Eogenesis

Introduction

Eogenesis (also called eodiagenesis) is the earliest stage of diagenesis in siliciclastic sedimentary rocks. It operates at very shallow burial depths - from a few metres to tens of metres - and largely under conditions controlled by the depositional environment. This distinguishes it from the deeper mesogenetic stage, which is governed by elevated temperature and pressure, and from telogenesis, which involves meteoric water flushing after uplift. Despite its shallow setting, eogenesis can make a permanent imprint on a sedimentary rock: bioturbation obliterates primary structures, early cements set the initial framework of the pore system, and the first suite of authigenic minerals is established. All subsequent diagenetic history is shaped partly by what happens at this stage.

Bioturbation

The principal diagenetic changes in the eogenetic regime include reworking of sediment by organisms (bioturbation), minor compaction and grain repacking, and mineralogical changes. Organisms rework sediment at or near the depositional interface through crawling, burrowing, and sediment-ingesting activities. Bioturbation can destroy primary sedimentary structures such as lamination and replace them with a variety of traces including mottled bedding, burrows, tracks, and trails. Organic reworking commonly has little effect on the mineralogical and chemical composition of sediments. Because burial depth is very shallow, sediments undergo only very slight compaction and grain rearrangement during this stage. ^[1]

Early Mineralogical Changes

Early diagenesis does bring about important mineralogical changes in siliciclastic sediments. Most of these changes involve the precipitation of new minerals. ^[1] The specific minerals that form depend on whether pore waters are reducing or oxidising.

Reducing (Marine) Environments

In marine environments where reducing (low-oxygen) conditions prevail, the formation of pyrite is particularly characteristic of eogenesis. Pyrite may form as cement or may replace other materials such as woody fragments. Other important early-marine mineral reactions include the formation of chlorite, glauconite (greenish iron-silicate grains), and illite/smectite clays. Iron oxides form in oxygenated marine pore waters, for example as red clays on the deep ocean floor. Potassium feldspar overgrowths, quartz overgrowths, and carbonate cements can also precipitate during early marine diagenesis. ^[1]

Oxidising (Nonmarine) Environments

In nonmarine environments, where oxidising conditions commonly prevail, little pyrite forms. Instead, iron oxides - goethite and hematite - are commonly produced, creating redbeds. Formation of kaolinitic clay minerals and precipitation of quartz and calcite cements may also take place in this environment. ^[1]

The contrast between marine-reducing and nonmarine-oxidising eogenetic mineral assemblages is a powerful diagnostic tool. Pyrite in a sandstone signals marine-reducing early conditions; red colouration from hematite or goethite signals oxidising continental conditions. These indicators remain useful even after deep burial has modified much of the original mineralogy.

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