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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS
Distinguished Lecture Tour
Abstract: Stages of Transformation of Carbonate Sands into Limestone and Dolostone:
Fuerteventura, Canary Islands
By
The island of Fuerteventura offers a unique possibility for studying
mineralogical, chemical and textural changes in Holocene and Pleistocene
skeletal carbonate sands which, due to eustatic sea level changes, have been
exposed to one or more diagenetic environments. An example is the diagenetic
development of a 30,000 year old dolomitized calcarenite from the Jandia
Peninsula.
Stage 1 (intertidal or "beachrock" stage) -- Precipitation
of magnesian calcite leads to the formation of beachrock in the inter-tidal
zone.
Stage 2 (supratidal marine evaporitic stage) -- Lowering of the sea
level (about 2-3 m compared with stage 1) has exposed the beachrock to the
supratidal zone which, however, still remains under the influence of the sea and
of the evaporating seawater. After the retreat of heavy seas, seawater
accumulated on the surface or within the pores of the beachrock, evaporates and
percolates through the calcarenite. The high Mg/Ca ratio (45) of the percolating
brines causes dolomitization of the magnesian calcite cement as well as of
allochems consisting of the same mineral (red algae, echinoderms).
Stage 3 (supratidal meteoric stage) -- When, by further lowering of
the sea level, the partly dolomitized beachrock is exposed to meteoric
(freshwater) conditions, the supra tidal meteoric stage is achieved. Aragonite
of the allochems is converted to calcite by wet transformation and calcite is
precipitated in the interstitual spaces of the calcarenite as second-generation
cement (cement B).
Other interesting examples are calcarenites (eolianites) which originally were
exposed to the meteoric diagenetic environment and are now in the intertidal zone;
a first generation cement consisting of calcite is followed by a second-generation
cement consisting of magnesian calcite!
Calcarenites overlain by basaltic lavas permit the study of hydrothermal alteration
of carbonate rocks. End_of_Record - Last_Page 5---------------