Diagenetic Evolution of Aptian Evaporites in the Namibe Basin (South&#X2010;West Angola)

Sedimentology (2015) 62, 204–233 doi: 10.1111/sed.12146

Diagenetic evolution of Aptian evaporites in the Namibe Basin (south-west Angola)

LAURENT GINDRE-CHANU*, JOHN K. WARREN†, CAI PUIGDEFABREGAS‡, IAN R. SHARP*, DAVID C. P. PEACOCK*, ROGER SWART§, RAGNAR POULSEN¶, HERCINDA FERREIRA** and LOURENCO HENRIQUE** *Statoil A.S.A., Sandsliveien 90, 5254 Sandsli, Postboks 7200, 5020 Bergen, Norway (E-mail: [email protected]) †Petroleum Geoscience Program, Department of Geology, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand ‡NTNU, Høgskoleringen 5, 7034 Trondheim, Norway §Statoil A.S.A., Belas Business Park, via A1, Luanda sul, Angola ¶BlackGold Geosciences CC, PO Box 24287, Windhoek, Namibia **Sonangol A.S. Direccßao~ de Exploracßao~ (DEX), Rua Rainha Ginga n.29/31, C.P. 1316, Luanda

Associate Editor – Adrian Immenhauser

ABSTRACT The widespread and dissected nature of the Angolan gypsiferous salt residuals offers a uniquely detailed view of the lateral and vertical relations inherent to secondary evaporite textures, which typify exhumed salt masses worldwide. Such secondary textures are sometimes misinterpreted as primary evaporite textures. Thin, metre-scale and patchy, dome-like gypsum accumulations are well-exposed within strongly incised present-day river valleys along the eastern margin of the Namibe and Benguela basins (south- west Angola). These sections are time equivalent to the main basinward subsurface evaporites (Aptian Loeme Formation) which mostly consist of halite. The gypsum (here called the Bambata Formation) is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during Late Cretaceous margin uplift and continues today. This halite underwent multiple episodes of diagenetic alteration between its deposition and its final exhumation, leading to the formation of various secondary gypsum fabrics and solution-related karst and breccia textures that typify the current evaporite outcrop. Four different diagenetic gypsum fabrics are defined: thinly bedded alabastrine, nodular alabastrine, displacive selenite rosettes and fibrous satin-spar gypsum. Current arid conditions are responsible for a thin weathered crust developed at thetopoftheoutcroppinggypsum,butthefabricsinthemaincoreofthecur- rent at-surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure. Alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones. Geomorphological and petrographic analyses, mostly based on the cross-cutting relations and crystallographic patterns in the outcrop, are used to propose a sequence of formation of these different fabrics. Keywords Angola, Aptian, Bambata Formation, diagenetic gypsum, dissolution, karstification, Namibe basin, South Atlantic.

INTRODUCTION consist of as yet undissolved salts (mostly halokinetic halite, which is called the Loeme Worldwide, Mesozoic evaporite outcrops are Formation). Outcrops of the gypsum beds yield mostly represented by thin gypsum and, to a les- several secondary hydrated fabrics that are inter- ser degree, anhydrite units, which display a preted here to represent solution-related broad spectrum of diagenetic textures, often diagenetic products, rather than evaporation- associated with less soluble residuals made up derived primary deposits. Such secondary of variably textured mudstones, claystones, sili- textures are sometimes misinterpreted as pri- ciclastics and carbonates. This lithological hege- mary evaporite textures. mony is a consequence of the high solubility of The great diversity of the diagenetic gypsum most buried evaporites. It means that, prior to fabrics seen at outcrop is characterized by par- their exhumation, most buried salt masses are ticular crystallographic patterns (alabastrine, inherently unable to reach the ground surface porphyroblastic, poikilotopic, fibrous satin spar) without being partially or completely removed and show distinct cross-cutting relations at the by dissolving ground waters (Braitsch, 1964; see land surface that provide the basis for the pro- also p. 455 in Warren, 2006). After being depo- posed sequence of formation. The breadth and sited, buried, possibly subject to halokinesis, continuity of outcrop exposure of evaporite and finally exhumed, most of the original residua along the Namibe Basin margin appear impure halite mass has been removed, leaving to be unique and allow the proposal of a com- behind cumulated bedded residuals composed prehensive model that integrates many of the of gypsum and fine-grained materials entrained diagenetic textures documented in more patchy in the former dissolving halite beds (Schreiber & exposures of evaporite residua and features Schreiber, 1977; West, 1979; Lambert, 1983; elsewhere. The aims of this study were: (i) to Warren et al., 1990; Babel, 1991; Uthan-aroon unravel the textural ambiguities by illustrating et al., 1995; Warren, 1997; El Tabakh et al., and examining in detail the various at-surface 1998; Hovorka & Nava, 2000; Warren, 2006). Dis- diagenetic gypsum fabrics and solution-related solution can begin syndepositionally and at landform features; and then (ii) to discuss their shallow depths where the halite is flushed by re- mode of formation across time as framed by fluxing plumes of under-saturated phreatic the uplift of the margin of the Namibe Basin. waters (Warren, 1997; Holt & Powers, 2010). The Additionally, this paper focuses on tentatively alteration continues during burial, as basinal, distinguishing the different dissolving mesoge- compactional and thermobaric waters come into netic and telogenetic water reservoirs that have contact with the salt edges, forming, respectively overprinted now-exposed evaporites along the at the top and bottom, a heterogeneous cap-rock Namibe Basin. From the time the halite mass and ‘upside-down’ caprock carapace (Hallager first began to dissolve, throughout its burial, et al., 1990; Uthan-aroon et al., 1995; El Tabakh until its final exhumation, these evolving et al., 1998). Typically, complete halite removal hydrologies drove the incremental alteration occurs during margin uplift, where exhumed and dissolution of the halite and the associated salt beds are entirely dissolved by re-entry into crystallization of the various diagenetic gypsum a zone of active phreatic aquifers and vadose textures. hydrologies (Longman, 1980; Warren, 2006). Consequently, some exhumed gypsum beds are not the result of primary evaporation of a satu- GEOLOGICAL SETTING rated brine; rather, they are the result of the par- tial dissolution of more soluble evaporites (i.e. Aptian salt basins in south-west Africa mainly halite). These evaporites are diagenetic cumulates, residuals with textures indicative of Rifting occurred in the South Atlantic Ocean recrystallized stacks and slumps, made up of the during the Late Jurassic – Early Cretaceous, less soluble components after complete halite forming a series of rifted half-graben sub-basins removal (Babel, 1991; Hovorka & Nava, 2000; distributed along the continental conjugate Warren, 2006). Brazilian and African margins (Rabinowitz & At the eastern margin of the Namibe Basin, LaBrecque, 1979; Torsvik et al., 2009). The mar- the Bambata Formation crops out as patchy gins underwent a period of relative tectonic well-stratified gypsum beds. Its subsurface, more quiescence during the Barremian – early basinward (seaward) equivalents dominantly Aptian, characterized by larger subsident ‘sag’

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 204–233 206 L. Gindre-Chanu et al. depressions that were active prior to the study, anhydrite, gypsum and dolomitic carbo- accumulation of widespread massive evaporites nate layers exposed landward in Namibe Basin deposited during the Aptian (Karner et al., 2003; were only briefly mentioned (De Carvalho, 1961; Karner & Gamboa,^ 2007). Trending along-strike Pautot et al., 1973) and no salt has yet been near the coast, the conjugate margins are reported seaward. It is assumed, however, that segmented by transverse fault zones that define the area north of the Walvis Ridge, where normal, oblique and transformed rifted end- Aptian mounded platform carbonates have been members (Guiraud et al., 2010). Sedimentation identified, constitutes the southern flank of the history, between rift initiation and deposition of African Salt Basin (Coterill et al., 2002). the Aptian evaporites, was broadly dominated The northern flank of the salt basin is located at by lacustrine siliciclastics and carbonates; with the Ascension fracture zone, north of the Gabon increasing marine incursions up to the base of Basin (Davison, 2007). Aptian evaporites are the main evaporite body (Grosdidier et al., 1996; well-known in West Africa as the Ezanga Forma- Braccini et al., 1997; Bate, 1999). tion in Gabon (Teisserec & Villemin, 1990), the Models predicting the timing of continental Loeme Formation in Congo (Bate et al., 2001), breakup for the South Atlantic Ocean vary the Massive Salt Formation in the Kwanza greatly in the literature (Rabinowitz & LaBrec- basins (Brognon & Verrier, 1966), the Dombe que, 1979; Szatmari, 2000; Karner et al., 2003; Grande Formation in onshore Benguela (Guiraud Fort et al., 2004; Hudec & Jackson, 2004; Moulin et al., 2010) and the Bambata Formation in the et al., 2005; Rouchy & Blanc-Valleron, 2006; Namibe Basin. These evaporites are interpreted Davison, 2007; Karner & Gamboa,^ 2007; Dias, as the capping unit of a regional transgressive 2009; Torsvik et al., 2009; Lentini et al., 2010). sequence where the onset is identified at the The question of whether salt accumulation base of the Pre-Salt Barremian – lower Aptian occurred prior to, during or after the break-up is Chela, Red Cuvo, Como or Gambo sandstones highly relevant to the understanding of the (Teisserec & Villemin, 1990; Bate et al., 2001; lateral connectivity of the salt basins both across Karner & Gamboa,^ 2007). On the basis of ostra- and along the margins. In spite of efforts by cod assemblages discovered in the uppermost petroleum companies to acquire new data sets, Pre-Salt strata, and the occurrence of planktonic the Aptian palaeogeography for the South Atlan- foraminifera (Hedbergella sp.) found in the over- tic Ocean and the distribution of salt basins burden of the Deep Sea Drilling Project (DSDP) still remain obscure. Nevertheless, their lateral 363 and 364 wells in the Benguela Basin, the continuity along the West African margin is con- evaporites are reasonably dated as not younger sidered likely, therefore defining a unique and than mid-Aptian (Caron, 1978; Grosdidier et al., single giant salt basin extending from northern 1996; Braccini et al., 1997; Bate, 1999). Interpre- Gabon to southern Angola (Rouchy & Blanc- tations of the Pre-Salt to salt contact vary signifi- Valleron, 2006; Davison, 2007; Dias, 2009). The cantly in the literature. It is often argued that Aptian evaporites mostly consist of massive the offshore transition is progressive (Bate, 1999; halite, except for the southerly South Gabon Bate et al., 2001). The Pre-Salt deposits pass Basin, where almost 750 to 800 m of interca- conformably upward into patchily distributed lated bischofite, carnallite and halite beds are anhydrite (for example, Penesaline 2 formation reported (Teisserec & Villemin, 1990). The eva- in the outer Kwanza Basin; Bate et al., 2001) fol- porites in Angola are assumed to be mostly lowed by the massive halite succession. The represented by halite in the outer offshore evaporites in all West African basins are over- Kwanza Basin (Von Herzen et al., 1972; Fort lain by post-break-up upper Aptian–Albian et al., 2004; Hudec & Jackson, 2004; Rouchy & mixed carbonates and clastics, which are inter- Blanc-Valleron, 2006) that once extended into preted to be deposited in shallow to deep mar- the inner onshore Kwanza Basin (Brognon & ine environments (Pinda Group and Verrier, 1966), even if today much of the origi- counterparts; Eichenseer et al., 1999). nal halite is highly halokinetic and a significant portion of the original salt mass and continuity The Namibe Salt Basin and the onshore has been lost via a combination of salt tectonic Aptian to Albian stratigraphy deformation and allochthon dissolution (Hudec & Jackson, 2002). The southward extension of The Namibe Basin, formerly called the the Aptian salt mass is poorly known due to the Mocamedes^ or Mossamedes Basin, is a narrow lack of subsurface data. Prior to the present and elongated marginal depression located in

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 204–233 Diagenetic evolution of Aptian evaporites 207 south-west Angola and northern Namibia, rang- mouth bars (Fig. 2). The sandstones are suc- ing from 50 to 100 km in width (east–west) and ceeded upward by organic-rich lagoonal ca 570 km long. It lies along an interpreted obli- micrites (Binga member) that pass upward into que rifted segment of the African margin of the progradational mixed deposits. Mixed sedi- South Atlantic, bounded to the north by the ments are capped by algal carbonates with transform faulted Benguela Basin (Guiraud thrombolites and rhodoliths, and locally et al., 2010) and to the south by the volcanic include oolites, peloids, gastropods, echino- Walvis Ridge, which extends adjacent to the derms and bryozoans. In some areas (for exam- major north-east to south-west oriented Namibe ple, Gaio), the algal oolitic limestones are transform fault. While exposed crystalline base- overprinted by solution-related karstification ment marks its eastern flank in onshore Angola, due to exposure. This succession is unconform- its basinward western border remains imprecise, ably cut by conglomeratic and coarse-grained because the continent–ocean boundary is still sandy alluvial fans of the Middle to Upper poorly defined. The Namibe Basin also consti- Albian Giraul Formation (Dondo Formation tutes the southernmost and narrowest Aptian equivalent in the Kwanza Basin). salt basin in the South Atlantic. The exposed Bambata Formation thickness The Bambata Formation is well-exposed in ranges between 0 m and 70 m, and averages the Namibe Basin, cropping out in clean sec- 50 m over the study area. Gypsum deposits are tions adjacent to rivers where it has been sur- mainly stratiform and conformable, mostly char- prisingly preserved from complete dissolution acterized by horizontal planar to undulating (Fig. 1). It forms patchy decametric to kilometric bedding, with local highly folded and faulted long dome-like confined pockets or hills, cross- strata (Fig. 3C and D). Locally, the gypsum is cut by a complex network of small gullies, completely removed. gorges and secondary valleys, offering active sol- Current landforms of the Bambata Formation utional conduits that get larger at the confluence are mostly a mosaic of medium sized topo- with major rivers (Figs 2 and 3). Gorges and gul- graphic mounds with edges bounded by inci- lies constitute upslope inter-connected narrow sions and fractures slightly downfolded by and linear upstream incisions, whilst down- gravity to form convex-up outer lips. The outer- slope valleys exhibit a more meandering and most gypsum layer yields many dissolutional wide configuration. Formation-confined frac- microforms including cavities, polygons, voids, tures and faults are common in the Bambata karren and cracks, but few morphogenic features Formation. resulting from recrystallization processes, such A typical at-surface Bambata Formation sec- as tumulis or pressure ridges, were observed. tion is composed of three distinct lithological At and immediately beneath the current land- units (Figs 2 and 4) and consists of: (i) the basal surface, the Bambata Formation gypsum experi- Unit A made of up to 50 m thick massive gyp- ences arid and desertic conditions, with less sum (inner gypsum layer); (ii) the intermediate than 10 mm of annual run-off (http://www.moca- Unit B made up of a 1 to 10 m thick clastic medes-namibe.climatetemp.info). The current marlstone and shale unit; and (iii) the upper dynamic evolution of the outcropping Bambata Unit C, consisting of up to 10 m of thinly bed- gypsum beds is not, however, only influenced ded gypsum (outer gypsum layer). These units by rainfall intensity. Being a coastal desert, the are widespread and can be regionally mapped region also has high humidity, an effect of the out and correlated over 170 km from Namibe northward-flowing Benguela longshore current city to Lucira (Fig. 1). Whilst the gypsum of whereby humidity averages 60% onshore, with units A and C is interpreted as purely diage- a maximum of 75% during the summer. Perva- netic, the marlstones of Unit B are considered in sive fogs and mists occur during mornings and this study as mainly depositional. The upper- nights, providing significant water supply to the most part of Unit C consists of a 1 to 2 m thick coastal area. Moreover, the condensation associ- tight gypsum carapace, termed the ‘weathering ated with daily temperature contrasts is also crust’ (Macaluso & Sauro, 1996; Ferrarese et al., quite common and constitutes a non-negligible 2002). fresh water input. Fog-water and condensation The Bambata Formation is overlain by the exceeds, in volume, the average annual rainfall Albian Pinda Group that consists, at the base, rate. Besides that, aquifers are fed by severe of dominantly fluvial to marginal marine clas- flooding during the humid season, which tics, including tidal reworked distributary occurs eastward to the Namibe desert between

Fig. 1. Map of the onshore eastern margin of the Namibe Basin, highlighting the locations where the gypsum Bambata Formation has been examined. The locations are, from south to north, Bero, Piambo, Ponta Negra, Gaio and Tumbalunda.

Fig. 2. (A) Stratigraphic succession displaying the uppermost part of the ‘Pre-Salt’ deposits, the Bambata Formation and the overlying lower Albian ‘Post-Salt’ Giraul Formation of the Namibe Basin. (B) and (C) Outcrop photographs illustrate the basal Unit A (gypsum), Unit B (marls) and Unit C (gypsum – outer gypsum layer) of the exposed Bambata Formation (Lucira section, 13°55042.44″S – 12°31046.09″E). Persons for scale in (C) are ca 18 m tall.

February and April and constitutes an addi- EVIDENCE OF ANCIENT AND RECENT tional water source to the area. SOLUTION-RELATED LANDFORMS AND KARSTIFICATION

METHODOLOGY AND STUDY AREA Broad karstified surfaces and solution-induced features have been identified throughout the All observations were carried out where the exposed gypsum across the study area. These Bambata Formation is exposed between Namibe metric to kilometric-scale features are located at city and Lucira (Fig. 1). From south to north, the the top, the base or even within the stratified gyp- gypsum cropping out in Bero, Piambo, Ponta Ne- sum beds and can be classified into three types, gra, Gaio, Tumbalunda and Lucira was exam- according to their size and morphology. These ined. In the Tumbalunda area, the formation has features consist of: (i) open fractures, conduits, been described entirely to reveal the transition incisions and local truncations (for example, with underlying strata, the vertical distribution Bero; Fig. 5B and C); (ii) small-scale steep-sided of lithofacies and diagenetic gypsum fabrics, and V-structures associated with brecciated materials also the nature of the stratigraphic contact with (for example, Piambo, Fig. 5D); and (iii) recent the overlaying Albian Pinda Group deposits semi-regional spoon-shaped immature shallow (Figs 2 and 4). Focus was also placed on the karstified depressions (for example, Gaio, Fig. 6; analysis of mesoscale solution-related morpho- ‘subsidence trough’, sensu Warren, 2006). logical features and the examination of gypsum In the Bero area, local funnel-shaped fractures textures and inclusions. All descriptions of and gullies entirely or partially cross-cut the micro-fabrics relied on the classification of Holli- gypsum, forming with inter-bed surfaces an day (1970) and Ortı Cabo (1977). Some gypsum interconnected network of conduits (Fig. 5B). samples were pulverized and then analyzed by The top of the formation is also locally cut by X-ray diffraction to identify the minerals present. decimetric to metric-scale sharp-edged incisions

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Fig. 3. Outcrop photographs showing the morphology of the exposed Bambata Formation preserved at: (A) Pia- mbo (14°41046.49″S – 12°2101.93″E) (the gypsum width averages 600 m); and (B) Bero (15°8034.29″S – 12°16057.89″ E). The width of the gypsum here is ca 500 m. (C) Outcrop photograph illustrating the contact between the Bam- bata Formation and the underlying succession in Tumbalunda area (14°0037.72″S – 12°30033.29″E). (D) Horizontal and conformable layering of the exposed well-stratified gypsum Bambata Formation exposed at Piambo (14°41046.49″S – 12°2101.93″E). that are entirely filled up by Albian conglome- 2005); they are often associated with chaoti- rates of the Pinda Group (Fig. 5C). None of these cally organized breccia blankets and are inter- morphological features evolved into mature preted as mature solution-collapse structures. karst, but remained efficient conduits for water Similar features have been described at the flow, in contrast to the Castile Formation in base of the upper Carboniferous to lower Per- Texas, where hypogene rather than epigene mian Wordiekammen Formation of central caverns dominate (Stafford et al., 2008). Spitsbergen (Eliassen, 2002). Well-exposed V-structures occur within the In the Gaio valley, spectacular north–south Bambata Formation in the Piambo area oriented elongate and coalescent bowl-like (Fig. 5D). V-structures are defined as being karstified depressions occur at the top of the karstification features in the subsurface caused gypsum (Fig. 6A to C). The depressions are up by the collapsed roof of a solution-related cave. to 600 m wide and 10 m deep, mostly filled by The structures are characterized upward by the recent alluvial unlithified fine-grained sand- downward flexure of the overlying strata, stones and marls that were transported down which create a local funnel-shaped (or into the karst via small converging and branch- V-shaped) morphological bowl at the karst ing fluvial systems initiated in the vicinity of entrance (Eliassen, 2002; Eliassen & Talbot, the gypsum-basement contact, which is exposed

Fig. 4. Detailed log of the Bambata Formation exposed at Tumbalunda (14°0037.72″S – 12°3033.29″E). The Bam- bata Formation is composed of three widespread units that consist from the base to the top: the gypsum Unit A, the marlstone Unit B and the gypsum Unit C. The contact between the basal Unit A and the underlying silty and muddy deposits is progressive. The transition between the uppermost Unit C and the ﬁrst deposits of the lower Albian Giraul Formation is also conformable.

B C

Fig. 5. (A) Exposure of a Campanian–Maastrichian growth syncline south of the Caranjamba River. Short and large wave-length folds associated with significant progressive unconformities occur at the rim of these mega-scale structures. The section is ca 7 km long and 250 m thick. (B) Funnel-shaped open fracture in Bero (15°014.88″S – 12°1041.34″E). The section is ca 12 m thick. (C) Top gypsum sharp-edged incision exposed at Bero (15°014.88″S – 12°1041.34″E). Arrows illustrate the base of the incision. The incision is ca 6 m wide and 2 to 4 m deep. (D) V- shaped solution structure or V-structure at Piambo (14°4054.27″S – 12°2051.78″E). Persons for scale are ca 18 m tall. towards the east (Fig. 6A and B). Heteroge- east-dipping raft-like fault blocks breached by neous solution-related collapse gypsum breccias well-developed wide and sub-vertical fault commonly rim the edge of the depression. The zones filled by reddish conglomerates and gypsum is overlain by steeply rotated and sandstone derived from the overlying middle to highly faulted lower Albian marine deposits upper Albian Pinda Group. The faults are inti- made up by tidally influenced cross-bedded mately connected to the karstified surface lying clastics, conglomerates and highly bioturbated at the top of the carbonates and act as vertical packstone to grainstone limestones rich in ben- flow corridors cutting through the lower Albian thic foraminifera (Miliolidae sp.). Karstification beds down to the roof of the gypsum. More- affects the top of the carbonates and is charac- over, the unit tends to take the concave shape terized by pinkish patchily distributed dolomi- of the underlying depression and displays a tized zones. The strata are organized into small slight thickening towards the centre.

Fig. 6. Example of a north–south-oriented elongated bowl-like karstified depression or ‘subsiding trough’ located at the top of the gypsum Bambata Formation at Gaio (14°024.14″S – 12°3027.31″E). Karstified surfaces are up to 600 m wide and 10 m deep and are mostly filled by recent unlithified fine-grained alluvial sandstones and marls. They converge down into the depression via small cross-cutting fluvial branching drainage systems initiated in the vicinity of the gypsum-basement contact, located towards the east Gaio. (A) Satellite image of Gaio area (source: Google Earth). (B) Oblique view of one of the karstified depressions at Gaio. The section is ca 1 km in size with gypsum 30 m thick to the right (east) and Albian clastics and limestones 20 m thick to the left (west).

DESCRIPTION AND INTERPRETATION Millimetre-thick lenticular-shaped coarse- OF SECONDARY GYPSUM FABRICS grained sandy pockets occur locally enclosed within the fine-grained gypsum layers (Fig. 7D). Locally, some layers display aligned undulations Thinly bedded alabastrine gypsum in map view, forming, respectively, crests and Description lows, giving the appearance of wavy or pseudo- The most common texture consists of thinly cross-bedded intersections on bedding planes bedded banded gypsum beds characterized by (Fig. 7E). The crests of such structures are mostly horizontal and concordant stacked lamina of made up of fine-grained gypsum crystal patches, very fine to fine-grained gypsum crystals while coarser grained crystals form the lows. In (Fig. 7A). The fabric represents ca 80% of the cross-section, these structures match with local outcropping gypsum textures observed at the layered boudinage and microfolded textures and land-surface. Millimetre to centimetre-thick lam- clearly are not bed-transport features. inae typically show internal plastic deformation in the form of undulated and folded structures, Interpretation with local small boudinage preserved (Fig. 7B). Similar laminated textures are commonly Gypsum beds form a series of massively orga- described as fine-grained alabastrine secondary nized thickening-up and thinning-up patterns. gypsum textures (Ogniben, 1957; Mossop & Depending on the insoluble residue and mud Shearman, 1973; Ortı Cabo, 1977; West, 1979; content, bed colours range from whitish, to grey Babel, 1991). In the Namibe Basin, the microcrys- to brown. The bedded fine-grained gypsum is talline aggregates and the coarser granoblastic often cross-cut by a network of polygonal porous textures correspond to the type 2 hydrated alabas- fractures (Fig. 7C). trine gypsum fabrics (sensu Holliday, 1970), In thin section, the dominant texture corre- whilst the non-uniform extinction components sponds to a microcrystalline equigranular aggre- could be attributed to type 1. The few anhydrite gate made up of a xenotopic mosaic of 10 to and dolomite inclusions show evidence of etch- 100 lm size anhedral gypsum crystals (Fig. 8A). ing and dissolution-related corrosion, which tend The gypsum forms an interlocking granoblastic to demonstrate that these relics underwent leach- meshwork, with local preferred directions of ing by under-saturated waters prior to gypsifica- crystal growth elongation. Under crossed polariz- tion (see also Schenk & Richardson, 1985, and ers, the intercrystalline boundaries show irregu- Lugli, 2001, for comparison). In contrast to por- lar curved growth-sutured edges. Some subhedral phyroblastic gypsum texture, alabastrine gypsum crystals can also be viewed dispersed within the fabrics may result from rapid re-crystallization of microcrystalline matrix, either as single crystals pre-existing fine-grained secondary sulphate crys- or as crystal aggregates. The gypsum crystals are tals under unstable hydration conditions (Holli- even coarser in places, ranging from 100 to day, 1967, 1970; Mossop & Shearman, 1973; Ortı 500 lm, and display sharper edges, forming a Cabo, 1977). It is likely that most of the alabas- well-sorted idiotopic to hypidiotopic granoblastic trine matrix was originally created by the hydra- texture (Fig. 8B). Very locally, some non-uniform tion of a former fine-grained anhydrite fabric (‘net extinction gypsum components (sensu Ortı Cabo, texture’; sensu West, 1979). However, the lack of 1977), rich in aligned needle-like anhydrite anhydrite relics and the present-day tight but dis- inclusions, are mixed with the microcrystalline ordered texture of the alabastrine matrix probably matrix or transitionally fringe the coarser porphy- represent several crystal growth and hydration roblastic gypsum crystals (Fig. 8C). phases, long after the initial anhydrite to gypsum Few scattered isolated lath-shaped anhydrite conversion. inclusions with local protrusions and irregular The undulated ‘crest and low’ structures edges are observed in some gypsum crystals locally preserved in some strata can give the (Fig. 8D). The matrix also encases some isolated appearance of wavy or cross-bedding. Similar disseminated dolomite crystals; they are pre- features resulting from internal plastic deforma- served as single subhedral crystals floating tion have been described (Schreiber et al., 1982; within the matrix or as intricating aggregates Schreiber & Helman, 2005) and can easily be (Fig. 8E). The borders of dolomite aggregates dis- misinterpreted as depositional current ripples. play a ‘stair-step’ pattern with re-entrants or Although, here, it is unclear whether the bedding irregular cleavage-related fringes, whilst their is related to banded nucleation sites, internal cores show a darkened peloidal micritic texture. plastic deformation (for example, micro-folding

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Fig. 7. (A) Horizontal thinly bedded alabastrine gypsum beds with interlayered satin spar veins and dispersed stellate-selenite clusters. Notebook is 29 cm long. (B) Common soft-sediment deformation structures and slight boudinage in the alabastrine gypsum host. (C) Solution-related polygonal network of fractures cutting through ala- batrine gypsum strata. Hammer is 33 cm long. (D) Remnant well-sorted coarse-grained sandy lens embedded into thinly bedded alabastrine gypsum beds. Clastic grains are dispersed in the host rock. (E) Interstratal ripple-like surface in plan view of a gypsum bed displaying aligned lows and crests, features formed, respectively, by ﬁne- grained and medium-grained gypsum crystallographic growth patterns.

A B

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Fig. 8. Photomicrographs of some alabastrine gypsum textures (cross-polarizers). (A) Example of a fibrous microcrystalline aggregate composed of a xenotopic mosaic of poorly defined elongated and interlocking gypsum crystals (<100 lm). (B) Example of an equigranular granoblastic alabastrine texture with few dispersed anhydrite remains. Note that the grain boundaries are straight and easy to distinguish. (C) Example of a non-uniform extinction component texture made up of poorly sorted coarse gypsum grains (>100 lm) rich in curved needle-like anhydrite inclusions. (D) Example of an etched rectangular anhydrite relic within an alabastrine gypsum texture. (E) Details of ‘stairstep’-edged dolomite aggregates floating within an alabastrine matrix. Note that some show clear curved re-entrants at the periphery of the crystals (arrow) illustrating leaching and alteration.

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 204–233 Diagenetic evolution of Aptian evaporites 217 and boudinage) or the influence of both, detailed The cores of the nodules consist of a hypidio- examination of layers excludes the possibility topic mosaic of anhedral to subhedral gypsum that these structures record a primary deposi- crystals, ranging in size from 50 to 100 lm tional brine flow. across, which form a granoblastic alabastrine Coarse-grained sandy pockets preserved texture, devoid of porosity. The gypsum crystals within the secondary alabastrine gypsum also contain a few scattered but highly altered anhy- developed during dissolution. These pockets drite relics. were probably formed by either: (i) the stacking The third group consists of centimetre across of insoluble clastics, originally trapped in eva- elongated botroydal nodules made up of hori- porites prior to dissolution; or (ii) a pervasive zontally oriented coarse fibrous gypsum blades allochthonous clastics influx; instead, the mate- (Fig. 9E). These coaslescent nodules are strat- rials being gravitationally transported by weath- abound and widespread following the current ering waters moving, via anastomose cavity bedding of the Bambata Formation. A section networks, throughout the actively dissolved gyp- through the most flattened nodular aggregates sum mass in the vadose zone. shows a core made up of horizontal seams of coarse satin spar gypsum veins. Nodular alabastrine gypsum Interpretation Description Sulphate nodules (anhydrite and gypsum) can Nodules can be grouped into three distinct sets form in a large variety of diagenetic realms (see according to their shape, size, clay content and summary in Machel & Burton, 1991), including relation with the alabastrine gypsum host. depositional sabkha-like settings (Dean et al., The first group is made up of horizontally 1975; West et al., 1979; Warren & Kendall, elongated and oval-shaped gypsum nodules, 1985), emerged massive salt pans (Marchal, which are characterized by pure white to opal- 1983; Lugli, 1999; Lugli et al., 1999), deep bur- like microcrystalline core textures (Fig. 9A); they ial environments (Machel & Burton, 1991; form decimetre to metre long botryoidal nodules Machel, 1993) and shallow telogenetic domains that are composed of clusters of smaller nodules influenced by pervasive fluid flow during uplift sometimes forming enterolithic structures. These (Ortı Cabo, 1977; West et al., 1979; Warren structures vertically displace the laminated ala- et al., 1990; Ortı et al., 2012). Widespread and bastrine gypsum host and typically occur coalescent anhydrite nodules can also occur as between layered strata as interstratal nodules. diagenetic products derived from sulphate-rich The main core normally consists of fine-grained water precipitation resulting from the dissolu- alabastrine texture passing outward transitionally tion of basal halite mass in a deep burial into centimetre-thick centripetally aligned clus- environment (‘upside-down caprock’; El Tabakh ters of coarser euhedral crystals (Fig. 9B). et al., 1998). The second group consists of whitish to grey- Secondary gypsum alabastrine core nodules ish massive centimetre to metre across irregular with peripheral tangential porphyroblastic crys- but stratiform meganodules, which occur as tals have been described in ancient exhumed aligned bedded bands within alabastrine host diagenetic gypsum deposits, and constitute part strata (Fig. 9C). Nodule borders are well-defined of the so-called ‘daisy wheel’ group of gypsum and rounded, locally characterized by coarser textures (West, 1975; West et al., 1979; Warren grained gypsum rinds and greyish mud coatings. et al., 1990). Such gradational crystallographic Meganodules mostly appear as floating cylindri- patterns, composed of fine-grained alabastrine cal features, whilst some display more concen- cores and coarser grained peripheries, encom- trically nucleated flower-like structures. The pass genetically different gypsum crystalliza- cores appear ‘impure’ with a flow-like appea- tion rates and phases (Holliday, 1970), which rance because quite significant volumes of are characterized by specific equilibrium- enclosed argillaceous materials are preserved. related hydration conditions during uplift Smaller nodules can join together to form larger (Mossop & Shearman, 1973). Indeed, daisy nodular aggregates. Typically, adjacent coarsely wheel gypsum is typically interpreted to form crystalline satin spar gypsum veins conformably in response to re-hydration under the specific surround the meganodules as a halo or gradually conditions of uplift hydrology when a nodular go across them giving the appearance of merging anhydrite unit enters the lowermost parts (Fig. 9D). of the telogenetic zone (Warren, 2006). In

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Fig. 9. (A) Example of a displacive pale oval-shaped botryoidal nodule, formed by the coalescence of smaller pure microcrystalline gypsum nodules. (B) Internal crystallographic texture of an interstratal nodule displaying alabastrine gypsum texture in the main core and a peripheral radial and tangentially aligned coarse-grained gypsum halo. (C) Example of aligned cylindrical meganodules (Bero). Note that the nodules are 50 cm high. (D) Detail of a displacive gypsum meganodule (ca 70 cm high and an average of 40 cm wide; Bero). (E) Horizontal botryoidal nodules formed by the suture of bladed coarse gypsum veins (Piambo). Hammer is 33 cm long. contrast, porphyroblasts result from slow crys- grained alabastrine fabrics occurs later within tal growth at near-equilibrium hydration condi- rapid crystal growth environments, typically tions, occurring ﬁrst at depths where stable nearer the surface in the active phreatic zone aquifers are located (stagnant phreatic zone, (Mossop & Shearman, 1973; Warren et al., sensu Longman, 1980). Formation of the ﬁner 1990).

The characteristics of nodular facies are diffe- arrangements of elongated tied crystals forming rent from those derived from a sabkha setting. different sizes of stellates (ranging from 05to Deformation of layering in gypsum beds above 3 cm in length; Fig. 11A and B) or mosaics of and below the growing nodule layers and the large crystal aggregates (Fig. 11C). The latter fact that most of the larger nodules result from tend to show open porosity between crystals (in- the coalescence of smaller ones with the conju- tercrystalline polyhedral porosity; Fig. 11C and gate merging of coarse satin spar veins imply a D) but also contain the highest proportion of displacive rather than replacive origin. Further- relic anhydrites. As previously described by Og- more, because the nodules do not display any niben (1957) and Ortı Cabo (1977), some of the compactional features, they should have formed selenitic gypsum stellates display a progressive in or after the unit experienced the deep burial outboard transition with the finer micro- environment. The displacive character, their crystalline alabastrine matrix, either directly or abnormal size (meganodules), the lack of associ- via a thin peripheral rim of non-uniform extinc- ated microbial carbonates and of capping ero- tion components (Fig. 11B). sion/dissolution surfaces or inter-nodular filling In the literature, these larger gypsum crystals matrix further diminish the possibility of pre- are termed porphyroblasts and the texture is typ- served depositional Aptian nodular textures ically described as poikilotopic (Holliday, 1970; prior to burial. It is difficult to determine with Artieda, 2013). In other papers, including here, accuracy the exact origin of the precursor anhy- this coarsely crystalline texture is also described drite nodules. Any fine-scale original texture as selenitic secondary gypsum (Ogniben, 1957). was probably homogeneously altered multiple The anhydrite inclusions present as aligned to times during gypsum conversion and recrystalli- less-aligned masses of laths encased in coarsely zation by the different hydration episodes that crystalline gypsum and typically adjacent to occur during exhumation. other larger gypsum crystals that contain few to minor anhydrite relics (Fig. 11D). These inclusion-free gypsum crystals are clearly aligned Displacive selenite rosette gypsum adjacent to the open pores. The anhydrite laths Description are commonly broken into smaller pieces with Elongated radial porphyroblastic gypsum crys- re-entrants and wafer-like cleavage-related tals can form individual round clusters that are fringes (Fig. 12A). Some smoothed isolated laths centimetres across. These internally centripetal show local thickness variations that are evi- clusters occur uniformly distributed throughout denced by birefringent zonations (Fig. 12B). some layers or groups of layers as stratiform As with the alabastrine matrix, some por- aggregates of connected patterns (Fig. 10A) or as phyroblasts enclose a significant amount of scat- floating individual crystals (Fig. 11A). Individual tered euhedral to subhedral corroded dolomitic centimetre-sized gypsum crystals do not show rhombs or aggregates. Thirty per cent of these preferred growth orientations, yielding multi- grains consist of dolospar to microspar coating a directional growth patterns. Some are vertically losangic to cubic porous core (Fig. 12C). squeezed, displaying elongated motifs (Fig. 10D). Numerous clusters cross-cut the crystallized Interpretation thinly laminated alabastrine gypsum matrix and, These isolated or grouped flower-like selenite in some cases, can also be cross-cut by another crystal clusters are mostly observed in gypsum generation of clusters. The patterns are locally outcrops and commonly described as ‘fleurs de highlighted and sharply cut by a dividing line, gypse’, ‘rosettes’, ‘stellates’, ‘sand roses’ or ‘dai- which is outlined by coloured stripes (Fig. 10B). sies’ (Cooke, 1941; Mossop & Shearman, 1973; Some form half-flowers and blades, nucleated Schreiber et al., 1976; Warren et al., 1990). In from overlying or underlying horizontal beds. the Namibe Basin, most of these clusters consti- Most of them appear to have grown displacively tute displacive features because they inherently upward or downward into the alabastrine gyp- grow in all directions within the alabastrine sum host from the interstratal joints (Fig. 10C matrix. The inclusion-free coarse crystalline and D). aggregates that are aligned with the open poly- In thin section, the clusters are seen to be hedral pores imply a possible micro-scale coarse-grained gypsum crystals characterized by replacement front involved in their formation. straight intercrystalline boundaries with local Porosity was created by dissolution of its pre- curved sutures. The textures vary as radial cursor (gypsum + anhydrite) and was followed

A B

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E F

Fig. 10. (A) Banded selenite gypsum ‘daisy’ beds. Pen is 15 cm long in (A), (B), (C) and (E). (B) Preservation of a fossil water table surface (arrow) underlined by a reddish band of insoluble residues. Note that the water table surface coincides with a truncation surface highlighted by the sharp contact of one of the precipitated rosette (‘daisy’) beds. (C) Example of selenite half-flower clusters (ii) nucleated from interstratal horizontal satin spar vein walls (i) and growing upward and downward throughout the alabastrine gypsum matrix (iii). (D) Example of sub- vertical fractures cross-cutting alabastrine gypsum beds and connected to horizontal interstratal joins filled by satin spar gypsum veins. Notice that selenitic flowers grew and expanded outboard from the fractures that acted as conduits for saturated fluid percolation. Pen is 14 cm long. (E) Example of a satin spar fibrous gypsum pattern filling an interstratal horizontal vein. The vein filling is composed of two seams of elongated and dense gypsum fibres separated by a central parting line. (F) Sequence of three horizontal accreting satin spar patterns enclosed into one single interstratal vein. Hammer is 33 cm long.

A B

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Fig. 11. Photomicrographs of porphyroblastic gypsum textures and fibrous gypsum satin spar (cross-polarizers). (A) Radial selenitic rosette enclosed into a finer grained alabastrine matrix. (B) Details of radial elongated porphyroblastic gypsum crystals of a stellate, rich in anhydrite and dolomite grains. Note that, here, the transition from the porphyroblasts and the alabastrine fibrous matrix is progressive. (C) Intercrystalline polyhedral porosity indicating a meso-scale dissolution front within a poikilotopic coarse-grained gypsum texture. (D) Example of inclusion-free gypsum rim lined adjacent to polyhedral porosity in a porphyroblastic texture. Anhydrite inclusion-rich porphyroblasts are separated from the inclusion-free gypsum rim by a dividing line parallel to the intercrystalline pores. (E) Example of a fibrous gypsum satin spar vein. (F) Details of a fibrous gypsum satin spar filling with anhydrite residue along vein walls.

Fig. 12. Photomicrographs of altered anhydrite and dolomite inclusions within selenitic gypsum (cross-polarizers). (A) Example of highly segmented anhydrite lath-shaped remains within a porphyroblast gypsum crystal. The crystal edge displays dissolutional features like cleavage-related fringes and re-entrants. The anhydrite crystal grains are dispersed as they were slightly removed. (B) Example of a smoothed and elongated rectangular anhydrite lath within a gypsum porphyroblast. Note that the leaching is emphasized by the curved edge and the local birefringent zones indicating different localized thicknesses along the lath. (C) Details of a right-angle prismatic porous core of a dolomite aggregate. by precipitation of gypsum in the resulting pore formerly precipitated halite pseudomorphs. Sim- space. This gives the appearance of a gypsum ilar early to late burial polyhedral authigenic rim to some crystals, but it is actually a dissolu- dolomite spars characterized by cloudy nuclei tion front that moved through the crystal and have been observed in other evaporites, floating precipitated pure gypsum. Similar but larger as individual crystals or aggregates either in and vertical metre-sized polygonal patterns are halite (Naiman et al., 1983) or with re-crystal- also observed at outcrops, indicating that the lized anhydrite in cap rocks at the top or the bot- dissolution fronts also operated from micro-scale tom of salt masses (Taylor, 1937; Goldman, 1952; to meso-scale (Fig. 7C). Machel, 1993; El Tabakh et al., 2003). The anhydrite inclusions display a large vari- The formation of the selenitic gypsum fabrics ety of highly corroded crystals testifying to abun- resulted in a series of near-equilibrium hydration dant but slow leaching prior to gypsification phases that enabled slow coarse-grained gypsum (Schenk & Richardson, 1985). The euhedral crystal growth, via free and isolated nucleation shapes of the dolomitic rhombs prove that they sites within a porous and fractured host rock grew in situ from a currently porous sharp-edged (Mossop & Shearman, 1973). A first generation of core, which might be interpreted to be small, clusters probably occurred when the evaporites

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 204–233 Diagenetic evolution of Aptian evaporites 223 passed upward through the stagnant aquifer, as outward from the central parting line. Under the also illustrated by the progressive transition microscope, these are characterized by a homog- observed between some porphyroblasts and ala- enous optical extinction. The upper seam is bastrine crystals. It is also likely that, when the commonly thicker than the lower seam. Like the host is tightly recrystallized and impermeable, surrounding strata, the veins can be folded or the rosettes may constitute the products of self- offset by fractures; the vein filling tends to fol- hydration by the water inherent in the host rock low the folds without thickness variations. The (Mossop & Shearman, 1973). The growth of the veins are mainly singular, composed of one rosette is also intimately linked with water table unique filling of two seams, but some are com- fluctuations within a formerly porous gypsum posite, consisting of two or three horizontally host at near-surface conditions. Most of the sele- accreted satin spar patterns (for example, Tum- nite crystal nucleation occurred along enlarging balunda section). Some veins show oblique fill- sub-horizontal interstratal joints and vertical ing patterns within horizontal interstratal veins, fractures in the alabastrine gypsum host, entrain- with some forming duplexes of stacked thrust- ing the selenite crystal growth to proceed in all like thin satin spar layers. directions within the phreatic pores, as is indicated by the contrast between fully centripetal Interpretation rosettes and the half-flower blades. Because most As Warren (2006) emphasized, satin spar forma- of the rosettes and the half-flower blades cross- tion may have resulted both from active and cut layers and lamina in the alabastrine matrix, passive hydrological processes, for which there it is reasonable to infer that the last generations could be several episodes of available under-sat- of rosettes grew after the formation of the alabas- urated waters input at different stages in the trine host. uplift. Meantime, substantial volumes of hyper- concentrated waters are necessary to precipitate dense pure gypsum fibres (Machel, 1985). It is Satin spar fibrous gypsum veins also argued that satin spar formation might have Description commenced at significant depths within the Whitish to yellowish satin spar gypsum occurs mesogenetic realm, where buried overpressured in horizontal interstratal and cross-cutting verti- waters might have acted as first concentrated cal to inclined veins, forming a three-dimen- waters sourcing fibrous gypsum crystallization sional mesh of coalescent and connected fibrous (Shearman et al., 1972). Although other models patterns (Fig. 11E and F). Vein thickness (space predict saturated waters derived from the host between the vein wall surfaces) ranges from a rock contraction and dehydration during burial few millimetres to 10 cm. Vein walls are mostly (for example, marls or others; Richardson, 1920), flat, parallel and conformable with small irregu- most models involve downward migrating dis- larities. In Bero, veins remain partially open, solving waters that come from the telogenetic yielding 7 to 10 cm wide convex open cavities, domain and that volumetrically constitute suit- whilst they are entirely filled in other sections. able external water reservoirs to allow satin spar Satin spar filling patterns consist of a greyish to gypsum precipitation. However, as Schreiber & creamy central thin dividing line (‘parting line’ Walker (1992) pointed out, the temperature according to Richardson, 1920), which is ori- greatly influences the solubility of saline brines ented roughly parallel to the vein walls, and and so the saturation of migrating telogenetic two isopachous seams filled by dense and paral- waters may change during burial. lel gypsum fibres, roughly perpendicular to the The geometry and mode of vein filling imply walls (Fig. 11E). Some veins enclose fragments particular mechanisms inherent to the sedimen- of host rock that float in the fibrous gypsum. In tary basin history between deposition and thin section, growth aligned, fibrous elongate exhumation. Overpressure-related hydraulic frac- and straight gypsum crystals infill individual turing (Shearman et al., 1972), gel contractions veins. Most satin spar samples lack significant (Von Gaertner, 1932), mechanical forces induced amounts of intercrystalline porosity, although by in situ gypsum crystallization (Bundy, 1956), some veins do show geopetal accumulations of collapse induced by dissolution of underlying anhydrite residues on the lower sides of the salt (Gustavson et al., 1994), and vertically ori- fractures (Fig. 11F). ented effective tensile forces (pore fluid pressure Within some of the horizontal veins, fibres exceeds overburden) related to external tectonic locally show slight convex curvature or bend regime (Machel, 1985; Cosgrove, 2001) have all

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 204–233 224 L. Gindre-Chanu et al. been cited as being major driving mechanisms spar precipitated during hydrofracturing (Gus- accounting for vein formation. tavson et al., 1994 versus Shearman et al., 1972). Because the horizontal vein filling consists of single to composite patterns, it is assumed that Solution breccia the dilation filling of the studied veins must have occurred as incremental vertically oriented Description stress-induced sequences (Ramsay, 1980). Fur- Breccia blankets are abundant throughout all thermore, the vertical effective tensile stress is at-surface sections in the Namibe Basin and all locally accompanied by a lateral shear com- display a broad range of clast sizes ranging from ponent along the veins, as indicated by fibre fine-grained, pebbles, cobbles to metre-sized curvatures and satin spar duplexes in the Tum- fragments at different stratigraphic levels into balunda area. The homogenous optical extinc- the Bambata Formation (Fig. 13A and B). tion characteristic of some curved fibres also Although their textures vary from homoge- implies a strain during crystal growth. The ques- neously crystallized to thinly laminated and tion of whether a shear component occurred lithified fabrics, the breccias are monomictic simultaneously with, or after, fibre growth is and made of gypsum. Clasts typically form a crucial because it will help to better constrain mosaic of whitish to pinkish matrix-supported the timing of satin spar vein formation with closely fitting to loosely fitting fragments that regard to the deformation of the gypsum in the appear as chaotically distributed bodies, without basin margin in its passage to exhumation. predictable lateral or vertical grading. The Machel (1985) argued that, even if the vertical matrix is composed of finely crushed and crum- effective tensile stress responsible for the int- bled mudstone, silt, shale and residual gypsum, erstratal vein dilation remains speculative, the and also contains a few authigenic quartz grains crystal bending post-dates the crystal growth, and undetermined insoluble material. Although and then the shear component is interpreted to most of the breccia clasts show irregular smooth result from a subsequent upward buckling. The rounded edges, sub-angular to angular rectangu- models of Tanner (1989, 1992) are favoured lar clasts are not uncommon. None shows evi- here, which depict the formation of intra- dence of specific coatings or calcitized rims. veining duplexes during flexural-slip folding, Internal fabrics in the breccia clasts do not differ implying a shear component along veins syn- from fabrics in the unfragmented portions of the chronously acting as the interstratal veins open host lithologies. Clasts constructed of gypsum and the gypsum fibres grow. The exact causes breccias are themselves common within the and the relative timing of vein formation and breccia proper, forming poorly sorted composite buckling could nevertheless be determined indi- bodies. Breccia accumulations typically occur vidually for any area by mapping the regional above solution-related karstified features, form- distribution of the veins. Extension oriented ing discordant horizontally bedded clast blan- orthogonal to bedding and local slumping/shear kets above intrastratal karstified surfaces (for are considered to be the dominant controls on example, Gaio), but also form chaotically imbri- spar vein orientation and development, and both cated trans-stratal clast bodies along fractures or are related to pervasive halite volume loss and on the steep limbs of the V-structures (for exam- are most intense once the salt mass passed into ple, Bero, Piambo and Tumbalunda). Although the telogenetic zone. stratabound breccias are not as equally pre- The veins with geopetalled cumulates were served as cross-cutting breccias, they seem to be probably created via fractional dissolution of connected genetically. Stratabound fragment precursor sulphates, leaving behind a residuum blankets grade transitionally downward into of less soluble materials (anhydrite). To form massive breccia accumulations preserved in this texture requires dissolution and fracture karstified voids and cavities. opening prior to cementation. It supports the notion of some near-surface satin spar in the Interpretation Namibe Basin being the result of a mostly pas- At-surface gypsum breccias constitute the ulti- sive crystallization process, created during mate solution-related products of the former extensional collapse as adjacent halite and per- record of depositional evaporites prior to com- haps anhydrite dissolved. This may define a tex- plete dissolution (Middleton, 1961; Stanton, ture that allows passive satin spar formation to 1966; Beales & Oldershaw, 1969). As the breccia be distinguished from subsurface gypsum satin blankets pass from interstratal karstified surfaces

Fig. 13. (A) Examples of solution collapse breccia blankets at Piambo (14°4046.49″S – 12°201.93″E). Note that the breccias show a broad range of clast sizes ranging from ﬁne-grained material to pebbles and cobbles. Some are composite breccia made up of matrix-supported fabrics with enclosed angular to sub-angular gypsum clasts, indicating a solution-induced re-brecciation mechanism. Pen is 15 cm long. (B) Example of a solution collapse breccia with poorly sorted metre-sized fragments at Piambo (14°4046.49″S – 12°201.93″E). Person for scale is ca 18 m tall.

into V-shaped structures in which they fill the tion typifies a karstified surface formed by the voids, evidence indicates that they were con- overprint of two separate gravitational-fluid dis- comitantly formed during the time that dissolu- solution mechanisms that operated at two diffe- tion was active. Breccias and the matrix were rent times: produced by hydrologically focused dissolution 1 The first mechanism occurred during early of the well-stratified secondary gypsum host. burial, where weathering waters responsible for The V-structures and associated trans-stratal the karst development in the top carbonates breccias formed when the horizontal bedding during the lower Albian sank pervasively was no longer supported, creating voids and downward through the strata, via the fracture cavities with subsequent collapse. The ‘rock- network, until they reached the halite and flour’ matrix is the accumulation of the finer initiated dissolution. Similar regional solution- grained residuals from the solution-related brec- induced karstified surfaces (including pipes, do- ciation, filling the interstitial voids and cavities lines, gentle folds or collapse breccia pockets) between the fragments. A re-brecciation mecha- occur at the top of Cenozoic and Mesozoic eva- nism is common when the degree of downward porites in Spain (Gutierrez et al., 2001) and in and sideward dissolution was relatively low. In the Carboniferous sulphates of central Spitsber- this case, it also implies that the transportation of gen (Eliassen & Talbot, 2005). fragments was local and took place over distances 2 The second mechanism was initiated later, measured in metres to tens of metres. during subaerial margin exhumation, when the Detailed examination of breccias reveals sedimentary succession had been significantly whether former evaporite units were closely eroded, exposing the gypsum to surface weather- mixed with less soluble lithological units prior ing. The concave karstified surfaces that had to dissolution (Friedman, 1997), and can also formed during the lower Albian were conse- help account for whether the breccias are quently exposed and acted as localized deposi- formed as a result of a unique or several dissolutional depressions, capturing sediments from the tion episodes (Warren, 2006). The fact that some recent drainage systems. Similar modern river of the breccias, often those located at the bottom capture along the feather-edge of exhumed eva- of the V-structures, are composed of gypsum porite masses is seen in the Ebro Basin of Spain, fragments indicates that more than one phase of the Nam Theun valley in Laos and the Little brecciation occurred during exhumation. Red River valley along the outcropping margin However, in Gaio, the solution-related land- of the Hutchison Salt in Kansas (Warren, 2006). form preserved at the top of the Bambata Forma-

DISCUSSION bedded halite without requiring thick edged sulphate precipitation; this is the case for most of the Upper Cretaceous marine-fed salt basins Buried salt removal or dissolution-related dominated by sulphate-depleted sea water deformational mega-structures (Lowenstein et al., 2001, 2003). This means that, Remote sensing, fieldwork analysis and detailed in such depositional conditions, the significance field mapping of overburden strata indicate broad of gypsum (or anhydrite) at the onset of the kilometric-long structures that affect distinct evaporite succession may be misinterpreted, stratigraphic levels. For instance, along the north especially along the edges, where flushing by and south sides of the Inamagando River mouth meteoric and basinal waters is most likely to (Fig. 1), Albian deposits are exposed in an east– occur (Kendall, 1988). The analysis of the gyp- west trending anticline where moderate-angle to sum fabrics of the Bambata Formation tends to low-angle listric normal faults, roll-overs and provide criteria for recognizing the marginal intra-formational unconformities are observed on telogenetic gypsification of a prevalent primary the flanks of the structure. Moreover, north and halite stockmass rich in sulphate impurities. south of the Caranjamba River, Campanian–Maa- This mechanism occurred throughout multiple strichian shallow marine sandstones are exposed stages of dissolution and recrystallization pro- in spectacular growth synclines, in which short cesses during uplift, rather than a pure replace- and large wave-length folds, associated with sig- ment of a primary depositional gypsum wedge. nificant progressive unconformities, occur at the No collected outcrop sample is composed of rim of these structures (Fig. 5A). Similar geome- pure anhydrite and all calcium sulphate samples tries can be respectively compared to salt-core with anhydrite present are dominated by gyp- domes or pillar and peripheral depressions that sum. The apparent at-surface laminations are develop at the flanks of growing salt diapirs (Hu- here believed to occur from dissolution fronts by dec & Jackson, 2011). Counterparts have been incremental groundwater flushing rather than by reported in Spain, where dissolution-induced depositional stratification. Similar patterns are subsidence structures recorded in the Mesozoic reported from salt domes and from the upper and Cenozoic sediments produce semi-regional part of some shallow basinal evaporite filling synforms, antiforms and collapse features where diagenetic anhydrite cap rocks form (Gutierrez et al., 2001). widespread and thick solution-related horizontal parallel bands (Goldman, 1933; Lambert, 1983; Hallager et al., 1990; Uthan-aroon et al., 1995; The origin of the at-surface Bambata gypsum Hovorka & Nava, 2000). Under the microscope, Formation in the Namibe Basin across the various meso-scale features described All ancient marine giant salt basins required in the preceding sections, a range of related-sul- hydrographic isolation and water saturation to phate textures indicate a set of near-surface dia- initiate evaporite precipitation. Such deposi- genetic mechanisms and processes that created tional conditions are usually attributed to rapid ever increasing proportions of purer gypsum regressive pulse, palaeogeographical confine- and decreasing amounts of residual anhydrite. ment and high evaporation rates, respectively, Anhydrite inclusions are best preserved in the due to global sea-level drop, plate-order tectonic porphyroblastic gypsum and, to a lesser extent, evolution and an arid climate (Hardie, 1991; into the alabastrine textures, but all reveal alter- Handford & Loucks, 1993; Warren, 2010). Pri- ation by abundant under-saturated fluids prior mary sulphate beds are typically expected to to gypsification (Schenk & Richardson, 1985). occur as a marginal wedge, preceding mega- The euhedral dolomite grains disseminated into halite deposits that subsequently migrate in the secondary gypsum textures suggest an in situ centre of the basin (Hsu et al., 1973; Tucker, precipitation from magnesium-rich derived dis- 1991; Warren, 2006). Among all Mesozoic salt solving evaporites (Naiman et al., 1983; El basins, the Mediterranean Sea best illustrates Tabakh et al., 2003). this depositional style during the ‘Messinian cri- The relative timing of porphyroblastic versus sis’ (Decima & Wezel, 1973; Rouchy, 1982; Dec- alabastrine gypsum is difficult to determine. A ima et al., 1988; Garcıa-Veigas et al., 1995; review of the literature suggests that porphyro- Clauzon et al., 1996; Butler et al., 1999; Rouchy blastic gypsum tends to precede alabastrine gyp- & Caruso, 2006; Lugli et al., 2010). However, sum in the uplift realm (when associated with sometimes, the evaporite filling initiates with anhydrite telogenesis) and that satin spar forms

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 204–233 Diagenetic evolution of Aptian evaporites 227 in fractures created by the dissolution and vol- between the secondary gypsum crystal size and ume loss associated with meteoric flushing and the equilibrium conditions and flow regime in uplift of a former anhydrite bed (Holliday, 1970; which hydration takes place (Holliday, 1970; Gustavson et al., 1994; Warren et al., 1990). The Mossop & Shearman, 1973; Longman, 1980), a Angolan examples also tend to suggest that sequence of formation of these different secon- alabaster and selenitic textures are typically dary gypsum fabrics can be evaluated with intermixed at outcrop and that differences in respect to margin evolution from the moment gypsum style may be more related to the degree the halite was deposited until today. A dynamic of gypsum supersaturation during anhydrite dis- diagenetic model is proposed to explain the for- solution. In this scenario, porphyroblastic gyp- mation of the gypsum of the Bambata Formation sum is favoured by sluggish water crossflows, cropping out at the eastern margin of the Nam- slower crystal growth rates and less saturated ibe Basin (Fig. 14). Additionally, it highlights pore fluids (with respect to gypsum), while gra- the influence of distinct dissolving water reser- noblastic gypsum is favoured by supersatura- voirs on the original halite, resulting in its com- tion, multiple crystallisation nuclei, faster plete cannibalization and the formation in time crystal growth rates and Ostwald crystal ripen- of distinct solution-related diagenetic sulphate ing. fabrics and features: The diversity of the at-surface secondary 1 It is reasonable to infer that the onset of gypsum fabrics in the Namibe Basin is the result effective dissolution occurred in the near of dissolution and re-crystallization as various surface, in the very shallow burial environment, invading under-saturated waters came into soon after the deposition of the Albian carbo- contact with soluble halite units. Dissolution nates (Pinda Group), when invading under-satu- episodes were probably first active syndeposi- rated waters came into contact with the halite tionally, but continued during burial, subsequent beds. The gravimetric flow was driven by the uplift and during the exhumation of the margin exposure of the Pinda carbonate platform. (Warren, 1997, 2006). Ongoing halite dissolution Weathering and salt wash-out was efficient implies that accumulation of residual anhydrite enough to affect the top of the halite and and insoluble material occurred in the deep bur- develop solution-related subsidence troughs dur- ial environment wherever and whenever waters ing the lower Albian. Salt deformation and/or under-saturated with respect to halite came into dissolution continued to be active later in the contact with the salt mass. These anhydrite burial as deformational growth structures in the accumulations were subsequently hydrated and overburden developed (for example, Campa- converted into gypsum during uplift, at the same nian–Maastrichian growth synclines of the time as any remaining halite was flushed out. Caranjamba River, Fig. 5A). The growth of granoblastic and porphyroblas- 2 The dissolution favoured the alteration of the tic textures from an anhydrite precursor, which halite edges as they came into contact with in turn was formed by fractionated removal of compactional and thermobaric pore-waters (War- halite, means that all of the textures seen at the ren, 1997). It resulted in a subsequent accretion micro-scale and meso-scale in gypsum outcrops of diagenetic massive fine-grained anhydrite are indicative of telogenetic processes. There is matrix and un-compacted nodular anhydrite no possibility of these uplifted-related re-precip- beds, preserved at the top and base of the halite itation and re-crystallisation sulphate textures (Uthan-aroon et al., 1995; El Tabakh et al., 1998; preserving primary (Cretaceous) depositional rel- Hovorka & Nava, 2000). Fission track analysis ics. Every mineral present (the gypsum, the carried out on the Pre-Salt and overburden sedi- anhydrite, the dolomite and the clay) are tho- mentary samples gives an estimate of ca 15to roughly mesogenetic or telogenetic and mostly 2 km for the maximum burial depth, correspond- related to the dissolution front or the cavity ing to less than 100°C in burial temperature. infill. 3 During uplift, partially dissolved halite and the anhydrite beds at the basin edges came into contact with stagnant phreatic aquifers of the Dynamic diagenetic model and chronology of lower telogenetic zone (sensu Longman, 1980). the various solution-related secondary This contact drove the complete removal of the gypsum of the Bambata Formation halite and the final accumulation of fine-grained Based on the petrographic and stratigraphic and nodular stratiform anhydrite beds. The con- observations that account for the narrow relation version of the anhydrite into gypsum may have

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 204–233 228 .Gnr-hn tal. et Gindre-Chanu L. © 04TeAtos Sedimentology Authors. The 2014 © 04ItrainlAscaino Sedimentologists, of Association International 2014 Sedimentology , 62

204–233 , Fig. 14. Dynamic diagenetic model and sequence of formation of the various solution-related secondary gypsum fabrics and landforms in the Aptian Bam- bata Formation exposed along the eastern margin of the Namibe Basin. Diagenetic evolution of Aptian evaporites 229 occurred during or after complete halite dissolu- enable pervasive water infiltration and perching tion. The current horizontal stratified pattern of over relatively long periods of time (years or the exposed gypsum beds typifies incremental tens of years). Relic water table surfaces, spar- dissolution, where the uppermost part of the sely distributed throughout the gypsum, show uplifted evaporites may have come into contact evidence that phreatic aquifers moved up and with horizontal flow lines of the aquifer front. down within the gypsum mass, after the alabas- That is, much of the anhydrite rehydration came trine gypsum precipitation. Stellate selenite from water flows in suprasalt rather than subsalt defines at least two generations of clusters and aquifers. As the uplifted dissolving halite and their distribution within the Bambata Formation gypsum mass started to buckle up and deform, reflects these vertical fluctuations in the aquifer. interstratal vein dilation and fracturing occurred, forming internal drainage conduits for sulphate-rich fluids (Tanner, 1989, 1992). Slug- CONCLUSIONS gish hydration, facilitated by the opened sub- horizontal veins, trans-stratal fractures and the The Aptian gypsum-rich Bambata Formation inherent pore network of the host, enabled the exposed along the current coast of the Namibe formation of ‘daisy wheels’ nodular fabric, a first Basin in south-west Angola provides an out- generation of rosettes, and fibrous gypsum crys- standing example of vanished evaporites. No tal growth into the interstratal veins (satin spar collected outcrop sample is composed of pure fabric; Shearman et al., 1972; West, 1979; War- anhydrite and all calcium sulphate samples with ren et al., 1990). Because the layering of the anhydrite present are dominated by gypsum. Bambata Formation may develop weakly porous The gypsum beds are mainly stratiform and interstratal joints, the overpressured saturated conformable, mostly characterized by horizontal water moves along the bedding boundary and planar to undulating bedding with local folds initiates the upward growth of nodules and their and faults. These beds include a great diversity subsequent sutures, reminiscent of bottom- of secondary fabrics, such as thinly laminated nucleated primary features (meganodules). alabastrine gypsum, selenitic rosettes, different 4 Deformation and flushing persisted as the displacive gypsum nodules, solution-related newly precipitated secondary gypsum beds monomictic breccia and interstratal fibrous satin reached fast-flowing under-saturated waters of spar gypsum veins. the active phreatic zone. The dispersive and All gypsum textures actually correspond to infiltrating water flow regime favoured rapidly secondary hydrated fabrics, shown here to be precipitated finer grained crystal growth (alabas- solution-related diagenetic products rather than trine) in a still permeable former gypsum matrix evaporation-derived primary deposits; they characterized by closely spaced nucleation sites. formed during uplift when various invading 5 Solution-related karstification and collapse under-saturated phreatic to vadose waters came breccia were probably most intense once the into contact with soluble salt units. Ongoing gypsum entered the vadose zone, where down- halite dissolution resulted in the accumulation ward-moving interstitial freshened (meteoric) of residual anhydrite and insoluble material that waters percolated and migrated through the occurred in the deep burial environment. These remaining pore network and along solution- anhydrite accumulations were subsequently enlarged fractures in the gypsum host. The for- hydrated and converted into gypsum during mation of solution-related collapse features, like uplift, at the same time as any remaining halite V-structures and karst, is indicative of a gravita- was flushed out. tionally focused dissolutional flow regime con- Anhydrite and dolomite inclusions are abun- taining waters that are periodically freshened dant in gypsum textures. Anhydrite relics are and that are under-saturated with the respect to best preserved in the porphyroblastic gypsum gypsum (Warren, 2006). and, to a lesser extent, in the alabastrine tex- 6 The preservation of the last sets of selenite tures, but all reveal alteration by undersaturated rosettes that cross-cut the laminated alabastrine fluids prior to gypsification. The disseminated gypsum beds implies a return to near-equili- euhedral dolomite grains suggest in situ precipi- brium hydration conditions enabling the precip- tation from magnesium-rich derived dissolving itation of coarse-grained gypsum crystals. This evaporites. still implies a sufficient permeable pore network In the telogenetic realm, the formation of por- within the gypsum beds that is sufficient to phyroblastic gypsum tends to precede alabastrine

© 2014 The Authors. Sedimentology © 2014 International Association of Sedimentologists, Sedimentology, 62, 204–233 230 L. Gindre-Chanu et al. gypsum because it is favoured by sluggish water Braccini, E., Denison, N., Scheevel, J.R., Jeronimo, P., crossflows, slower crystal growth rates and less Orsolini, P. and Barletta, V. (1997) A revised chrono- saturated pore fluids, while granoblastic gypsum lithostratigraphic framework for the pre-salt (lower Cretaceous) in Cabinda, Angola. Bull. Centres Rech. Elf necessitated less stable hydration conditions, Explor. Prod., 21, 125–151. faster crystal growth rates and supersaturation. Braitsch, O. (1964) The temperature of evaporite formation. Satin spar gypsum veins form in fractures cre- In: Problems in Palaeoclimatology (Ed. A.E.M. Nairn), pp. ated by the progressive dissolution and volume 479–490. Wiley, New York. loss associated with meteoric flushing and uplift Brognon, G.P. and Verrier, G.R. 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