In the Ebro River Valley in the northeastern part of the Iberian Peninsula, a playa-lake area covering 65 sq km of Tertiary sedimentary rocks is known as "Los Monegros." Many of the lakes coincide with east-west fault lines, in some of which the scarps are 8 to 10 m high. Year round sampling of 15 of the lakes has been conducted.

In the emerged zone of the playa efflorescences of 5-cm long thenardite crystals occur locally with salts such as halite and bloedite and minor amounts of calcite, dolomite, mica, quartz, and others.

On the highest part of the emerged zone, pulverulent salt crusts are formed of thenardite, gypsum, and halite. The saline crusts as well as the efflorescences are weak and ephemeral, appearing and disappearing quickly, depending on climatic factors such as rain and wind.

The silt which constitutes the habitual sediment of the playa lakes is formed mainly of gypsum with a variable proportion of quartz, micas, calcite, and dolomite. The imbibition waters of the silt precipitate halite and/or bloedite. This silt is commonly on top of a rich layer of organic matter, in which it is not unusual to find halite crystals. In some playa lakes the silt is covered by an algal mat, reddish in hue, with abundant cracks, air bubbles, and tepees.

There are two types of precipitates in the playa lakes: one of halite with thenerdite and bloedite subordinate, and the other of thenardite with bloedite, gypsum, and halite and, in some, traces of glauberite. Salt crystals locally accumulate in bar form several centimeters thick parallel with the coast line.

Hydrochemically, the water in the playa lakes has the following characteristics: (1) the concentration of SO₄ is much greater than that of Ca⁺²; (2) the concentration of Cl^- is less than that of Na⁺; (3) the concentration of anion sulfates, not counting those which theoretically combine with calcium cation, is greater than the concentration of sodium cations not counting those which theoretically combine with chloride anion; (4) the relation: [(K⁺) + (Mg⁺²)/(Na⁺)] = 0.30 - 1.

End_of_Article - Last_Page 835------------