INTRODUCTION

The Bocana de Virrila is located on the coast of northwestern Peru in the Sechura Desert (Fig. 1). It is a relict river channel which forms an estuary with an open connection to the Pacific Ocean. The topographic features of this ancient river bottom and the excessive aridity of the region have combined to form an interesting example of the deposition of evaporites in an arid climate. It is the purpose of this report to describe where and how these processes are taking place and to confirm some of the hypotheses postulated by other workers concerning the deposition of salts in a restricted estuary where evaporation exceeds precipitation plus runoff.

The report summarizes a reconnaissance investigation of this interesting area made by A. G. Fisher and the writers in August, 1955. No detailed study has ever been made.

DESCRIPTION OF CHANNEL

Beginning near the village of Sechura in the Sechura Desert near Paita, Peru, a broad area slightly lower than the surrounding flat Pleistocene beach deposits extends southward about 30 kilometers, encounters a ridge of flat Miocene shale beds, then curves westward out to the Pacific Ocean. During exceptional years when rain falls on the western slopes of the Andes Mountains, this whole lowland is covered by large sheets of water derived from the spill-over of the Piura River, which normally enters the sea west of Sechura. For example, during the rainy

End_Page 2467------------------------------

year of 1925, geological maps made at that time show the Piura River flowing along this course. However, during normal years, the upper part of this low area is generally dry except for a few shallow evaporating pans. Thin salt and gypsum crusts surround these areas. In the subsurface, gypsum has been observed in numerous shot holes at depths of 30 feet, presumably formed during recent time. Small mining operations for salt and gypsum have been carried out for years throughout this area. No vegetation is present. Although it is some distance from the sea, the area is very near sea-level.

Closer to the sea, the broad area becomes more constricted to form a channel that gradually falls below sea-level. This channel forms an estuary called the Bocana de Virrila, about 20 kilometers long and 2 kilometers maximum width. As this lower part of the channel contains a year-round supply of sea water, considerable quantities of salt and gypsum are being deposited at the present time.

Fig. 1. Location of Bocana de Virrila, Peru.

End_Page 2468------------------------------

DEPOSITIONAL ENVIRONMENTS

In accord with the classification used by Sloss (1953), the depositional environments of the Bocana have been classified as normal marine, penesaline, and saline. Each of these environments is quite distinct geographically and is represented by characteristic mineral associations and water compositions. The nature and location of the environments may change seasonally, and certainly change in those occasional years when there is abnormal rainfall on the coast of Peru. The present paper describes the condition as it was in August, 1955.

The area extending from somewhere just inside the mouth of the Bocana to a sill near the upper reaches of the lagoon has been designated as penesaline and is intermediate in salinity between normal marine and saline waters. According to Sloss (1953), the penesaline environment is characterized by deposition of evaporitic carbonates, interbedded with anhydrite. The waters from the Bocana are a milky, yellowish green. Water depths probably do not exceed 2 meters. Surface waters had a temperature of about 23°C. Throughout the penesaline environment, no indications of sodium chloride or calcium sulphate being deposited were to be found. Black muds covered the floor of the estuary near shore. Farther from shore, a white marl ¼-½ inch thick covered the floor of the estuary. D rectly below were found soft black muds approximately 1 foot thick. This mud in turn overlies gypsum crystals. This gypsum might indicate that higher salinities had previously prevailed much closer to the mouth.

The saline environment of the Bocana occurs at the extreme upper end of the estuary. A small sill composed of several small islands and shallow sand bars greatly increases the physical restrictions of the dense, outflowing brines. This sill was located approximately where the two forks of the upper part of the Bocana meet. Just inside the sill, black muds are found on the bottom. The margins of small islands which occur in the area contain small gypsum crystals. Many of these crystals have a leached, indistinct appearance, as if they have partly gone into solution several times. This is probably due to fluctuations in the horizontal salinity gradient caused by tides, changes in rate of evaporation, or possibly some other reason.

Most of the halite and gypsum deposits are found at the extreme margins of the saline environment. Water depths do not exceed 1 foot. Large interlocking crystals of gypsum up to ½ inch in length have been observed. The growth of these interlocking crystals has bowed up the floor of the estuary, in some places thrusting large folds of the crystals completely above water level. The transition zone between the principal gypsum and halite deposits is very indistinct as both halite and gypsum crystals appear the same when seen under shallow depths of water. The color of the water in the saline environment, especially in the upper reaches where the vast majority of the evaporites are being deposited, is a bright pink. This pink color is thought to be due to red algae in the water. The ame color has been noted in other evaporating pans on the western coast of Peru.

End_Page 2469------------------------------

The only forms of life noted in the saline environment are red and green algae and insect larvae. The temperatures in this environment ranged between 25° to 27°C., somewhat warmer than in the lower reaches of the Bocana.

COMPOSITION OF WATER SAMPLES

Analyses of six water samples taken from the different environments are shown in Table I. Sample locations are shown on the map (Fig. 2). An analysis of normal sea water taken from Clarke's "Data of Geochemistry" is also presented for comparison (Table I). The columns are arranged in order of increasing salinity from left to right. The absolute concentration of the ions is given in parts per million of total water sample. The relative amount of each ion is given in percentage of total solids.

Although a detailed sampling project of the entire estuary would be much more desirable, these preliminary figures readily bring certain facts to light concerning relative ion concentrations in the different environments.

Calcium ions show an increase in concentration in the penesaline environment and a decrease in the saline environment. Percentage-wise, however, calcium ion concentration drops steadily from normal marine to saline conditions. This is because the calcium is precipitated in the lower reaches of the estuary in the normal and penesaline environments, which increases the relative percentage of the more soluble ions, such as sodium, remaining in solution.

Sodium ions show a gradual increase in concentration until the innermost samples are reached; then a sharp decrease is noted. Sodium is not precipitated until the highly saline environment is reached.

Table I. CHEMICAL ANALYSES OF BOCANA DE VIRRILA WATER SAMPLES

End_Page 2470------------------------------

Magnesium ions show a slight increase in concentration until the innermost saline environment is reached, where a sharp increase is noted both in absolute concentration and relative percentage of total solids. Potassium ion concentration is fairly constant until the innermost end of the estuary is reached, where a sharp increase is apparent. The relative percentage is about the same in all samples. Thus neither of these ions is being precipitated appreciably under present conditions, but both are being concentrated by loss of less soluble ions and by evaporation. Still more restricted conditions would be necessary before they could be deposited.

Among the anions, the chloride ions increase in absolute concentration from the lower part of the estuary to the extreme upper end. However, their relative percentage first increases slightly as a result of the precipitation of calcium sulphate, and finally decreases markedly in the place where sodium chloride is precipitated. This comes also as a result of concentration of magnesium and sulphate ions in the last stages.

The sulphate ions show a steady rise in absolute concentration, increasing in the upper reaches of the estuary. There is a slight decrease in relative amount in the lower reaches where gypsum is precipitating. The bi-carbonate ions increase in concentration toward the upper end, but decrease in relative per cent.

Fig. 2. Depositional environments, Bocana de Virrila, Peru.

End_Page 2471------------------------------

Summarizing these results, we find that at first there is a simple increase in concentration of the individual ions above that of normal sea water. As precipitation of calcium carbonate, calcium sulphate, and sodium chloride takes place successively, the relative concentration of these ions decreases while other ions, such as magnesium, potassium, and sulphate increase. Should further concentration of the salts take place due to evaporation, magnesium sulphate and magnesium chloride would probably be precipitated, with the last bitterns being rich in potassium and chloride ions.

LITHOLOGIC DESCRIPTION OF CORES FROM BOCANA VIRRILA

At locality A in the upper reaches of the estuary, the stratified evaporite deposits were sampled to a depth of about 3 feet by driving 3-inch pipes into the sediments. The cores contain interstratified beds of halite, gypsum, and unconsolidated sandstone, indicating that both water level and salinity have fluctuated in the past. Following is a description of the cores.

Table

End_Page 2472------------------------------

Table

PHYSICAL AND CHEMICAL PROCESSES

The order of the precipitation of the evaporites in the estuary conforms closely to Usiglio's order of precipitation. He has shown that calcium carbonate will be precipitated from solutions with salinities ranging from 72 to 199 parts per thousand. The gray marls from the floor of Bocana Virrila were found to be underlying waters with salinities ranging from 88 to 103 parts per thousand. Usiglio showed that calcium sulphate would be precipitated from waters with salinities ranging between 199 and 353 parts per thousand. A water sample taken above gypsum deposits in the estuary totalled 354 parts per thousand. Finally, Usiglio showed that sodium chloride would be deposited when the salinities reached 457 parts per thousand. A water sample taken above halite deposits from the estuary co tained a salinity of 355 parts per thousand. Thus, a good correlation exists between the laboratory data prepared by Usiglio and actual concentrations found above the various salts from Bocana Virrila.

The presence of black muds covering the floor of the estuary in the lower part of the saline environment is believed to furnish a valuable clue in explaining the frequent association of black shales with evaporites. Black muds from the Bocana Virrila were observed to occur below thin crusts of gypsum in the saline environment. In the transition zone between the penesaline and saline environments, black muds were observed on the floor of the estuary, and presumably are being deposited at the present time. The pH of the surface waters measured 7.8 while the bottom waters directly overlying the black muds were slightly acidic and measured 6.6. The acid conditions may be brought about by sulphate-reducing bacteria which reduce the sulphates to sulphides, thus freeing hydrogen sulphide gas which tends to acidize the water.

COMPARISON WITH OTHER LOCALITIES

Super-saline waters are not uncommon in many places throughout the world. The majority of those found along coasts, such as the Bocana Virrila, contain modified sea water. As the sodium chloride crystallizes out they become enriched in magnesium sulphate like that in the Bocana Virrila. Their actual chemical composition, however, varies rather widely, depending on local circumstances

End_Page 2473------------------------------

and the contribution of water from other sources. The published analyses most closely resembling that of the Bocana Virrila are from the Gulf of Karaboghaz on the east side of the Caspian Sea (Clarke, 1924, p. 169) where a similar inflow from the sea area occurs, although on a much larger scale than here. The same disappearance of calcium, decrease in sodium chloride, and increase in magnesium sulphate is noted. The water of the Dead Sea is quite different, containing practically no sulphate and substantial amounts of calcium. The interior lakes of the Rocky Mountains and Great Basin areas of the United States are still different, containing sulphate and carbonate as the predominant anions and calcium in quantities comparable with sodium among the anions. These lakes were not derived irectly from the evaporation of sea water.

The possibility that evaporite deposition could occur as a result of a counter-current, with less dense brine entering from the sea at the surface, concentrating by evaporation, and returning seaward along the bottom was suggested by P. B. King in 1942 to explain evaporite deposition in the Permian of Texas and New Mexico. Scruton in 1953 pointed out that counterflows of this type had been observed repeatedly in estuaries and river mouths, and might have been a dominant factor in the deposition of evaporites. The counterflow has not yet actually been observed at the Bocana de Virrila, but it may be going on.