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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Sandstone pebbles can be found on many of the Gulf Coast beaches of southern Louisiana. Preliminary analyses indicate cement composition to be high-magnesium calcite with 10 to 15 mole % magnesium carbonate. Petrographically, the cement appears in the form of blades and fibers. SEM observations, however, indicate a complex arrangement of stacked euhedral to subhedral crystals.
Field observations have led to the discovery of the lithified sandstone in situ. Cementation occurs along a narrow band between beach dunes and a salt-water marsh on the leeward side of the dunes. Apparently lithification occurs at or near the surface where high temperatures and salinities exist in a supratidal environment. X-ray analyses indicate a cement composition of high-magnesium calcite. The mole percent of magnesium carbonate ranges from 20 to 50%. Although mole percentages are high, X-ray analyses do not indicate the presence of well ordered dolomite. Petrographically, the calcite appears both as a "microspar texture" and fiber-radiate rim cement. As with the reworked sandstone pebbles, SEM observations indicate the presence of stacked euhedral to subhedral crystals that aver ge 0.25 µm in diameter. These aligned or stacked crystals form pseudofiber bundles and blades. Preliminary studies indicate that the in-situ high-magnesium calcite is unstable and probably undergoes molecular leaching when exposed to normal sea water. This is exemplified by the composition of the reworked sandstone pebbles.
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