Telogenesis

Introduction

Telogenesis (telodiagenesis) is the final diagenetic stage, and it operates in a fundamentally different chemical environment from mesogenesis. Where mesogenesis is governed by hot, saline, reducing pore waters at high pressure and temperature, telogenesis begins when rocks are uplifted by mountain-building and unroofed by erosion. The mesogenetic pore fluids are flushed and replaced by oxygen-rich, acidic meteoric (rain) waters of low salinity. This switch reverses many mesogenetic processes and introduces new ones. The mineral assemblage that was stable under burial conditions is no longer in equilibrium, and widespread alteration follows. Telogenesis grades continuously into subaerial weathering as the rocks are eventually exposed at Earth’s surface.

Processes and Changes

Sedimentary rocks that have undergone deep burial diagenesis may subsequently be uplifted by mountain-building and unroofed by erosion. Uplift brings mineral assemblages - including new minerals formed during mesogenesis - into an environment of lower temperature and pressure in which mesogenetic pore waters are flushed and replaced by oxygen-rich, acidic meteoric waters of low salinity. ^[1]

Under these changed conditions, the following telogenetic changes can occur: ^[1]

• Secondary porosity creation: Dissolution of previously formed cements and framework grains by meteoric waters. ^[1]
• Porosity reduction: Alteration of framework grains to clay minerals (e.g., potassium feldspar to kaolinite) plugs pore space. ^[1]
• Silica or carbonate cementation: Depending on the nature of the meteoric pore waters, new silica or carbonate cements may precipitate. ^[1]
• Iron oxide formation: Oxidation of iron carbonate minerals and other iron-bearing minerals produces goethite and hematite. ^[1]
• Pyrite → gypsum: Oxidation of sulfide minerals (pyrite) forms sulfate minerals (gypsum) where calcium is present in pore waters. ^[1]
• Dissolution of unstable minerals: Pyroxenes and amphiboles may be dissolved by meteoric waters. ^[1]

Relationship to Weathering

The processes of telogenesis grade into those of subaerial weathering as sedimentary rocks are eventually exposed at Earth’s surface. ^[1]

This continuum from telogenesis to weathering means there is no sharp boundary between the two regimes. The distinction is partly one of scale and position: telogenesis operates in the shallow subsurface where rocks are still intact, while weathering operates at the exposed surface where physical disaggregation accompanies chemical alteration. Both share the same meteoric fluid driver and many of the same geochemical reactions.

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