Meteoric Water Flow and Diagenesis

The introduction of meteoric water has a profound effect on carbonate sediments and their potential as reservoir rocks. A proportion of the rainwater (meteoric water) falling on land infiltrates into the groundwater. The flow of groundwater ultimately is limited by the rate of recharge by rainwater, which determines the water table gradient. As long as the water table is above sea level the groundwater has the hydrodynamic potential to flow beneath the beach and out into the basin beneath the seafloor, floating on top of the more saline basin porewater (Fig. 5.47). The freshwater lens is floating like an iceberg in the sea. With a groundwater density of 1.00 g/cm3 and the more saline water 1.025 g/cm3, the ratio between the groundwater head and the depth of freshwater penetration is theoretically 1/(1.025 − 1.00) = 1/40. A groundwater head of just 10 m can drive freshwater to a depth of up to 400 m below sea level. Shallow water carbonates deposited in coastal environments and around islands will thus in

However, coastal carbonate environments are usually rather dry and while carbonate platforms may have more rainfall, the islands on them may be small compared to the size of the platform. Both these factors tend to reduce the flux of meteoric water into marine carbonate sediments, although it may still be very significant, particularly when the sedimentation rate is low. More distal and pelagic facies may avoid this flushing altogether.

On land and in the nearshore parts of the basin meteoric water may also be undersaturated with respect to calcite, in which case caverns are likely to develop. When meteoric water flows through recent carbonate sediments it will be undersaturated with respect to aragonite but become rapidly supersaturated with respect to calcite. Aragonite will therefore dissolve first and calcite will precipitate. Gradually the meteoric water will reach equilibrium with low-Mg calcite, and calcite cement in the form of large crystals (blockshaped cement) may be precipitated (Fig. 5.48).

This cement is very different from marine cements precipitated from modified seawater (without sulphate). On land, the sediments above the water table are located in the vadose zone, where the pores are alternately filled with water and air as a consequence of intermittent meteoric water percolation. Partial desiccation results in an unequal distribution of the porewater with it primarily held near grain contacts, by capillary forces; as a result there will be a preferential cementation of pore throats giving a rounded pore geometry. This cement type is called meniscus cement. Porewater will also collect on the underside of grains as pendant droplets and precipitate cement in this form, called pendant cement. Both meniscus cement and pendant cement are characteristic of partial cementation in the vadose zone.