Fe₂O₃ and less Al₂O₃ and therefore indicates that the mechanism of formation of this mineral must be different (Baldermann et al. 2017).

The warm and humid climatic conditions during the Paleogene thermal events enhanced the rate of continental weathering (Hessler et al. 2017). Consequently, an increase in the supply of K, Fe, Si, Al, Fe, and Mg ions into the shallow marine environment through riverine input likely to have raised the alkalinity of oceans (Fig. 7). Experimental results indicate that highly alkaline seawater promotes the formation of glauconite (Harder 1980). Extensive physical reworking of all varieties of continental rocks during the attendant marine transgression further facilitated the release of nutrients to the seawater (Peters and Gaines 2012). The enhanced riverine input during the warm climatic intervals could have provided the required Fe for the formation of glauconite in the shallow marine environment.

The formation of iron-bearing authigenic phases is regulated by the depositional redox condition and the iron reduction reactions (El Albani et al. 2005; Meunier and El Albani 2007; Taylor and Macquaker 2011). Experimental results indicate that sub-oxic condition is a prerequisite for glauconite formation (Harder 1980). Fe occurs as sulfide in reducing conditions in the sulfidic anoxic zone (Berner 1981), while goethite and chamosite forms in oxygenated seawater (Kimberley 1979; Rudmin et al. 2019). However, shallow and intermediate seas presumably became oxygen-deficient during the hyperthermal events (Nicolo et al. 2010; Schulte et al. 2013; Sluijs et al. 2014). The extensive occurrence of Paleogene black shale within the shallow marine Tethyan domain bears testimony to this (Gavrilov et al. 2013; Schulte et al. 2013). Micropalaeontological data of Kutch in India also supports oxygen-deficient shallow marine environments during PETM, ETM2, and ETM3, all of which are characterized by the high abundance of rectilinear benthic foraminifera that is known to be tolerant to low oxygen (Nigam et al. 2007; Khanolkar and Saraswati 2015, 2019). Multiple factors possibly led to this hypoxia on the Paleogene shelves (Fig. 7). The enhanced bioproductivity related to the abundant supply of nutrients created the sub-oxic condition in the shallow marine domain (Sluijs et al. 2014). Widespread hypoxia in marginal marine environments has been documented from Cretaceous global anoxic events (Sluijs et al. 2014). Significant warming during the hyperthermal events led to the discharge of freshwater and nutrients, causing a stratified seawater column and thereby promoting the suboxic to anoxic conditions. Sluggish deep-water circulation further aided the seafloor oxygen depletion (Ridgwell and Schmidt 2010). The glauconite-bearing green shales of Kutch in early and middle Eocene correspond to ETM2 and MECO. These glauconitic shales are typically characterized by tiny, triserial planktic foraminifera Jenkinsina Columbiana and Streptochilus martini (Kroon and Nederbragt 1990; Kimoto et al. 2009; Khanolkar et al. 2017). Their abundance reaching up to 35% of planktic foraminiferal count suggests high runoff and upwelling conditions in these intervals. Therefore, the availability of abundant continent-derived Fe, as well as the development of the sub-oxic conditions in shallow seas boosted glauconite formation during warm climatic intervals.

A slow rate of sedimentation generally helps the reduced iron to be incorporated into the glauconite structure (Odin and Matter 1981; Meunier and El Albani 2007; Amorosi 2012; Banerjee et al. 2016a, 2016b). The enhanced supply of siliciclastics because of heightened continental weathering during the hyperthermal events should also have raised the sedimentation rate. Although the absolute sea-level rose only about 20 m to 30 m during the hyperthermal events, its rapidness might have led to sediment starvation in shallow seas (Sluijs et al. 2014). The occurrence of around 90% glauconite coinciding with the three warm climatic intervals marked in Fig. 1 indicate that temperature, redox condition of the depositional environment, rapid transgression as well as the availability of abundant nutrients possibly superseded the effect of possible excessive clastic supply into the marine basin related to enhanced weathering during the ‘greenhouse world’.

Factors promoting phosphorite deposition during the Paleogene time

Phosphorite is a common associate of glauconite in the Paleogene (Glenn and Arthur 1990; Kouwenhoven et al. 1997; Kechiched et al. 2018; Metwally and Mahfouz 2018; Banerjee et al. 2019). Conditions favourable for the precipitation of phosphorite and glauconite broadly overlap, requiring depletion in clastic supply and oxygen-depleted seawater. Hypoxic and anoxic bottom seawater facilitates the recycling of phosphorus from organic matter. Sub-oxic Paleogene shelf waters, therefore, remained the favourable sites of formation for both glauconite and phosphorite. Schulte et al. (2013) reported the formation of phosphorite during the recovery phase of the PETM. Phosphorite–glauconite association in modern and ancient sediments forms within a narrow zone lying between upper slope (Fe- and P-poor, TOC enriched) and outer shelf (Fe- and P-enriched), in close vicinity of the oxygen minimum zone (Banerjee et al. 2019 and references therein). Palaeolatitude is also another factor that controls global P-cycle (Soudry et al. 2006). Low latitudes favouring open circulation prefer the accumulation of phosphorite (Cook and McElhinny 1979; Soudry et al. 2006). During most of the Paleogene, the northern part of the African continent remained close to the equator (Figs. 3, 4, 5, 6).

Factors influencing lignite deposition

Paleogene lignite deposits are predominantly of strand plain origin (Prasad et al. 2013), and they remain confined to the tropical zone along the palaeo-Tethyan margin (Figs. 3, 4, 5, 6; Chattoraj et al. 2009; Egger et al. 2009; Samanta et al. 2013a, 2013b). Lignite deposits form at the top of smaller order shallowing-upward cycles, below the marine flooding surfaces within an overall transgressive deposit (Prasad et al. 2013). Whereas, the occurrence of glauconite coincides with the marine flooding surfaces (Banerjee et al. 2012a, 2012b). A humid climate presumably facilitated the growth of the tropical rainforest during the warmer climatic intervals of Paleogene. Accumulation of abundant vegetal matter in a stagnant marginal marine environment possibly led to lignite formation. Coal deposits during the Paleozoic formed in tropical climates under high rainfall (Cecil et al. 1985). A low rate of clastic input coupled with wet climatic conditions and vegetation cover facilitated coal formation (Cecil 1990). However, glauconite, as well as phosphorite are rarely associated with Paleozoic coal deposits.

Coal/lignite is particularly abundant in several Indian Paleogene basins including Cambay (Prasad et al. 2013; Samanta et al. 2013b), Kutch (Khanolkar and Saraswati 2015 and references therein), Rajasthan (Raju and Mathur 2013) and in Assam-Arakan basin (Saikia et al. 2009). The high abundance of lignite within the Indian Paleogene possibly relates to the formation of a tropical rainforest that leads to the rapid deposition of organic matter and higher land plants into the marginal marine environment (Prasad et al. 2013). Extensive development of marsh-bay complexes characterized the Indian subcontinent that remained close to the equator during the Eocene (Figs. 4, 5) (Prasad et al. 2013).

Formation of oolitic ironstone during the Paleogene

Glauconite forms an important component within the Paleogene oolitic ironstone deposits along the globe (van Houten 1992). Depositional conditions required for the formation of oolitic ironstone, glauconite, and phosphorite are broadly similar (van Houten 1992; Todd et al. 2019). Although most Paleozoic oolitic ironstones involve upwelling, Cenozoic deposits are controversial in terms of origin. A warm climate and marine transgression favour the formation of oolitic ironstone (Todd et al. 2019). The warm climate facilitates continental weathering and supplies abundant Fe into the shallow marine ocean (see Todd et al. op. cit.). The particulate riverine Fe is trapped mostly in lagoons, estuaries and flood plains before reaching the deep ocean during the rapid transgression (Poulton and Canfield 2011). Further, submarine volcanism, related to ocean floor spreading provides abundant Fe²⁺ into the marine realm. The upwelling current carries additional P⁴⁺ and Fe²⁺ from the deeper ocean and facilitates the formation of phosphorite, glauconite and oolitic ironstone. Microbial respiration/oxidation of organic matter is further responsible for the formation of anoxic and hypoxic water column in the upwelling front favouring the formation of francolite (and/or pyrite) and Fe-silicates, respectively (Todd et al. 2019). A more oxygenated water column results in the formation of Fe-(oxyhydr)oxide constituting the ironstone facies. Several studies indicated that the formation of oolitic ironstone is favoured immediately after ocean hypoxia (Schulte et al. 2013; Bekker et al. 2014). Therefore, an increase in productivity and related oxygen deficiency provides abundant ferrous iron in shelf waters, thus facilitating massive ironstone deposits during the warm climatic intervals (Homoky 2017; Konhauser et al. 2017). While the pyrite can be formed in hypoxic and anoxic seawater, chamosite and/or berthierine formation is favoured in hypoxic seawater conditions (Berner 1981; Taylor and Macquaker 2011; Todd et al. 2019; Rudmin et al. 2020). Rudmin et al. (op. cit.) established a link between volcanism and oolitic ironstone formation from the Siberian basins. Widespread volcanism in north Atlantic during the early part of Paleogene might have facilitated hypoxic seawater.

Conclusions

The review of existing literature establishes a link between Paleogene warming events and authigenic mineralization, with the following conclusions.

• 1) A review of global occurrences of Paleogene glauconites broadly correspond to warm climatic intervals that witnessed multiple hyperthermal events.

• 2) The widespread occurrence of glauconite across the globe in the late Paleocene and early Eocene relates to a combination of factors including global sea-level rise, hypoxic shelf, and warm and humid climate. A slow rate of sediment accumulation within the transgressive shallow seas facilitated the formation of glauconite on the seafloor.

• 3) The depositional conditions of phosphorus and oolitic ironstone are broadly similar to those of glauconite, and therefore, the abundance of these two deposits follows a similar trend.

• 4) Although the occurrence of authigenic deposits was largely influenced by global climatic conditions, factors like upwelling current and palaeolatitude led to the formation of phosphorite and lignite regionally.

• 5) Palaeo-latitudinal settings also influenced the occurrence of authigenic deposits. The deposition of phosphorite and lignite deposits was favoured in low palaeolatitudes.

• 6) Paleogene lignite formed mostly in coastal environments and their formation is facilitated in warm and humid climate. These deposits marked smaller order regressions within an overall transgressive deposit of warm climatic intervals.