DIAGENESIS OF THE BAR AIL SANDSTONES

Transcription

1 CHAPTER-VII DIAGENESIS OF THE BAR AIL SANDSTONES 7.1 INTRODUCTION Diagenesis is any chemical, physical, or biological change undergone by sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism. It is a process gone through from loose sediment transportation deposition - consolidation sedimentary rock. Dapples (1972) studied the diagenesis of sandstones on the basis of mineral association and recognized four oxide series among sandstones, viz., aluminia-limemagnesia-iron oxide series, silica-lime-magnesia-iron oxide series, silicaaluminia-iron oxide series, and silica-aluminia-lime-magnesia series. From the type of reaction involved three main stages are considered in a progressive development of sandstones, namely, redoxomorphic, locomorphic and phyllomorphic stages. Newly deposited sediments are characterized by loosely packed, uncemented fabrics; high porosities; and high interstitial water content. As sedimentation continues in subsiding basins, older sediments are progressively buried by younger sediments to depths that may be reaching tens of kilometers. Sediment burial is accompanied by physical and chemical changes that take place in the sediments in response to increase in pressure from the weight of overlying sediments, downward increase in temperature, and changes in pore water composition. The changes act in concert to bring Petrography and Geochemical study o f the Barad Sandstones m and around Ukhrul, Manipur, India: Chapter- VII 152

2 about compaction and lithification of sediment, ultimately converting it into consolidated sedimentary rock. Thus, sand is lithified to sandstone, unconsolidated gravel is eventually lithified to conglomerate, and siliciclastic mud is hardened into shale (mudrock). The process of lithification is accompanied by physical, mineralogical, and chemical changes. Loose grains packing gives way with burial to more tightly packed fabrics having greatly reduced pore spaces by precipitation of cements. Minerals that were chemically stable at low surface temperatures and in the presence of environmental pore water become altered at higher burial temperatures and changed pore water compositions. Minerals may be completely dissolved or may be partially or completely replaced by other minerals. Thus, porosity, mineralogy, and chemical composition may be changed to various degrees during burial diagenesis. The study of diagenesis in rocks is used to understand the tectonic history they have undergone; the nature and type of fluids that have circulated through them. Diagenetic changes reflect mainly the depositional conditions of the sediment Stages and Realms of Digenesis Diagenesis takes place at temperature and pressure higher than those of the weathering and depositional environment but below those that produce metamorphism. There is no clear boundary between the realms of diagenesis and metamorphism and it is a gradational boundary. However, commonly considered diagenesis occurs at temperature below about 250 C. Petrography and Geochemical study o f the Baratl Sandstones m and around Ukhrul, Manipur, India. Chapter- VII 153

3 Diagenesis begin immediately after deposition and continues through deep burial and eventual uplift. Burial subjects the sediments to conditions of pressure and temperature markedly different from those that exist in the depositional environment. Increase in geostatic (rock) pressure, hydrostatic (fluid) pressure, and temperature as a function of depth also change porefluid composition. Various authors (Boggs, S. Jr. 2006) have suggested that sediments go through three to six stages of diagenesis. Perhaps the most widely accepted stages of diagenesis are those proposed by Choquette and Pray (1970), viz., (i) Eodiagenesis, (ii) Mesodiagenesis and (iii) Telodiagenesis. Some authors (Burley and Worden, 2003) refer to these stages simply as, eogenesis, mesogenesis, and telogenesis Eogenesis (Shallow burial) It refers to the earliest stages of diagenesis, which takes place at very shallow depths (a few meters to tens of meters) largely under the conditions of depositional environment. The principal diagenetic changes that take place in the regime include reworking of sediments, minor compaction and grain repacking and mineralogical change Mesogenesis (Deep Burial) It is the stage of diagenesis that takes place during deeper burial, under conditions of increasing temperature and pressure and changed pore-water compositions. The load pressures caused by deeper burial significantly increase the tightness of grain packing with concomitant loss of porosity and thinning of beds. Increase pressure at the contact point between grains also Petrography and Geochemical study o f the Barad Sandstones m and around Ukhrul, Manipur, India: C h a p ter- V II 154

4 increases the solubility of the grains at the contact, leading to partial dissolution of the grains. This process is referred to as pressure solution or chemical compaction. Chemical compaction further reduces porosity and increase bed thinning. Under the influence of physical and chemical compaction, aided by cementation, the primary porosity of sands is reduced during deep burial. Compaction also causes bending of flexible grains such as micas. Mechanical compaction and pressure solution cause porosity loss in quartzose sandstones mainly at burial depths less than about 2 km (Siever, (1979) because the combined effects of compaction, pressure solution and small amount of quartz cement produce stable grain packing arrangements. Porosity loss at greater depths is primarily resulted from quartz cementation (Siever, 1979). Burley and Worden, (2003) suggest that some porosity loss owing to compaction can continue to depth of at least 5 km. An increase in temperature of 10 C during burial can accelerate chemical reaction rates to double or triple. Thus, mineral phases that were stable in the depositional environment may become unstable during deep burial. Increasing temperature favors the formation of denser, less hydrous minerals and also causes an increase in solubility of most common minerals. The most important of these processes are cementation, dissolution, and replacement. Cementation refers to the precipitation of minerals into the pore space of sediment, thereby reducing porosity and bringing out lithification of the Petrography and Geochemical study o f the Barail Sandstones m and around Ukhrul, Manipur, India. C hapter- V II 155

5 sediment. Silica cements are the most common; however feldspars, iron oxide, pyrite and many other minerals may also form as cements. Quartz cementation is favored by high concentration of silica in pore waters and by low temperatures. Silica may also be imported from other areas of the sedimentation basin during episodes of fluid flow related to deep- basin mineral dehydration or tectonic activity (Siever, 1979). Dissolution of framework silicate grains may occur during deep burial under conditions that are essentially the opposite of those required for cementation. Rock fragments and low stability silicate minerals, such as plagioclase feldspars, pyroxenes and amphiboles may dissolve out as a result of increasing burial temperature and presence of organic acid in pore water. Selective dissolution of less stable framework grains or parts of grains takes place during diagenesis by intrastratal solution. Dissolution of framework grains and cements, increases porosity, particularly in sandstones take place through interstratal solutional activities. It is believed that much of the porosity that exists in sandstones below burial depths of about 3 km is secondary porosity, created by dissolution processes Telogenesis (Late stage) Telogenesis refers to the late stage diagenesis that accompanies or follows uplift of previously buried sediments into the regime of meteoric water. These processes bring mineral assemblages, including new minerals formed during mesogenesis, into an environment of lower temperature and pressure Petrography and Geochemical study o f the Barail Sandstones in and around Ukhrul, Manipur; India Chapter- VII 156

6 and in which mesogenetic pore water are flushed and replaced by oxygenrich, acidic meteoric (rain) water of low salinity. Under these changed conditions, previously formed cements and framework grains may dissolve (creating secondary porosity) or framework grains may alter to clay minerals. The presently studied Barail sandstones suffer from distinct diagenetic changes and have been identified and discussed below. 7.2 Diagenesis of the Barail Sandstones In the present case certain diagenetic changes are clearly visible which reflect conditions mostly ranging from eogenesis to mesogenesis. The minerals have undergone some changes during and after their deposition. Presence of pressure solution features such as grain point contacts and sutured grain boundaries, formation of two or more generation of quartz overgrowth, formation of concavo- convex contact between grains indicating dissolution, kink bend of mica in quartz grains are the important features which show diagenetic changes in the Barail sandstones. Eogenesis statge is represented in the studied sandstones by quartz overgrowth (Fig.7.1), kink band in mica (Fig.4.1t), bending in quartz (Fig.7.2) and compaction of sediments. Silica cementation, silicification of undeformed fossil shell (Fig.7.3) and stylolites indicates mesogenesis stage. Mesogenesis stage is also represented by concavo-convex grain boundaries, compaction of sediments Petrography and Geochemical study o f the Barail Sandstones m and around Ukhrul, Manipur, India Chapter- VII 157

7 through realignment of grains under pressure, removal of matrix, extensive overgrowth of mineral grains and removal of unstable heavy minerals. Also, intrusion of quartz veins in the eastern sandstones as evidenced in the field indicates mesogenesis. The study shows the western sandstones have mostly undergone eogenesis followed by mesogenesis, whereas the eastern sandstones well attained the mesogenesis stage. The study on diagenesis indicates that the older eastern sandstones have suffered deep burial as compared to the younger western sandstones. Petrography and Geochemical study o f the Barad Sandstones in and around Ukhrul, Manipur, India Chapter- VII 158

8 Fig-7.1 : Quartz overgrowth, corrugated and sutured grain boundaries of quartz. Fig.7.2: Bending in quartz grains and stylolitic line. Fig-7.3 : Silicified fossil shell of foraminifera. Petrography and Geochemical study o f the Rarail Sandstones in and around Ukhrul, Manipur, India: Chapter- VII ] 59