goodall et al 2000 - surface and subsurface sedimentary structures produced by salt crusts

Surface and subsurface sedimentary structures producedby salt crusts

TIMOTHY M. GOODALL*, COLIN P. NORTH and KENNETH W. GLENNIEDepartment of Geology and Petroleum Geology, University of Aberdeen, King's College,Aberdeen AB24 3UE, Scotland, UK (E-mail: [email protected])

ABSTRACT

The growth and subsequent dissolution of salts on or within sediment may

alter sedimentary structures and textures to such an extent that it is dif®cult to

identify the depositional origin of that sediment and, as a result, the sediment

may be misinterpreted. To help to overcome such problems with investigating

ancient successions, results are presented from a comprehensive study of the

morphology and fabrics of three large areas of modern salt ¯ats in SE Arabia:

the Sabkhat Matti inland region and the At Taf coastal region, both in the

Emirate of Abu Dhabi, and the Umm as Samim region in Oman. These salt ¯ats

are affected by tidal-marine, alluvial and aeolian depositional processes and

include both clastic- and carbonate-dominated sur®cial sediments. The

ef¯orescent and precipitated salt crusts in these areas can be grouped into

two main types: thick crusts, with high relief (>10 cm) and a polygonal or

blocky morphology; or thin crusts, with low relief (<10 cm) and a polygonal or

blister-like appearance. The thin crusts may assume the surface morphology of

underlying features, such as ripples or biogenic mats. A variety of small-scale

textures were observed: pustular growths, hair-like spikes and irregular

wrinkles. Evolution of these crusts over time results in a variety of

distinctive sedimentary fabrics produced by salt-growth sediment

deformation, salt-solution sediment collapse, sediment aggradation and

compound mixtures of these processes. Salt-crust processes produce features

that may be confused with aeolian adhesion structures. An example from the

Lower Triassic Ormskirk Sandstone Formation of the Irish Sea Basin

demonstrates how this knowledge of modern environments improves the

interpretation of the rock record. A distinctive wavy-laminated facies in this

formation had previously been interpreted as the product of ¯uvial sheet¯oods

modi®ed by soft-sediment deformation and bioturbation. Close inspection of

laminations seen in core reveals many of the same sedimentary fabrics seen in

SE Arabia associated with salt crusts. This facies is the product of salt growth

on aeolian sediment and is not of ¯uvial origin.

Keywords1 Adhesion ripples, adhesion warts, evaporites, playa, sabkha.

INTRODUCTION

Salts, especially halite, commonly have only ashort-lived existence at or near the surface, yettheir growth on or within the sediment may so

greatly modify pre-existing sedimentary struc-tures as to make recognition of the primarygenesis of the sediment extremely dif®cult. How-ever, it is suggested by the present authors that,with a full knowledge of the formation of modernsalt crusts, it is relatively straightforward toidentify the original depositional processes.

For example, it has been possible to recognizethat successions with enigmatic wavy lamination,

*Present address: Production Geoscience Ltd, NorthDeeside Road, Banchory, Kincardineshire AB31 5YR,Scotland, UK (E-mail: [email protected])

Sedimentology (2000) 47, 99±118

Ó 2000 International Association of Sedimentologists 99

originally interpreted as dewatered alluvialdeposits, are actually inland salt-¯at sedimentsdeposited by aeolian processes with contempora-neous salt crust growth. This is the case for partsof the Lower Triassic Sherwood Sandstone Groupof offshore NW England (Cowan, 1993; Herries &Cowan, 1997) and parts of the Lower PermianRotliegend Group in the North Sea (Goodall,1995). In these particular examples, which arereservoirs for hydrocarbon gas, the change inunderstanding of the origin of these facies hasprofound implications for planning the exploita-tion of the gas.

A review of the salt crusts and related sedimen-tary structures developed on three extensive areasof modern salt ¯ats in SE Arabia is presented here:the Sabkhat Matti region, within the Emirate ofAbu Dhabi from the coast to the Saudi Arabianborder, the At Taf coastal region of Abu DhabiEmirate to the east of Sabkhat Matti, and the Ummas Samim region in Oman (Fig. 1). Sabkhat Mattiis the largest area of continuous inland salt ¯ats inArabia. This study demonstrates that salt crustswithin ¯uvial or aeolian sediments produce dis-tinctive sedimentary fabrics.

Examples of salt crust-related sedimentarystructures in the literature are often fragmentary,mineral-speci®c (e.g. Watson, 1983) or location-speci®c (e.g. Fryberger et al., 1983). Smoot &Castens-Seidell (1994) published an exemplaryaccount of sedimentary features produced by saltcrusts, drawing extensively on examples in Saline

Valley and Death Valley, California, USA, wherethe salt ¯ats are situated in hydrologically closedintermontane basins surrounded by alluvial fans.This paper aims to extend and complement theaccount of Smoot & Castens-Seidell (1994). Thesalt ¯ats studied in this paper experience a greatvariety of sedimentary processes (tidal-marine,alluvial and aeolian), as well as a range ofgroundwater geochemistries, and include bothclastic-dominated and carbonate-dominated sub-strates.

Use of the descriptive term salt ¯at is preferredby the present authors instead of the Arabicword `sabkha' or the Spanish word `playa',although both these terms are commonly usedin the geological literature to describe salt-encrusted plains. The terms sabkha and playahave been given geological de®nitions that are atvariance with their literal translations, and theirusage is subject to confusion (Goodall & Al-Belushi, 1997). Similarly, the term saline pan isavoided, because of con¯icting de®nitions (Shaw& Thomas, 1997). Excluded from this study arethose regions where there is signi®cant netpreservation of salt deposits (e.g. Lowenstein &Hardie, 1985), for there will be plenty of directevidence in the rock record from which tointerpret their genesis.

GEOMORPHOLOGICAL SETTINGOF THE SALT FLATS

The dominant wind system today across theArabian Peninsula is the winter `Shamal', whichblows to the south and then SW. The major dunesystems in Arabia owe their origin to a windsystem that followed much the same orientation(Glennie, 1994) before the rise in sea level in theArabian Gulf that followed the last glacial maxi-mum cut off the supply of aeolian sediment to theUnited Arab Emirates (UAE). Flooding of the AtTaf coastal strip (formerly known as the TrucialCoast) has produced the classic supratidal salt¯ats (Evans et al., 1964; Kinsman, 1969) (Fig. 2a).The coastal region inland from the supratidal salt¯ats is being progressively de¯ated, leaving a lagof coarse-grained sandsheets and slipfacelessdunes. A new system of small dunes is beingcreated and advancing inland and downwind,beginning near the coast as barchans, but chang-ing downwind into linear forms and, eventually,piling up against the older transverse megadunesin the southern part of the UAE (Glennie, 1994,1998; Bristow et al., 1996; Pugh, 1997).

Fig. 1. Location and setting of the study areas. SM,Sabkhat Matti; UaS, Umm as Samim; AD, Abu Dhabicity; UAE, United Arab Emirates.

100 T. M. Goodall et al.

Ó 2000 International Association of Sedimentologists, Sedimentology, 47, 99±118

The exact origin of the Sabkhat Matti depres-sion is unclear. The west margin has a pro-nounced linear geometry, which is parallel to thestrike of the Qatar arch, suggesting structural

control, but major faults are not evident fromseismic sections through this area (Goodall,1995). Large palaeochannels, which have alongthem archaeological evidence for wetter times(McClure, 1984), converge on Sabkhat Matti fromacross Saudi Arabia (Anton, 1984; Dabbagh et al.,1998). Interbedded conglomerates and sands of¯uvial origin, with northward-directed troughcross-bedding, are found in the northern part ofSabkhat Matti (Fig. 2a). The sands have yieldedluminescence ages of 40 and 147 ka, undoubtedlyin the Quaternary and so not related to the clearlyalluvial sediments of Miocene age that areexposed on the coast (Goodall, 1995; Bristow &Hill, 1998). The alluvial gravels are now mostlycovered by a veneer of aeolian sand, and post-glacial sea-level rise has produced a groundwaterrise leading to cementation of the alluvial gravelsby evaporites.

The very low gradients in the Sabkhat Mattiand At Taf regions allow storm-driven marine¯ooding to reach inland typically up to 2 kmbeyond the limit of normal high spring tides, and¯ooding occurs further inland after occasionalheavy winter rain. This ¯ooding gives rise toshallow ponds of brine, which persist for a fewweeks each winter until the water evaporates.Evaporation to dryness of these brines results inthick (1±5 cm), residual salt crusts, which char-acterize the surface of large areas of the supratidalsalt ¯ats.

In the Sabkhat Matti region, the supratidal salt¯ats grade southwards into inland salt ¯ats withno distinct break. The inland salt ¯ats extendsouthwards for about 130 km and, in this dis-tance, the land surface rarely rises more than80 m above sea level (Fig. 2a). There is littleinformation available on rainfall for the inlandareas of Sabkhat Matti, but measurements takenon the coast show that average annual rainfall isless than 40 mm.

The Umm as Samim salt ¯ats in Oman have avery different geomorphological setting (Fig. 3a).Fringing the Oman Mountains, which rise to over3000 m above sea level, are extensive alluvialfans. To the south of the mountains is a broadde¯ation plain of ¯at-lying Miocene and oldermarine strata, much of which is covered withde¯ation lag gravels. The salt ¯ats of Umm asSamim occur in a low-lying area between thealluvial fans to the north, the aeolian dunes ofthe Rub' Al Khali to the west and south, andthe de¯ation plain to the east. The main source ofwater to Umm as Samim is subterranean ¯owthrough the Umm Er Radhuma limestone, a

Fig. 2. (a) Geomorphological setting of Sabkhat Matti(the area left and centre of the diagram) and the At Tafcoastal regions in the United Arab Emirates (UAE),showing the main sediment types at the surface. (b)Approximate distribution of the main crust macro-morphologies in the Sabkhat Matti region, derived fromLandsat imagery and checked by ground observations.

Salt crust sedimentary structures 101


Tertiary unit that underlies much of Oman (AlLamki & Terken, 1996). Recharge to this aquifer,and hence ¯ow into Umm as Samim, is partlyfrom the Oman Mountains in the north, butmostly from the Dhofar Mountains over 500 kmto the south (Macumber et al., 1998).

In its setting, Umm as Samim bears somesimilarities to the inland drainage rift basins of

Saline and Death Valleys in California (Smoot &Castens-Seidell, 1994), but it differs markedly inscale. The Umm as Samim salt ¯ats are over anorder of magnitude larger than the salt ¯ats of theUSA rift basins (see inset in Fig. 3a), and thefringing alluvial fans are up to two orders ofmagnitude larger in downdip width.

SALT CRUST GENESIS

Salt crusts form by two main processes, ef¯ores-cence and precipitation. We believe that it isuseful to discriminate between these two, becausethe process substantively controls the morphologyof the resultant crust. Ef¯orescent crusts accumu-late by direct crystallization onto sediment grainsas a result of the evaporation of brine adhering tothose grains. The brine may be groundwaterdrawn up by capillary forces, or may result fromdew or ocean spray. Salts nucleate directly ontosubstrate, either at the sediment±air interface orwithin sediment pores, and the result is generallya powdery or puffy texture (hence, the termef¯orescence, from the Latin for blossoming or¯owering). Precipitated crusts form by theevaporation to dryness of ephemeral ponds ofrainwater or storm-driven marine brines. Saltscrystallize within the progressively concentratingbrine, usually at the water±air interface, and thenfall to the bottom of the ponded brine to form alayered crust. Both processes occur in the salinepan model of Lowenstein & Hardie (1985), withthe precipitated crusts forming beneath standingwater, and ef¯orescent crusts forming on areasexposed to the air.

The salt that ends up in the saline crusts of thestudy area may be sourced in several differentways. Groundwater is commonly already rich insolutes before reaching the salt ¯ats. Along thecoastline, groundwater chemistry approachesmarine water compositions, and sea water maybe driven directly onto the land by storms, both assurface ¯oods and as aerosol sprays. Meteoricwaters, rain or dew, may become enriched in twoways. When water collects in ephemeral ponds, itfully or partially dissolves any pre-existing crustsand so forms a concentrated brine when itevaporates. Such brines have a composition thatis highly modi®ed because of the fractionaldissolution of the older crusts (Kendall, 1984).In addition, salt may arrive as windblown dust(Wood & Sanford, 1995).

The important features that distinguish inlandsalt ¯ats, such as those of Sabkhat Matti, from

Fig. 3. (a) Geomorphological setting of Umm as Samim(UaS) in northern Oman, showing the main sedimenttypes at the surface. Inset top right: Death Valley,California, USA, at the same scale as the main diagram.(b) Approximate distribution of the main crust macro-morphologies in the Umm as Samim region, derivedfrom Landsat imagery and checked by ground obser-vations.



supratidal salt ¯ats are (Patterson & Kinsman,1981; Hardie, 1984):

(1) The inland salt ¯ats are beyond the in¯u-ence of marine ¯ooding and the associatedshoreface deposition of carbonate sediments.The groundwater is dominated by meteoric,continental chemistries.

(2) The supratidal ¯ats experience mixing ofmarine waters from coastal ¯ooding withmeteoric, continental groundwaters. This givesrise to mixed groundwater chemistries.

The supratidal salt ¯ats are more susceptible to¯ooding and have thicker white or grey salt crustsas a result of the large volume of halite (crusts upto 3±5 cm thick) precipitated at the surface fromsupersaturated standing brines. Areas that areslightly elevated near the coast (as a result of pre-existing topography) also have salt crusts, butthey tend to be covered by water only duringsevere ¯oods. Here, the crusts are dominantlyef¯orescent and thinner, and can be grey incolour where they incorporate detrital carbonatesediments during their formation. The salineminerals either accumulate as surface crusts orform within the surface sediments as displacivecements. Growth of surface crusts by ef¯ores-cence may continue long after all surface waterhas been evaporated.

Groundwater may discharge at the surface fromnatural springs and seeps, and because of con-centration near the surface as a result of evapo-transpiration. Evaporative pumping is the termgiven by HsuÈ & Siegenthaler (1969) to the processwhereby saline groundwater brines are drawn upbecause a hydraulic gradient is created by surfaceevaporation. The process has been demonstratedexperimentally by HsuÈ & Siegenthaler (1969), butit is hard to prove its viability in the ®eld.Macumber (1991) argues that evaporation willonly cause drawdown of the water table and thatregional ¯ow paths are de®ned by the hydraulichead in the aquifer system.

The inland salt ¯ats of the Sabkhat Matti regionare rarely ¯ooded, and they are subsequently

characterized by thin (<1 cm), ef¯orescent saltcrusts that accumulate from the evaporation ofsaline groundwater. The crusts are usually brownin colour, because they incorporate a high pro-portion of sand, silt and clay during their forma-tion. In some inland parts of Sabkhat Matti,groundwater issues at the surface as smallsprings, which may locally result in thicker crustsof precipitated salt.

SALT CRUST MORPHOLOGIES

A wide variety of salt crusts have been recognizedin SE Arabia. In attempting to classify crust types,we have tried to keep the number of divisions to aminimum, yet still re¯ect the variation in genesisand signi®cant differences in sedimentary fabric.Where possible, the descriptive terminology ofSmoot & Castens-Seidell (1994) has been adopted.At the macroscale, the amount of topographicrelief, measured laterally over distances of a fewtens of centimetres to metres, increases with thethickness of the salt crust. The crusts can begrouped into thick crusts with high surface relief(>10 cm) and thin crusts with low surface relief(<10 cm) (Table 1, Fig. 4). At the microscale,there are four signi®cant styles of surface texture(Table 2, Fig. 5).

The spatial distribution in the study areas ofthe main crust macromorphology types is shownin summary in Figs 2b and 3b, which are basedon interpretation of Landsat imagery calibratedby ground surveys. It must be noted that thesemaps are approximations only, and the distribu-tion is subject to seasonal change. The scale ofthese maps does not allow the ®ner detail anddistribution of speci®c morphological subtypesto be shown. Comparing the crust type distribu-tions (Figs 2b and 3b) with the maps showingdepositional substrate on which the crusts haveformed (Figs 2a and 3a) reveals that substrate haslittle effect on crust morphology, and that geo-morphological setting is the dominant control-ling factor.

Table 1. Types of salt crust observedin SE Arabia (see also Fig. 4).

1. High relief (>10 cm) (a) polygonalthick salt crusts (b) blocky: (i) saucer-shaped

(ii) slab-like

2. Low relief (<10 cm) (a) polygonalthin salt crusts (b) blister

(c) imitativeef¯orescent crusts:

(i) rippled (subaqueous)(ii) biogenic mat



High-relief, thick salt crusts

Thick salt crusts with large surface relief (>10 cm)are produced dominantly by precipitation, but aremodi®ed by ef¯orescence. In the Sabkhat Mattiregion, they occur only in areas where the watertable is high and that are susceptible to ¯ooding

after sporadic heavy rainfall. This generallyrestricts them to a zone, parallel to the coast,between the supratidal ¯ats and up to about20 km inland. In contrast, thick, high-relief crustsare very common across the Umm as Samim salt¯ats, even though this region is seldom subjectedto surface ¯ooding. These salt ¯ats receive large

Fig. 4. Macromorphology of thesalt crusts. a±c are high-relief thickcrusts: (a) polygonal (scale: spadeon left); (b) blocky, saucer-shaped;(c) blocky, slab-like. d±g are low-relief thin crusts: (d) polygonal(scale: 25-cm bar at lower right); (e)blister (scale: coin 2á5 cm at lowerright); (f) imitative after subaque-ous ripples (scale: 25-cm bar atlower left); (g) imitative after bio-genic mat.

`Popcorn' surfacesHairy, ef¯orescent surface haliteSalt crust wrinklesSmall-scale erosional structures: microyardangs

ribs (truncated cross-bedding)

Table 2. Types of micromorphologyof the salt crusts.



volumes of brine from below by the discharge intothe region of groundwaters ¯owing under artesianpressure through the underlying Umm er Rad-huma Limestone (Macumber et al., 1998).

Two types of high-relief, thick salt crusts havebeen distinguished: polygonal crusts and blockycrusts. The variations in morphology of suchcrusts primarily re¯ect the maturity of the crustand the amount of time it has been developing.The development sequence can be seen in avariety of stages by comparing the crusts devel-oped on fresh substrate after seismic survey trackshave been bulldozed across Umm as Samim.

Polygonal crusts

The initial development of high-relief crusts,composed principally of halite, is into a distinc-tive pattern of ridges that are polygonal in planform (Fig. 4a). The diameter of the polygonsusually ranges from 1 m to 4 m. The formationof these crusts begins when the salt ¯ats are¯ooded by either heavy rainfall or marine¯ooding, or where there is a high rate ofgroundwater discharge. Evaporation causes thebrines to become supersaturated with salts,mainly halite. Halite begins to crystallize onthe surface of the brine pool as rafts of laterally

linked tabular crystals or hoppers (Dellwig,1955). The crystal rafts may be blown to theedges of the pools or may become too heavy to¯oat and sink to the bottom (Handford, 1991).Those that sink form the nuclei for the growth ofcubic halite crystals. As the brine continues toevaporate, the pools become shallower, andwind-induced waves may cause the crystals onthe bottom to be worked into straight-crested,symmetrical ripples. By the time the brines havecompletely evaporated, a loose aggregate of up to3 cm of halite rafts may have accumulated onthe ¯oor of the pool. This crust is initiallyplanar, and the individual crystals are clearlyvisible but, after a few weeks, the original cubicform of the surface crystals is lost throughdissolution by desert dew and through abrasionby wind-blown sediment.

Shearman (1970) noticed that repeated phasesof ¯ooding and desiccation on salt ¯ats causes thehalite crusts to develop characteristic crystaltextures: chevron-shaped trails of ¯uid inclusionsmark crystal growth surfaces, which are disruptedby irregular dissolution cavities (Fig. 6), althoughchevron texture in halite is not unequivocallydiagnostic of growth at the surface (Chipley &Kyser, 1994). The crystals form layers, separatedby truncation surfaces that may in some places be

Fig. 5. Micromorphology of thesalt crusts. (a) `Popcorn' surface.(b) Hairy, ef¯orescent surfacehalite. (c) Salt crust wrinkles.(d) Small-scale erosional features ±microyardangs. (e) Small-scaleerosional features ± ribs (truncatedcross-bedding) (scale: hammer atlower right).



marked with a thin layer of mud. Such surfacesrepresent a period of desiccation that was fol-lowed by a ¯ood event.

Although the salt crusts are initially white,wind-blown dust soon starts to adhere to thedamp hygroscopic surfaces of the halite crystals,resulting in the crust turning darker with age.White, ef¯orescent halite commonly forms alongthe margins of some of the polygons and in thepolygonal fractures caused by the evaporation ofgroundwater brine drawn up to the surface bycapillary pressure. The ef¯orescent halite is very®ne grained and has a porous, powdery, texture.

As elucidated by others before us, the polygo-nal pattern is the result of fracturing as a result ofvolume reduction brought about by either thermalcontraction or desiccation. By analogy with poly-gon formation in periglacial settings and basaltcolumns, thermal contraction is the preferredcausative agent of some workers (e.g. Lachen-bruch, 1962; Tucker, 1981). Comparison withpolygonal cracks in clay soils, which undoubtedlyform as a result of water removal (producingcollapse of the clay mineral crystal lattices)(Abuhejleh & Znidarcic, 1995; Konrad & Ayad,1997), has stimulated others to attribute theoccurrence of this phenomenon in salt crusts toa desiccation origin (e.g. Neal et al., 1968). Tucker(1981) argued strongly against a desiccationorigin for the sediment-®lled ®ssures he observedup to 6 m deep in Triassic salts in NW England.He reasoned that it is impossible to form a trulyopen crack in salt, of the type needed for

sediment to be able to enter, purely by desicca-tion. This is because porewater within precipi-tated halite would be NaCl-saturated, and anyfurther loss of water would precipitate yet morehalite, so sealing any void. Continued precipita-tion of halite within a bed would lead to a netexpansion of the surface layer and cause the bedto buckle into tepee structures. Tucker (1981) alsoargued that, although fractures are common in thecrests of salt tepees and over-riding can occur, theboundaries between tepees are essentially closed,which would not allow deep penetration ofsediment from above as he observed in theTriassic examples. The debate is further cloudedby a recent paper (Muller, 1998) that uses adesiccation analogue model (in a starch medium)to investigate polygons clearly of thermal origin(in basalts). More fundamental work is required.

The present study produced no new data toresolve this matter. It did con®rm that, afterinitial fracturing, growth of ef¯orescent salt crys-tals or inclusion of sediment in the fracturescreates a space problem during subsequentexpansion (thermal or hydration), so inhibitinglateral movement and causing the crust to buckleupwards and rupture into irregular, salt-thrustpolygons. During repeated cycles of this process,the polygon edges are forced upwards intodramatic tepee structures or pressure ridges.

Blocky crusts

If the high-relief polygonal salt crusts are allowedto develop uninterrupted for a number of years,they eventually produce a high-relief blockysurface, features also described by others beforeus (Hunt & Washburn, 1966; StoÈcklin, 1968;Stoertz & Ericksen, 1974; Eugster & Hardie,1978). The continuing precipitation of ef¯orescentsalt along the margins of some of the polygonscauses them to form thick, saucer-shaped slabs ofsalt (Fig. 4b). Further growth of the ef¯orescentsalt can up-end and even overturn some sectionsof these polygonal slabs, so that the crust even-tually assumes a rough blocky morphology(Fig. 4c). Small stalactites of halite are commonon the undersides of the overhanging slabs.

High-relief, blocky salt crusts are common inthe central region of the Umm as Samim, but theyare absent in the Sabkhat Matti region. This isprobably because of the difference in geomorphiccontext and a consequent difference in the timebetween `disturbance' events. The high-reliefpolygonal salt crusts in Sabkhat Matti are partiallydissolved most years by either rainfall inland or

Fig. 6. Photomicrograph of salt crust from NW SabkhatMatti showing chevron-textured, ¯uid inclusions fromperiods of crystal growth and dissolution cavities fromtimes of ¯ooding of the salt ¯ats. Scale: bar at lowerright is 1 mm.



marine ¯ooding near the coast. This prevents theirfurther development into high-relief blocky saltcrusts. In contrast, in the Umm as Samim, the saltcrusts are seldom subjected to surface ¯oodingbut, instead, receive large volumes of brine frombelow. In the Umm as Samim, these high-reliefblocky salt crusts can reach thicknesses of up to1á5 m (Heathcote & King, 1998).

Smoot & Castens-Seidell (1994) recognizedanother type of high-relief salt crust in SalineValley and Death Valley, USA, up to 1 m thick,with 40±50 cm of surface relief consisting ofclosely spaced irregular pinnacles and deep pits.These highly irregular surfaces are characteristicof especially thick salt crusts and are likely to bethe result of selective dissolution by rain. Rain-water collects and enlarges small depressions inthe surface of the crust, resulting in a deeplypitted appearance (Kendall, 1992). Crusts withthis type of morphology have not been observedin either the Sabkhat Matti region or Oman.

Low-relief, thin salt crusts

In the Sabkhat Matti region, thin salt crusts with asurface relief of less than 10 cm are the mostwidespread crust type, but they are much lesscommon around Umm as Samim, where they arerestricted to the margins of the salt ¯ats. Suchcrusts are usually less than 1 cm thick and have ahigh proportion of included sediment and adher-ing aeolian dust, which makes them dark-coloured(brown or grey). Seasonal rain causes these cruststo dissolve partially, and the ensuing evaporationleads to the precipitation of small patches of whitesalts in hollows on the crust surface. After a fewmonths, the white salt is darkened again byadhering dust. The surfaces of the crusts aretypically very irregular and variable in relief.

In Sabkhat Matti, the different crust morphol-ogies are not exclusive to particular areas, buttend to have a patchy distribution. The reason forthis characteristic is not known, but it is probablylinked to variations in local groundwater dis-charge. In some areas, the different crust typespass abruptly from one to another over distancesof a few metres.

Three types of low-relief, thin salt crusts havebeen distinguished: polygonal crusts, blistercrusts and imitative ef¯orescent crusts.

Polygonal crusts

The low-relief salt crusts may exhibit polygonalridge patterns, but the polygons generally have

small diameters of 10±30 cm (Fig. 4d). The diam-eter of the polygons probably varies with thethickness of the salt crust, because the thicknessaffects the mechanical strength of the crust. Suchvariations in diameter are also seen in desiccationcracks in muds. As observed also by Smoot &Castens-Seidell (1994), the smallest diameterpolygonal salt crusts correlate with sand-rich salt¯ats. In Sabkhat Matti, the low-relief polygonalsalt crusts are characteristically limited to patchesless than 100 m2 in area and, along their margins,they grade into other, more irregular crust mor-phologies.

The polygonal ridges of the low-relief crustsform in a similar way to those of the high-reliefcrusts. The low-relief salt crusts, however, arecomposed of ef¯orescent salt that crystallizes onthe surface from saline groundwater rather thanby the precipitation of salts by the evaporation ofstanding brines. For polygons to form and retain acoherent pattern on thin crusts, the halite must beprecipitated in a uniform manner, at a relativelyconstant rate and on a ¯at surface.

Blister crusts (pustular, puffy or dome-like)

Some of the particularly thin, low-relief saltcrusts include a higher proportion of underlyingsediment and adhering aeolian dust. These crustsoften exhibit an irregular network of puffy saltblisters (Fig. 4e).

Blisters can be formed by two different pro-cesses, one purely physical, the other imitative ofpre-existing morphology. Blisters or pustules canbe produced by processes similar to those thatform the polygonal networks of pressure ridges(i.e. crust expansion as a result of halite crystal-lization and thermal expansion and contraction).For blisters to form on part of a salt crust, thehalite must be precipitated in an irregular mannerand at different rates. This causes the salt crustsurface to be characterized by a random networkof rounded pressure ridges rather than the moreorganized polygonal pattern. The close proximityof the pressure ridges to each other interruptsgrowth. This causes the polygonal network to loseits integrity and become more pustular.

Imitative ef¯orescent crusts

Low-relief, ef¯orescent salt crusts seldom form oncompletely ¯at sediment surfaces. In manyinstances, the surface is already covered by windor subaqueous ripples, and occasionally by bio-genic mats. The crystallization of ef¯orescent salt



partially preserves the pre-existing surface mor-phology by leaving a cast of its original form(Fig. 4f and g). Imitations of ripple structures arecommonly deformed into more hump-like shapeswith continued crystallization of the ef¯orescentsalt crust (also reported by Smoot & Lowenstein,1991, p. 227).

In some instances, pustular salt crusts in theSabkhat Matti region mimic the surface morphol-ogy of thin, short-lived biogenic (algal or cyano-bacterial) mats (see also Smoot & Castens-Seidell,1994). In the UAE, biogenic mats are mainlypresent on intertidal ¯ats, but they can also existin inland desert areas for short periods (a fewweeks) when interdune areas are ¯ooded byrainwater. Anaerobic activity beneath the mattends to produce bubbles of gas at the sediment±mat contact, which causes the surface of the matto deform into blisters. The mats in inland areasstart to die once the rainwater ponds begin toevaporate and the water starts to become hyper-saline. Before the biogenic mat decomposes, it canbe encrusted with either settling halite crystals oref¯orescent halite. After the new salt crust driesout and is exposed, its surface features re¯ect theoriginal morphological characteristics of thepre-existing blistered biogenic mat. The crustsmay be slightly modi®ed later by lateral expansionassociated with further halite crystallization.

Micromorphology

The salt crusts observed possess a variety ofmicromorphological (<1 cm) forms (Fig. 5) thathave the potential to be preserved in ancientsuccessions.

`Popcorn' surfaces

On the inland salt ¯ats of the Sabkhat Mattiregion, ef¯orescent halite crystals commonlyform nodular aggregates about 1±4 cm across(Fig. 5a). Eugster & Hardie (1978) aptly namedthese features `popcorn' surfaces. The origin ofthese popcorn-like growths is dif®cult to discern,and no explanation has been published.

Hairy, ef¯orescent surface halite

The ef¯orescent salt crusts of SE Arabia aretypically ®nely crystalline and have a porous,almost powdery, texture. Some also have delicate,needle-like hairs of halite covering their surface(Fig. 5b). The hairs all lean in a downwinddirection, which suggests that their formation is

related to the desiccating action of the wind. Thewind assists in the process by drawing out surface®lms of brine from the crust, so forming smallprotuberances of ef¯orescent halite. Brine iscontinually blown to the end of the hair-likeprojections, where it evaporates, precipitatinghalite and so lengthening the hairs.

Salt-crust wrinkles

Small-scale, salt-encrusted ridges or furrows(Fig. 5c), referred to as salt-crust wrinkles, areformed as a result of the modi®cation of the surfacemorphology of pre-existing surface features.

Fine, powdery halite may crystallize in the topfew millimetres (<5 mm) of the surface sediment,which has the effect of destroying the originalsubsurface structures. At the surface, however,the halite cements the sediment only lightly, andits effect on the surface morphology is subtle. Theaddition of the halite delicately wrinkles thesurface morphology of any pre-existing sedimen-tary features, such as wind ripples. This wrink-ling effect is usually at a millimetre scale.Fryberger et al. (1984, ®g. 9) recorded featuressimilar to salt-crust wrinkles in the Jafurah regionof Saudi Arabia, which they describe as `irregulartopography ¼ resulting from the cementation ofthe sands by evaporites'.

Biogenic mats may produce small-scale surfacefeatures as well as in¯uence the macroscopicscale to produce the blister-type, low-relief crustsdescribed above. Biogenic mats are composed ofdelicate ®laments, which bind the surface lamin-ations and produce a certain amount of cohesionand elasticity. Anaerobic activity beneath the matproduces small bubbles of gas at the sediment±mat contact. In ¯ooded interdune areas, theaction of wind-driven waves as the pools driedout has been seen to tear thin biogenic mats topieces. Not only did the tears themselves producemicrorelief, but the scraps of mat were crumpledand wrinkled by further agitation of the water.The micromorphology of the wrinkled mats wassubsequently preserved by the deposition of ®ne-grained ef¯orescent halite crystals. The crustmorphology can, however, be completely modi-®ed by further halite crystallization andassociated thermal expansion, so that its biogenicorigins become hard to discern.

Small-scale, erosional salt crust structures

Fryberger et al. (1984) noticed that the surfaces ofthe damp salt ¯ats in the Al Jafurah region of



Saudi Arabia commonly had irregular erosionaltopography. They believed that this was the resultof differential erosion of evaporite-cementedsands. Two of the features that they documentare also present in the inland salt ¯ats of SabkhatMatti.

Some areas of inland salt ¯ats in the SabkhatMatti region are covered by small patches ofstreamlined, erosional features a centimetre or soin length and height, which are referred to here asmicroyardangs (Fig. 5d). Fryberger et al. (1984)suggested that these features are formed by thewind shaping the evaporite-cemented sand andsilt into small, streamlined protrusions. Similarfeatures have been observed in the present studyforming on the surface of rain-dampened dunesand, where there is no in¯uence of evaporitecementation (Fig. 5d), suggesting that the saltcrust features are also the product of aeoliande¯ation differentially eroding sediment in subtlydifferent stages of cementation. Similar structureshave been described as `asymmetric adhesionwarts' by Olsen et al. (1989). Our observationssuggest that differential erosion rather than adhe-sion produces these structures.

The dominant surface feature of some dampinterdunes in SE Arabia are sets of ribs or ridgesthat are sinuous in plan form and 1±2 cm high(Fig. 5e). These are the surface expression oflightly cemented, truncated dune cross-bedding.Fryberger et al. (1984) recognized that similarfeatures in the Al Jafurah region were producedby differential resistance to wind scour of thedune cross-bedding. Examples in Sabkhat Mattishow fabric-selective cementation of the differentlamination types within the dune cross-bed sets.The ®ner grained, wind-ripple laminae are bettercemented and resist the scouring action of thewind. They form ridges or ribs, while the windremoves the more poorly cemented sand of theintervening coarser grained, grain¯ow laminae.

SEDIMENTARY FABRICS PRODUCEDBY SALT CRUSTS

Salt crusts are widespread surface features onboth supratidal and inland salt ¯ats, but are rarelypreserved intact. Even when such crusts becomeburied by sediment, the groundwaters are usuallyundersaturated, especially with respect to halite,and dissolve the evaporites within the crusts.Saturated and oversaturated groundwaters arelikely to be present only towards the centre ofbasins with a closed groundwater system.

Although not preserved themselves, salt crustsinteract during formation and burial with thesurface and subsurface sediments to produce awide range of distinctive sedimentary features.Correct identi®cation of the salt crust genesis ofobserved sedimentary structures can providesigni®cant evidence to aid environmental inter-pretation. An important point to bear in mind,however, is that there is not always a simple one-to-one correspondence between a particular crustmorphology and the resultant fabric. Many struc-tures are the product of multiple phases of saltcrust growth. The crusts may be of different typesin each phase, and so partially obliterate anydiagnostic features. It is crucial, therefore, to haveknowledge of the possible primary types of crustas, armed with this information, it is still possibleto recognize the in¯uence of individual formativeprocesses even in compound, multiphase crusts.

Hardie et al. (1978) and Kendall (1984) stressedthat the evaporite minerals in the surface crustshould not be taken as an indication of the type ofevaporite minerals that will be preserved in theunderlying sediments. So, care is required whenusing studies of modern environments to aididenti®cation of the sedimentary fabrics pro-duced by salt crusts. It is possible that the regionis in net degradation, and the observed crust issitting on structures produced by earlier deposi-tional and salt growth processes.

Salt growth sediment deformation

Where ef¯orescent salt crusts form on areas ofsandy sediment, especially areas of highly per-meable wind-blown sand, the top few millimetresof the sediment become incorporated into thecrust. This process destroys any pre-existinglamination within the sediment. In contrast,thicker salt crusts formed on less permeablesediments tend to deform the original laminaerather than destroy them, so that the laminaebecome increasingly concave upwards, re¯ectingthe continued growth of polygonal depressions atthe surface.

Salt crusts growing on surfaces already occu-pied by ripples and biogenic mats initially re¯ectthe original morphologies of the pre-existingfeatures. With further halite crystallization andassociated thermal expansion and contraction,the original ripple laminae are deformed, so thatthey form convex lenses of sediment whoseinternal laminae are oversteepened and some-times exhibit small-scale faulting. The growth ofcrusts causes horizontal laminae, such as those



beneath biogenic mats, to buckle so that theybecome convex upward.

Salt-solution sediment collapse

Solution collapse features occur when earliersurface salt crusts, or evaporite accumulationsbeneath the surface, are dissolved. This mayresult from ¯ooding or heavy rain. The rapidremoval of a surface crust leads to the deforma-tion of any sediment incorporated within it.Sediments above a dissolving subsurface crustare likely to collapse into broad hollows. Smoot &Castens-Seidell (1994) noticed that the dissolu-tion of surface crusts by ¯ood waters tended toproduce a compact clay layer with numerousmicrofaults and fractures ®lled with ¯ood sedi-ments.

Solution collapse features are more pro-nounced in areas that either do not have crustsor have thin crusts. There is a higher likelihood ofdissolution being re¯ected at the surface whereprior crusts are most incompetent. After winterrains, solution collapse features are widespreadon the surface of the salt ¯ats in Sabkhat Matti.They are present in two main forms: ®rst, as small(<1 m diameter), bowl-shaped depressions(Fig. 7); and secondly, as larger (1±4 m diameter),more irregular hollows, which have microfaultedand brecciated margins. Both are produced by thedissolution of evaporites from the near-surfacesediment. The larger solution hollows tend ini-tially to be ®lled with water. The water dissolves

the surface and subsurface evaporites, and thehollows develop steep sides. Once the water hasdrained away or evaporated, these solution hol-lows can be up to 50 cm deep and 4 m indiameter.

Solution collapse hollows are notably prevalentin the area around a natural spring in SabkhatMatti. Such a spatial association between solu-tion features and springs has also been seen inSaline Valley, USA, and it is therefore tempting tosuggest a causative link between the hollows andthe artesian upwelling. But solution hollows areuncommon in Umm as Samim, even though it isclear that artesian upwelling is the dominantwater supply process in this region.

Solution collapse features may also form bythe dissolution of salt nodules that had grownwithin sediment, salts referred to as `soil evapo-rites' by Smoot & Lowenstein (1991). The pre-cipitation of displacive, nodular salts probablyoccurs at times when the level of the capillaryfringe remains static for an extended period.Halite nodules are subsequently dissolved byeither rising groundwater or in®ltrating rainwa-ter. This leaves lenticular dissolution patches,about 10±20 cm long and 2±5 cm wide, ofdisrupted sediments (Fig. 8).

Sediment aggradation

The surfaces of salt crusts are hygroscopic andusually have surface relief such as pressure ridgesor blisters. These two attributes combine to make

Fig. 7. An extensive area of bowl-shaped depressionson the surface of an area of inland salt ¯ats, SabkhatMatti, produced by the dissolution of subsurface eva-porites after heavy rain. Scale: the hole at lower right(black) is about 50 cm across.

Fig. 8. Dissolution of salt nodules that had grown dis-placively within aeolian cross-bedded sands leavesirregular, lenticular patches of silt and sand (arrowed).Trench section, Sabkha Matti. Scale: short black bars atlower right are each 1 cm.



the salt crusts quite ef®cient sediment traps. Thehygroscopic nature of halite causes wind-blownsilt and dust to adhere to the surface of the saltcrust. When the crust dissolves, it leaves behind aresidue of poorly sorted, silty mud.

Low-relief, thin crusts with a blister morphol-ogy (Fig. 4e) trap wind-blown or water-depositedsediment in two different ways. Sediment collectson top of the crust in hollows and can also betrapped beneath the crust. This occurs becausethe crests of salt ridges and blisters are thin andbrittle, and often collapse, providing a shelteredlocation for deposition of sediment. In areaswhere the level of the water table is rising(perhaps after rain elsewhere), the surfaces ofthese sand and silt lags are then quickly encrustedwith new ef¯orescent salt. With continued cry-stallization of salt, the new patches of crusteventually develop pressure ridges and/or blis-ters, and the cycle is repeated (Fryberger et al.,1984). After burial, these deposits are preservedas sandy lenses surrounded by a muddy matrix ofhygroscopically trapped silt- and clay-size grainsleft behind as the salt crusts progressively dis-solve. The result is an irregular wavy lamination,which is illustrated clearly in the trench sectionsshown by Fryberger et al. (1983) in their ®g. 26Cand D. The same process is responsible for theformation of irregular, `¯oating' patches of sand ina poorly sorted mud, which has been described as`sand-patch fabric' by Smoot & Castens-Seidell(1994).

Compound salt crust fabrics

Few of the sedimentary features present in inlandsalt ¯at successions are the product of a singleprocess. In most cases, they are compoundfeatures, resulting from a combination of aggra-dation, surface deformation and solution col-lapse.

Such multiprocess derivation is demonstratedby one particular surface feature, common inSabkhat Matti, which consists of subcircularpatches of sand (typically 10±60 cm in diameter)separated from each other by a muddy, evaporiticmatrix (Fig. 9). These features were nicknamed`leopard spots' by Glennie (1991) because thedark sand patches separated by lighter colouredsaltier areas resemble the spotted coat of aleopard.

It is believed that these features are initiallyproduced by aggradation, when wind-blown siltand sand collect in wide depressions (either saltgrowth polygons or solution hollows) on the

surface of a former salt crust (e.g. Fig. 7). Duringtheir later development, these features are mod-i®ed by surface deformation, solution collapseand solution loading. The internal laminaewithin the sand patches are turned upwards atthe edges, which suggests that the underlyingsalt continues to dissolve after deposition of thesand (Fig. 10). The internal laminae of the sandpatches are also separated into discrete packagesby truncation surfaces, suggesting a phased orcyclic development. The truncation surfaces areinterpreted as being the result of seasonalsubsurface solution (solution loading).

Fig. 9. Subcircular patches of damp sand interspersedwith evaporites (`leopard spots' of Glennie, 1991). Thisis the characteristic surface of large areas of the inlandsalt ¯ats in Sabkha Matti.

Fig. 10. Trench cut into an area of subcircular sandpatches (Fig. 9) revealing that the sand forms lobateprojections into the underlying muds and evaporites.Scale bar at lower right with 1-cm divisions.



Salt crusts and adhesion structures

Some workers interpret all wavy lamination interrestrial sands as the product of adhesionripples. Recent work on core material from thePermian Rotliegend Group beneath the southernNorth Sea (Goodall, 1995) has shown that majorparts of the succession studied were not domin-ated by aeolian adhesion processes but wereproduced by salt crusts, a difference that hasprofound implications for the prediction in thesubsurface of porosity and permeability. It can beseen, with hindsight, that this situation may havecome about because earlier publications by Glen-nie (1970, 1972) referred repeatedly to adhesionripples and largely overlooked the possibility ofwavy laminations related to salt crusts. Theseworks in¯uenced the interpretations of subse-quent workers, and inappropriate identi®cationof adhesion ripples has continued, despite thelater appearance in the literature of examples ofsalt-related wavy laminations (McKee, 1982; Fry-berger et al., 1983, 1984). One of the earliestdescriptions of salt-related wavy laminations isthat of Nagtegaal (1973), from the Namib Desert,but this has generally gone unnoticed becausethis feature was, confusingly, termed adhesionripple lamination.

To set the record straight, it should be empha-sized that salt crusts are at least as important asadhesion ripples in producing wavy laminationin sediment. To assist in clarifying the situation,the following observations from our experiencesin SE Arabia are offered.

Adhesion ripples (or climbing adhesionripple structures)

Typical adhesion ripples producing pseudocross-lamination (Hunter, 1973; Kocurek & Fielder,1982) are rarely found on the inland salt ¯ats ofSE Arabia. For adhesion ripples to form, thesurface must be free of encrusting evaporites, sothat the sand and silt can adhere naturally to arelatively smooth surface. The surface has to bekept damp by a relatively slow and constantcapillary rise in moisture, so that the adhesionripple process is maintained. Although the sur-faces of the salt crusts are damp and encourageadhesion, the growth of evaporites disrupts boththe surface morphology and the internal structureof the delicate `pseudo'-ripple bedforms. Therecognition in the rock record of large amountsof adhesion-rippled sandstone, such as that seenin the Upper Cambrian Galesville Sandstone in

south-central Wisconsin, USA (Kocurek &Fielder, 1982), therefore attests to a fairly restric-ted set of environmental conditions andprecludes a salt ¯at setting. However, conditionsmay be locally suitable for adhesion rippledevelopment on salt ¯ats, as adhesion pseudo-cross-lamination was developed on a small areaof inland salt ¯at in Sabkhat Matti (Fig. 11).

The pseudocross-laminae themselves maysubtly record different rates of capillary rise.Hunter (1973) recognized that, in some instances,naturally formed adhesion pseudocross-laminaewere curved, convex up. He believed that thisoccurred in response to declining rates of rippleclimb as the depositional surface becomes progres-sively drier and the rate of adhesion is reduced, aninterpretation con®rmed experimentally byKocurek & Fielder (1982). In Sabkhat Matti, manyof the adhesion pseudocross-laminae are, how-ever, concave up not convex up (Fig. 11), which isinterpreted here to re¯ect a trend of increasingrather than decreasing moisture availability.

Quasi-planar adhesion lamination(or adhesion plane bed lamination)

Flat lamination produced by adhesion (termed`quasi-planar adhesion lamination' by Hunter,1980; or `adhesion laminations' by Kocurek &Fielder, 1982) is not widespread on the inland

Fig. 11. Close-up of trench wall cut into adhesionpseudocross-lamination. The set of arrowed cross-laminae are concave up, indicating that the rate ofvertical aggradation adhesion steadily increased, per-haps in response to a progressive increase in moisturecontent of the depositional surface. Above the arrowedlaminae are small dome-shaped adhesion wart struc-tures that are accreting almost vertically. Scale: shortblack bars at lower right are each 2 cm.



salt ¯ats of SE Arabia. For quasi-planar adhesionlaminae to form, the surface must be free ofencrusting evaporites, so that the sand and siltcan adhere to a relatively smooth surface. Quasi-planar adhesion laminae are likely to develop inareas where the capillary rise of moisture is tooslow for the formation of adhesion ripples (Hunt-er, 1980), or where grains are falling vertically(e.g. grainfall in the lee of a dune) onto the surface(Kocurek & Fielder, 1982), and the sand and siltsticks to the damp surface in a more uniformmanner. Although the laminae are planar, theycan be distinguished from plane bed subcriticallyclimbing translatent wind ripple strati®cation(Hunter, 1977) because quasi-planar adhesionlaminae are ®ner and more irregular.

Adhesion wart structures (verticallyclimbing adhesion ripples)

A structure resembling the adhesion wartsdescribed by Reineck (1955) is present on theinland salt ¯ats of SE Arabia as small domes oroval bumps about 1 cm in size. According toReineck (1955), these features form in areas wheresand and silt is sticking to a damp, evaporite-freesurface during strong, frequently shifting winds.The variable orientation of the wind was believedto encourage vertical rather than lateral accretionof sediment. The irregular morphology of thesurface was thought to cause the vertical sedi-ment accretion to produce in-phase, open-archedlaminae (Fig. 12). Kocurek & Fielder (1982) werenot able to replicate such features experimentally

under the conditions described by Reineck(1955), although they did produce a similarlooking feature by vertical grainfall onto a dampirregular surface.

In Sabkhat Matti, there are some cases in whichstructures resembling these àdhesion warts' canclearly be seen developing from steeply inclinedadhesion pseudocross-laminae. If the rate ofcapillary rise increases particularly rapidly, thensteeply inclined adhesion pseudocross-laminaewill reach a state where they are accreting almostvertically. When this happens, the `pseudo'cross-lamination is lost and replaced by in-phase, near-horizontal lamination characterized by regularlyspaced small domes, which were previously theadhesion ripple crestlines (Fig. 11). This transi-tion demonstrates an increase in sediment surfacewetness. The features observed in this study arevertically climbing adhesion ripples, and it isrecommended that the term àdhesion wart' bedropped in future.

Evaporitic adhesion structures

Evaporitic adhesion (the term used by Kocurek &Fielder, 1982) is clearly a widespread surfacefeature on the inland salt ¯ats of SE Arabia. Itoccurs when sand and silt sticks to the surfaces ofsalt crusts. Small changes in the hygroscopicabilities of the crust cause the sediment to stick inan uneven manner. Although evaporitic adhesionis a widespread feature at the surface, the internalstructures have a low preservation potential,because subsequent salt crust deformation andassociated dissolution act to destroy the originalmorphologies.

Kocurek & Fielder (1982) explained the originof evaporitic adhesion structures by the followingsequence of events. Sand and silt are blown ontothe crusts during the daytime, while the crusts aredry. During the night and the following morning,the humidity in desert areas typically increasesand moistens parts of the crust. Sand and silt aretrapped hygroscopically at this time and incor-porated into the damp portions of the crust. Thewind may shape parts of the lightly evaporite-cemented sediment into streamlined elongateridges (microyardangs; Fig. 5d). The successivesediment accretion, coupled with its unevenerosion, produces irregular, wavy laminae. Theseare the àdhesion ripples' of Nagtegaal (1973,®g. 2a).

It is considered by the present authors to beunlikely that this set of processes could resultin any signi®cant thickness of wavy-laminated

Fig. 12. Trench section showing adhesion wart struc-tures (arrowed, centre), which leave stacked in-phase,open-arch laminae. Above and below are incipientadhesion pseudocross-lamination structures. Scale:short black bars at top are each 2 cm.



sediment. Instead, it is suggested that wavylamination is the result of sediment accumulationin areas of thin, low-relief salt crusts that haveblister morphology (the salt ridges of Frybergeret al., 1983), as described earlier. This is also aview now being adopted by G. Kocurek.

ANCIENT SALT FLAT SUCCESSIONS

To illustrate the value of a detailed understandingof modern salt ¯at environments when studyingthe rock record, one example will be reviewedbrie¯y in which the recognition of salt crustfeatures led to a signi®cant revision of theinterpretation of the depositional environment.This revised interpretation was possible eventhough the only material available was a 10-cm-diameter borehole core. The Triassic OrmskirkSandstone Formation in the East Irish Sea Basinis the uppermost formation of the SherwoodSandstone Group (Jackson et al., 1987). Betweenone-third and one-half of the formation consistsof ¯at-lying beds with an irregular, wavy sedi-mentary fabric (Meadows & Beach, 1993). Theselithofacies are interbedded with thin aeolianunits and thicker ¯uvial channel sandstones(Cowan, 1993; Herries & Cowan, 1997). Sectionsof this wavy fabric lithofacies up to 50 m thick areseparated by variable thicknesses of aeoliansandstone with millimetre-to-decimetre-thick lay-ering that displays clear aeolian grain texturesand lamination types.

The setting of the East Irish Sea Basin duringthe Early Triassic was initially thought to havebeen dominated by ¯uvial deposition that tookplace under semi-arid climatic conditions (Colter& Barr, 1975; Bushell, 1986). The wavy-laminatedfacies was previously interpreted as the productof ¯uvial sheet¯oods modi®ed by soft-sedimentdeformation and bioturbation (Colter & Ebbern,1978). But close study of the wavy-laminatedunits in core from wells in the Morecambe GasField has revealed many features that match saltcrust fabrics observed in the modern salt ¯ats ofSE Arabia. As a result, these units are reinter-preted as salt ¯at deposits, which, in turn, hasenabled the recognition of ®eldwide, correlatable,drying-upward depositional patterns (Herries &Cowan, 1997).

One of the most distinctive and abundantsedimentary fabrics shown by the wavy-laminatedlithofacies consists of sand lenses up to about4 cm thick separated by millimetre-scale siltylaminae or drapes (Fig. 13a). Although the major-ity of the sand-rich laminae extend across theentire width of the core, they occasionally pinchout. These sand lenses are interpreted as sedimentaggradation fabrics characteristic of low-relief,thin salt crusts. Wind-blown sand has beenincorporated into the spaces beneath the broken-topped ridges of polygonal morphology crusts(Fig. 4d) or into broken domes of halite of blistercrusts (Fig. 4e). Thinner sand layers and lensesare produced by wind-blown sand accumulatingwithin surface irregularities on top of the low-relief, thin crusts, such as on the leeside of ridges.

Fig. 13. Wavy-laminated lithofacies of the Ormskirk Sandstone Formation in core from the Morecambe Field (arrowsshow the way up). (a) Typical appearance of silty laminations and sand lenses aggraded in areas of low-relief, thinsalt crusts. Scale divisions at upper left are in centimetres. (b) Large sand lens formed by sand blown into a hollowsalt blister, with subsequent lamination lapping onto it, showing that the lens is preserving the original morphology.Black square at upper left is 1 cm. (c) Large sand lens with internal dipping laminae. Scale divisions at upper left arecentimetres.



The wavy silt layers are dust that initially adheredto the salt crust surface as evaporitic adhesionstructures. The dust has been concentrated into®ne-grained envelopes or drapes that fully orpartially enclose the sand lenses following thepost-burial dissolution of the salt crusts.

The sand lenses formed by in®lling of thehollows beneath blister salt crust surfaces usuallyhave near-horizontal lamination lapping ontotheir sides (Fig. 13b), which indicates that theoriginal surface relief imparted by salt blisters orridges is being preserved. The presence of con-vex-upwards laminae in some of the sand patchessuggests that the sand was blown into the blistersand accumulated by grainfall in the shelter of theblister. Some of the internal laminae have unidi-rectional dips, which suggests that the sandentered from one side of the feature rather thanthrough the top (Fig. 13c). Dome-like blisters onthe surfaces of modern salt crusts in SE Arabiamay have openings on their tops but, morecommonly, they have openings on their sides, asa result of selective abrasion by saltating sandgrains of the upwind side of the blisters. Thewind subsequently blows sand through theseopenings into the space beneath the blister.

These salt crust sediment aggradation fabricsoften comprise successions stacked at the centi-metre-to-decimetre scale. The repetition of len-ticular sandy laminae with silty drapes may bethe re¯ection of wetter and drier seasonal varia-tions. The sand-rich laminae accumulated byaeolian deposition during drier conditions andthe silty drapes re¯ect the intermittent formationof salt crusts during wetter periods (either bysubtle water table rise or by the recrystallization

of halite after rainfall). Fryberger et al. (1983,®gs 26 and 28) have also documented the pres-ence of similar, stacked salt crust fabrics in bothancient and modern salt ¯at sediments.

Other sections of the wavy-laminated litho-facies show crinkly silty laminae that are inter-preted as imitative ef¯orescent crusts, in this casere¯ecting the micromorphology of biogenic mats(Fig. 14a). Thin biogenic mats are common inephemeral pools. After these pools dry out andthe mats decompose, the surface is usuallyencrusted with ef¯orescent halite, which pre-serves the original morphology of the mat. Theresulting salt crust morphology may be subjectedto further deformation by lateral compression ashalite continues to crystallize. The intimateassociation between biogenic mats and salt crusts,coupled with the low preservation potential ofboth the organic and evaporitic material, meansthat it is sometimes impossible to say whether thecrinkly silt laminae represent the mats or thecrusts. A criterion that is useful for distinguishingbetween the two origins is the presence of small-scale wrinkles, especially sharp-crested features(Fig. 14b). Modern salt crusts in SE Arabiapossess similar morphological features only whenhalite has crystallized on the surface of a pre-existing biogenic mat.

Crinkly silt layers occur most commonlywithin units of the Ormskirk Sandstone thatare relatively argillaceous (Fig. 14c). The asso-ciation of these laminae with muddier intervalsis signi®cant because it supports interpretationas products of both biogenic mat and salt crustgrowth. The presence of biogenic mat structuresindicate that these crinkly laminated units were

Fig. 14. Crinkly silt laminae produced by imitative ef¯orescent crusts in the wavy-laminated lithofacies of theOrmskirk Sandstone Formation, in core from the Morecambe Field. Scale divisions are in centimetres. (a) Typicalappearance, with biogenic mat wrinkles arrowed. (b) Sharp-crested silt layers (arrowed) diagnostic of biogenic matwrinkles modi®ed by salt growth. (c) Crinkly laminae within an overall more argillaceous part of the formation.



deposited on low-lying salt ¯ats that wereoccasionally ¯ooded.

SUMMARY

The morphology and fabrics of salt crusts aredescribed from three large areas of salt ¯ats in SEArabia in order to assist with interpreting ancientsuccessions that may have been in¯uenced by saltgrowth. Salt crusts form either as ef¯orescentcrusts that accumulate by evaporation of ground-water discharge or as crusts precipitated by theevaporation to dryness of ephemeral ponds ofstorm-driven marine brines or rainwater. Thecrusts can be grouped into thick crusts with highsurface relief (>10 cm) and thin crusts with lowsurface relief (<10 cm). At the microscale, thereare four signi®cant styles of surface texture. Thethick crusts develop mostly in regions subject toprolonged discharge of groundwater that arerarely ¯ooded at the surface, such as the centralparts of Umm as Samim in Oman. Thin crustscharacterize regions that are frequently, if brie¯y,inundated, such as the inland parts of SabkhatMatti in the UAE.

Although the salt crusts themselves are rarelypreserved, their formation and developmentleaves a distinct record in the sedimentary fabrics.Salt growth sediment deformation, salt solutionsediment collapse, sediment aggradation andcombinations of any or all of these producefeatures that, with care, allow recognition of anancient sedimentary succession as one thatdeveloped as a salt ¯at. This is illustrated byexamples from the Lower Triassic Ormskirk Sand-stone Formation of the East Irish Sea Basin. Wavylamination may be produced by adhesion ripples,but may equally result from sediment trapping onthin salt crusts. The frequency of occurrence ofadhesion ripples has been overstated in the past. Itmust always be remembered that the sedimentarystructures observed in the rock record may havehad a multiphase development.

ACKNOWLEDGEMENTS

The work of T.M.G. was supported ®nancially byNederlandse Aardolie Maatschappij BV. Theauthors are most grateful to the Abu DhabiCompany for Onshore Oil Operations (ADCO)for logistical support for ®eldwork in the UnitedArab Emirates. This paper has been greatlyimproved through the thoughtful and diligent

reviews by Gary Kocurek and Joseph Smoot, andthe authors thank them for their contributions.

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Manuscript received 12 January 19984 ;revision accepted 3 June 1999.



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