l•½onomic Geology Vol. 74, 1979, pp. 432-447

A Comparison of Evaporite Facies in the Late Paleozoic Amazon and the Middle Cretaceous

South Atlantic Salt Basins

PETER SZATMARI, RENATO S. CARVALHO, AND IVAN A. SIM6ES

Abstract

Large evaporite basins are characterized by a cyclic alteration of sediments deposited under an arid climate in marine, lagoonal, and subaerial environments, respectively. In the Permo-Carboniferous Amazon Basin, marine calcarenites are abundant, lagoonal sedi- ments consist mostly of coarse halite, and nodular anhydrite was formed below the dry salt flats. In this basin, dominantly marine conditions passed into lagoonal, and ulti- mately the lagoon broke up into disconnected salt lakes. In contrast, at least in the Sergipe embayment of the Cretaceous South Atlantic salt basin, marine sediments are absent, coarse halite is restricted to "tidal"-type feeding channels, and the marginal la- goons, lakes, and salt flats are characterized by a wide array of carnallite tachyhydrite facies. These differences reflect the greater isolation of the South Atlantic graben from the world ocean.

Introduction

OWING .to their great sensitivity to environmental conditions, evaporites are among the best indicators of the faci.es in which they are deposited. Unfortu- nately, a lack of continuous coring and no chemical

Fro. 1. Location map. Scrgipe is only a marginal bay of the large South Atlantic salt basin.

uniformity of the principle evaporite often prevents full use of this trait. It is th.erefore particularly fortunate that this is not true of the Brazilian Sergipe deposits (South Atlantic) and, to a lesser extent, of those of the Amazon evaporite basin (Fig. 1). These deposits have been extensively explored and abundant core material shows these evaporit. es are represented by an unusually wide range of eva, porite rocks.

Amazon Basin

The Amazon Basin is a larg, e Paleozoic basin in northern Brazil containing a thick sequence of an- hydrite and rock salt. The .evaporit.e sequence itself is of lower Permian age; it is underlain by fusulinid upper Carbonif. erous (StephanJan) shallow water limestones and sandstones and overlain by upper Permian to Lower Triassic red shales deposit.ed in a continental-lacustrine environment. Thus the evapo- rit,e sequence is a regressive one, transitional betw.een an underlying open marine sequence and an overlying continental one. The evaporit. e-depositing area was subjected to both marine and continental influences. Marine limestone wedged in and int. ercalated with the eva.porites œrom the open ocean or I•eosyncline along the western margin of the present South American continent, and fan deltas built up from the Guyanan, Brazilian, and W.est African cratons that enclosed the basin from the north, south, and .east, respectively. Eolic dune sands which derived th,eir material from these fan deltas built up along the margins (Fig. 2).

The texture of the .evaporit,es reflects their ex- posure to marine and continental influences and shows as well the gradual transition from marine to

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EVAPORITE FACIES, SOUTH ATLANTIC SALT BASINS 4.41

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Fro. 4. Polished core slabs from Amazon Basin showing progressive subaerial anhydritiza- tion of marine sediments.

A. Small anhydrite nodules appear in the sediments. B and C. Increasing anhydrite growth constricts the original sediments to a thin inter-

concretional network.

D and E. When there is no more space left for horizontal growth, the near-surface nodules grow vertically; strongly anhydritized, upheaved terrane is flooded by undisturbed, anhydrite- free marine sediments.

continental conditions. The sequence is cyclic, as-most evapori. te sequences are, and each succeeding cycle is represented by more continental facies than the •previ- ous one. Thus, in the lower cycles (Fig. 3), the dominant evaporites are the product of subaerial transformation of normal marine bioclastic carbonate

sediments in a sabkha environment. Nodular an-

hydrites resulted, with biocalcarenite forming a net- work between the nodules as well as intercalating in the form of subaerially unaffected layers (Fig. 4).

In the middle part of the sequence, halite is the dominant evaporite rock (Fig. 5). Nodular an-

442 SZATMARI, CARVALHO, AND SIMILES

A

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FIa. 6. Cores showing coarse halite formed during the early stages of halite precipitation. The large crystals in A and the irregular bedding in B are both due to repeated dissolution of the salts by undersaturated sea water and their subsequent recrystallizatior•

hydrite and limestone are restricted to the basal part of each cycle, becoming thinner and thinner in each succeeding one. The halite is typically lagoonal, coarse, and relatively pure; its rather low Br con- tent (about 70 ppm) as well as its large grain size (Fig. 6) indicate that the concentration of the brine did not advance-beyond the point of saturation with NaCI. Not until the end of the u, ppermost lagoonal cycle (VII), as the l'agoon disintegrated into isolated lakes, did the -basin brine reach a high enough con- centration to deposit, first, fine-grained halite with a high Br content and, then, sylvinite (Figs. 7 and 8 A-D).

The uppermost, or continental part of the evaporite sequence, is characterized by essentially lacustrine sediments. In the lakes occupying the .basin oxygen- rich (red-green) muds with abundant evidence of periodic desiccation were deposited alternatively with halite, characterized by intense recrystaffization as a result of frequently changing salt concentrations and the repeated desiccation of the lakes (Fig. 8 E and F). Most of this halite had been washed into the lakes from subaerially exposed salt beds of earlier cycles. Finally, evapori.te sedimentation was reduced to the recurring development of subaerial anhydrite

concretions in lacustrine and alluvial muds washed

in from the newly formed Hercynian Mountains that had cut off the Amazon Basin from the Pacific.

Sergipe

The colorful hypersaline evaporite sequence of Ser- gipe is in sharp contrast to the rather unpretentious sequence of the Amazon Basin, characterized by eva. porites deposited from relatively low-concentra- tion brines. Sergipe is not the centraI part of the evaporite basin (that is, under the waters of the South Atlantic Ocean) but is only a marginal bay of the large Red Sea-type South Atlantic evaporite basin .that formed during the Middle Cretaceous split of South America from Africa. As in the Amazon

Basin, the evaporite sequence is transitional' be- tween a continental and a marine group, but in Ser- gipe the continental sequence is below and the marine sequence above the eva.porites, reflecting the gradual invasion of .the South Atlantic rift valley by the sea. The evaporite sequence of Sergi.pe is thus transgres- sive, in contrast to the regressive sequence of the Amazon Basin.

Although the evaporite sequence in Sergipe is cyclic, its cyclicity is considerably less marked than

EVAPORITE FACIES, SOUTH ATLANTIC SALT BASINS 443

in the Amazon sequence. This reinforces the idea that .the well-defined cyclicity of the late Paleozoic sediments, whether evaporites or coal measures, may

be due to eustatic changes resulting from contem- poraneous glaciation. No known glacia.tion occurred during the Cretaceous, and this may explain why the

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Fxo. 8. A. Slabbed core of cycle VII showing bottom growth of coarse halite crystal overlain by fine-grained halite. Massive precipitation of very fine crystals dominated this depositional stage of cycle VII.

B. Slabbed core of cycle VII showing perfect lamination of very fine halite. C. Slabbed core of white sylvinite from cycle VII. Only selective leaching of the KCI along

the core margin betrays any lamination. D. Slabbed core of basin margin sylvinites. The sylvite crystals are hematite-stained (black

in photo) in contrast to the white halite. E. Slabbed core of halite immediately overlying sylvinite in cycle VII after sylvinite deposi-

tion. This material was deposited during periodic flooding of the salt layer by muddy fresh water, causing the recrystallization of salt cubes in the washed-in mud (top).

F. Slabbed core showing continuation of conditions in E except that there is little mud and much halite_ Recrystallization after flooding left only a discontinuous mud network (white) between the halite crystals.

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0 ? , ,. + .+ +--+ '•-•/ L LOWER CRET4CEOUS THE SERGIPE EVAPORITE SEOUENCE

• NiHYORITE L L[•"• I•ffE "1• HALITE WITH SYLVINITE BEDS --•.:• CALCAREOUS - HALltiC SHALE T T•'"• TACHYHYO•ffE ':' :':• SANDSTONE '•al• CARNALLITE +f•:• METAMORPHIC BASEMENT

Fro. 9. The transgressive basin-margin evaporite sequence of Sergipe. This consists essentially of a single cycle, passing from halites overlying the basal shales to increasingly soluble carnal- lites and tachyhydrites, and then back through sylvinite and halite to anhydrite and finally to normal marine sediments.

evaporite sequence of Sergipe consists of only a single large cycle, .though it is modulated by several smaller ones (Fig. 9).

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The basal part of the eva.porite sequence reflects the gradual entry of sea water into the area. It is characterized by calcareous muds and coarse-grained lagoonal halite as well as by algal stromatolites con- centrated along tidal channels (Fig. 10).

As -the original fresh-water lakes of the South Atlantic graben became replaced .by salt-water la- goons of the advancing sea, salt concentrations rose sharply. In Sergipe the lagoonal areas (depressions) became sites of carnallite deposition (Fig. 11), while low-concentration halite continued to form along the tidal channels. In both areas, the halite is markedly different from the relatively low-salinity coarse la- goonal halites of the Amazon Basin. In the channels, it shows a typical skeletal texture, resulting from rapid axial growth in the dense brines. These upward- pointing skeletal halite crystals whose shorter axes point to the sides sometimes grew to a height of several centimeters (Fig. 12A).

In the lagoon areas, new halite-saturated water was brought in only occasionally by the waves and tides. Since the lagoon water was highly concentrated in MgC12, as shown by the presence of carnallite, the halite precipitated abruptly, with abundant nucleation and consequently very small grain size. This finely crystalline (sucrose) halite is finely laminated and intercalated with the carnallite (Fig. 12 B and' C).

The carnallite itself also shows characteristic

changes in texture in response to changes in concen- tration. At first, when the Mg and Ca concentration in the brines was still relatively low, it formed slowly, dissolving and recrystallizing readily, result- ing in large nodular crystals (3-7 cm) surrounded

EFAPORITE FACIES, SOUTH ATLANTIC SALT BASINS 445

by distorted laminae of fine halite (Fig. 12 D). At least part of these crystals formed, like the anhydrite nodules in the Amazon Basin, in subaerially exposed lagoonal sediments, except that in Sergipe the la- goonal sediments consisted of halite and the inter- stitial brine was close to saturation with carnallite.

With the rise of the Mg concentration in the la- goon, the crystal size of the carnallite diminished as recrystallization becalne less common. Gradually, nucleation became so abundant that only carnallite of small crystal size (1-3 ram) was deposited, still intercalated with occasional halite laminae deposited from halite-saturated brines carried by the former tidal channels. In the channels themselves, almost no carnallite formed.

Finally. when the inflow through the channels reached its lowest level and carried only hypercon- centrated residual brines, the carnallit.e gave place to, although occasionally still intercalated with, tachy- hydrite. In Sergipe, tachyhydrite forms a mono- mineralic evaporite rock which is chemically analo-

gous with but considerably more soluble than carnal- lite. Until its discovery by R. Hite in Thailand, the South Atlantic and primarily Sergipe were the only areas where this mineral was known to form larg.e deposits (in layers up to 100 m thick). With the precipitation of tachyhydrite, the gradual increase in brine concentration came to an end. A renewed in- crease in the inflow of salt-saturated water to the

area, probably related to an improved connection of the South Atlantic with the world ocean, put an .end to the deposition of highly soluble evaporites. These inflowing brines, unsaturated in Mg, leached much of the Mg from the carnallite, dissolved tachyhydrite, and precipitated much of the KC1 liberated from the former as sylvinite. The resulting sylvinit.e beds oc- cur in several horizons (Fig. 12E), directly overly- lying the tachyhydrite as well as farther above it, thus reflecting the gradual transgression of the Mg-un- saturated brines over the terrane. In the lowermost

beds, lateral facies changes from carnallit. e to sylvinite are common The associated halites show an initially

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FEEDING CHANNELS WITH HALITE AND ALGAL STROMATOLITES

CARNALLITE DEPOSITING LAGOONS

INTERLAGOON FLATS AND EROSIONAL BOUNDARY SOUTH ATLANTIC

(ARACAdU 2.5 KM) 0 I 2 3 KM

Fro. 11. Paleogeography of the Sergipe evaporites. Feeding channels bring in halite from the salt-saturated South Atlantic graben and deposit skeletal halite as well as, during occasional dilution, atgal stromatolites. The lagoons, together wih the frequently emergent inter-lagoon flats fed by the channels, became the sites of, first, carnallite and then tachyhydrite deposition.

446 SZATMARI, CARVALHO, AND SIM{•ES

FxG. 12. A. Pennate skeletal halite crystals which grew in very shallow water of a feeding channel.

B. Slabbed core of finely laminated, finely crystalline halite, deposited in a carnallitic la- goon. The •rowth of a large carnallite crystal (lower left) deflects and ruptures the halite lamination.

C. Slabbed core showing growth of single-crystal carnallite nodules in finely laminated lagoonal halite.

D. Slabbed core containing large carnallite crystals which grew in a fine halite network. E. Slabbed core from a sylvinite bed showing the intricate, amoebalike intergrowth of halite

and sylvite crystals. F. Slabbed core showing halite crystals grown at the lagoon bottom. This material overlies

sylvinite and reflects the falling concentration of lagoon brines after sylvinite deposition and presages the end of evaporite formation as normal sea water flooded the widening South Atlantic ocean.

high, and then gradually decreasing, Br concen- tration, reflecting the increasing inflow of diluted brines (Fig. 12F). Finally, as normal conditions

were stabilized in the widening South Atlantic ocean, massive halite precipitation ceased, giving place to the deposition of shales and limestones in a normal

EVAPORITE FACIES, SOUTH ATLANTIC SALT BASINS 447

marine environment. In these beds, the only macro- scopic evaporite mineral is supratidal anhydrite.

Summary

As this brief analysis demonstrates, the texture, structure, and composition of the evaporites permit a facies interpretation at least as detailed as can be obtained by such a study of the more conventional sediments. Besides the evident advantage the study has for the .exploration of potassium and magnesium salt deposits, it can be used as a sensitive indicator of the paleogeographic and tectonic changes taking place not only in the ar.ea studied but also in the supplying sea.

Acknowledgments

The authors wish to express their thanks to the management of P.etrobr/ts Mineraq•to S. A. and its predecessors, especially to Josd A. Martins, Alvaro R. Pontes, Jorge N. de Castro, and Jo5o N. de Figueiredo, as well as to Paulo V. Belotti, Carlos Walter M. Campos and Jos• Marques Neto of Petrobr/ts, for creating the conditions conducive to producing this study. The geologists studying the deposit have benefited from the advice of G. Richter- Bernberg, Norman C. Wardlaw, D. Kinsman, and R. Hite.

P•.rROBRAS MINERACAO AVENIDA PRESIDENTE VARGAS 583

RIO DE JANIERO, BRAZIL January 23, 1979

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