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Porosity changes induced by salt weathering of sandstones, Bohemian Cretaceous Basin, Czech Republic.

INTRODUCTION

Pore spaces in coarse detrital rocks function as dominant pathways for fluid migration and transport of solutes. In the early stage of rock evolution, they convey diagenetic/hydrothermal fluids, which are responsible for the distribution of cement in sandstone bodies in a sedimentary basin. With the continued uplift of the basin fill, introduction of cooler, oxidized meteoric waters may induce changes in cement distribution, hence also pore space rearrangement. After exhumation to the earth surface, and even shortly before, the rock becomes exposed to various atmospheric effects (physical, chemical, biological) including the water cycle of precipitation--saturation --evaporation. The effects of diagenetic changes (notably the cement distribution) become accentuated during the weathering process.

Evaporation of pore waters on the rock surface, and shallow under the surface, has the largest impact on the formation of rock crusts and small-scale landforms in sandstone landscapes. The two processes in operation, salt weathering and case hardening, have been first described by Beyer (1912) in the Sachsische Schweiz in Saxony. Beyer recognized the role of pore waters in the formation of salt efflorescences and rock crusts, and identified alum salts ("Alaun") as the principal mineral contributing to sandstone deterioration. He also noted the presence of superficial crusts "sintered" by gypsum, and speculated that these crusts prevent pore waters from penetrating to the rock surface.

Despite the evidence brought about by Beyer, processes responsible for the origin of cavernous and pitted surfaces on sandstone outcrops have become a subject to a wide discussion until 1970s when convincing experimental work has been carried out (Evans, 1970; Goudie et al., 1970) and the effect of saline solutions has been acknowledged.

In inland settings, salt weathering is typically associated with surface crusting (Robinson and Williams, 1987). An association of salt weathering with surface crusting represents a basic condition to produce well-defined honeycomb pits and tafoni. The strengthened layer on sandstone surface is reinforced by precipitation of substances (iron, silica, salts) contained in the pore waters. Generally accepted models (cf. Turkington and Paradise, 2005) presume that the process of surface crusting proceeds simultaneously with the process of rock weakening in a shallow subsurface. Breaching of the strengthened layer would allow local evaporation of pore waters, which is otherwise prevented/retarded by the crust. Destructive effect caused by the growth of salt subflorescences concentrates to the deepest parts of the honeycomb pits. Salt efflorescence's, on the other hand, concentrate to the periphery of the honeycomb pits. Their destructive effect is considerably smaller and, in fact, may turn into a constructive one. In case of a missing hardened surface, the distribution of honeycomb pits on the sandstone surface is generally controlled by primary sedimentary structures of the rock, i.e., by the presence of clay laminae or grain-size contrasts. The pits are arranged in lines parallel to bedding planes or cross-bedding foreset laminae, or tend to follow bioturbation patterns. On the surfaces of massive, homogeneous sandstones, very initial forms of honeycombs are distributed irregularly; during the subsequent phases of development, random patterns turn into ordered ones. Shapes and typical distances of the pits depend, e.g., on grain size, inclination of the surface and insolation (Mikulas, 2001).

An important role of salt weathering in honeycomb formation is accepted for sandstone outcrops lying within the reach of wave splash and salt spray at seashores (e.g., Mustoe, 1982, 2010; Pye and Mottershead, 1995; McBride and Picard, 2004), even though such process is only rarely accompanied by surface crusting.

Data from mercury intrusion porosimetry serve as a good indicator of changes in the pore space in detrital sedimentary rocks (Gregg and Sing, 1982). They reflect many aspects related to sedimentary and diagenetic processes (grain packing, sedimentary texture and cementation) as well as to rock disintegration and weathering. As yet, their use has been mostly limited to the conservation of historical buildings and monuments (Rossi-Doria, 1983), e.g., for the distinguishing of rock provenance (Fitzner and Snethlage, 1982; Sramek, 2009). Siedel (2010) combined porosity measurements of sandstone building blocks from the Zittau area with analyses of salt subflorescences. He concluded that sandstone partly impregnated with iron oxyhydroxides is more vulnerable to granular disintegration and cavernous weathering because of its enhanced microporosity.

Sandstones which underwent the same diagenetic history generally show similar patterns in pore size distribution. Using a set of samples from the Bohemian Cretaceous Basin, Sramek et al. (1992) demonstrated that building sandstones from a specific area share common porosity characteristics, notably the total pore volume vs. median ratio, and differ from those in other areas. Differences in pore size distribution among sandstones from the same area, or even from the same site, should be therefore largely a function of the different styles of weathering and their intensity. This assumption was tested by Pcikryl et al. (2007) who studied pore-size distributions in sandstone samples affected by salt weathering from natural outcrops in the Bohemian Switzerland area, Bohemian Cretaceous Basin.

In order to determine the validity of such approach over a wider geographic area and a variety of products of salt weathering/case hardening, we studied pore-size distributions in different sandstone types in the Bohemian Cretaceous Basin and compared them with those in the overlying superficial layers affected by weathering. Special attention was given to: 1, changes in pore-size distribution in the zone of honeycomb pit formation, 2, changes in poresize distribution in armoured surface crusts bearing no honeycomb pits.

GEOLOGICAL AND GEOGRAPHICAL SETTING

The Bohemian Cretaceous Basin (BCB) extends from Saxony, Germany, towards ESE across most of the Czech Republic. Fluvial, lacustrine and estuarine sediments were deposited in the Early and Mid Cenomanian (from ca. 98 Ma). From the Late Cenomanian onwards, the basin was flooded by a shallow epicontinental sea lying between the boreal ocean in the NW and the deep-water Tethyan basin in the SE. The youngest preserved sediments are Santonian in age (ca. 85 Ma). Detrital material for Cenomanian sediments was derived from multiple elevations in the crystalline basement. In the Turonian and Coniacian, however, with the tectonic uplift of blocks beyond the northern basin margin, most of the material was transported to the basin from the granite-dominated West Sudetic and East Sudetic islands in the north (Skocek and Valecka, 1983). Partly overlapping wedge-shaped packages of coarse detrital sediments of shallow marine origin, max. 150-350 m thick, extend from the northern basin margin towards the basin axis (Ulicny et al., 2009). Internal arrangement of the sandstone bodies shows upward-scoarsening (i.e., upwards-shallowing) cycles of several orders of magnitude (metres to tens of metres in thickness) separated by flooding surfaces. A typical cycle consists of bioturbated fine-grained clayey sandstone, passing upwards to horizontally stratified or cross-bedded medium- to coarse-grained sandstone (Adamovic, 1994). Most of the cycles are topped by conglomerate beds.

Sandstones of the BCB are composed almost exclusively of quartz, with less than 5 vol. % feldspar grains and mica flakes. Accessory heavy minerals (<1 vol. %) are mostly represented by tourmaline, zircon and rutile, although staurolite, garnet or anatase are also present in elevated amounts near source areas in the Cenomanian sediments (Skocek and Valecka, 1983).

The most common clay mineral in the finegrained clayey sandstones is kaolinite. Its content rarely exceeds 10 vol. %. In strongly bioturbated intervals, clay minerals are concentrated to wall linings of tunnels formed by the activity of in-fauna, or to the passive fill of burrows. They are also present in occasional smears and laminae generated by bottom currents. Secondary clay minerals are produced by disintegration of feldspar grains during diagenesis.

Five regions dominated by sandstone can be distinguished within the BCB (from W to E): Bohemian-Saxonian Switzerland (Lower to Upper Turonian), the Luzicke hory/Zittauergebirge Mts. (Middle Turonian to Coniacian), the Ralska pahorkatina including the Kokorin area (Middle to Upper Turonian), the Bohemian Paradise (Upper Turonian to Coniacian) and the Broumov area extending to the Gory Stocowe in Poland (Middle Turonian to Coniacian). Lowland regions have warm or moderately warm climate with average annual temperatures of 7-8 [degrees]C and annual precipitations of less than 700 mm (Ralska pahorkatina, Bohemian Paradise). Others are rather cold (e.g., 6 [degrees]C in Gory Stolowe--max. altitude of 919 m), with annual precipitations between 700 and 1100 mm (Gory Stolowe, Luzicke hory Mts.). Temperature inversions are common in narrow gorges. Bases of cliffs are shaded by tree vegetation, thus preserving higher moisture content compared to bare, dry cliff tops.

Formation of large-scale landforms is generally controlled by tectonic deformations, such as tilting of blocks and low-angle folding. Valley courses follow zones of intensive fracturing, and "rock cities" commonly form in areas with orthogonal jointing. Valley profiles show recessive steps and notches at the level of cycle boundaries, which implies that lower parts of cliff faces are mostly composed of fine-grained sandstone types. High variability of smallscale weathering forms is supported by differences in sedimentary textures and structures, by the variable intensities of sandstone cementation and types of cement, and by the favourable humid temperate climate (Adamovic et al., 2006, 2010).

METHODS

Rock samples were taken from each site to represent all weathering products: unweathered sandstone, sandstone with salt subflorescences, rock crust, and salt efflorescences. Polished thin sections of rocks and polished cross sections of the rock surface were made for petrographic study. These were studied using the Olympus BX51 polarizing optical microscope. Scanning electron microscopy (SEM) was applied to rock surfaces and surfaces of freshly broken rock crusts using the Cameca SX 100 electron microprobe (secondary electrons), or in combination with energy-dispersive X-ray spectra (EDX) using Tescan Vega 3 scanning electron microscope at the Institute of Geology AS CR in Prague. Selected samples of salt efflorescences were cast in a polyester resin, and polished cross-sections were created. They were studied at the Institute of Rock Structure and Mechanics AS CR in Prague, using the Leica polarizing microscope with Leica DC 200 camera in reflected light, and in scanning electron microscope with energy dispersive X-ray microanalysis (SEM/EDX; Quanta 450, EDAX).

Mineral phases of subflorescences and some efflorescences were identified using X-ray diffraction (XRD) on the Philips X'pert X-ray diffractometer at Institute of Geology AS CR in Prague (analyst J. Dobrovolny). Mineral phases of most efflorescences were identified using a PANalytical X'Pert PRO diffractometer at Institute of Inorganic Chemistry AS CR in Prague-Rez in a transmission mode.

Skeletal densities (densities of solid phase) were determined by means of helium pycnometry at Institute of Chemical Process Fundamentals AS CR in Prague, using the Accupyc 1330 Micrometrics helium pycnometer (analyst H. Snajdaufova), with 5 cycles of purging, equlibration rate of 0.58 Pa/s = 0.005 psig/min.). Statistical evaluation provides accuracy down to 0.001 g/[cm.sup.3] . Measurements of the total intrusion volume, median and average pore radii and bulk density of the samples were performed at the same institute using a high-pressure mercury Micrometrics AutoPore III porosimeter (analyst H. Snajdaufova). This instrument allows high-pressure mercury intrusion of up to 400 MPa (corresponding to pore radii of 1.5 nm).

DESCRIPTIONS OF SAMPLING SITES

The sampling sites were selected after a careful field examination in order to cover the different types of sandstone (grain size and silt/clay content, sedimentary structures). Quartzose sandstones (quartz over 95 vol. %) and clayey sandstones (clay proportion over 5 vol. %) were identified. Sampling was performed on surfaces of rather uniform inclination (80-85[degrees]) at similar heights above the cliff base (0.5-2.0 m) as it has been proved by previous observations that these factors strongly influence the processes on sandstone surfaces (cf. Mikulas, 2001).

Of the eight sampling sites (Figs. 1 and 2), five lie in the Ralska pahorkatina region in north-central Bohemia: in the Kokorin area (Dobren 1 and 2, Plany dul, Kobylka) and the Doksy area (Hradcany). All of them belong to the Jizera Formation (Middle to Upper Turonian). Two sites, Rohliny and Zaborci, lie in the Bohemian Paradise region in NE Bohemia, in the Teplice Formation of the BCB (Upper Turonian to Coniacian). The site of Cernuc lies in central Bohemia and the sandstone represents the Peruc-Korycany Formation (Upper Cenomanian). Within the BCB, the sites belong rather to warm or moderately warm sandstone regions with annual precipitations of less than 700 mm. The outcrops lie in forested areas with the exception of Cernuc, which is an old quarry face sparsely shaded by vegetation.

[FIGURE 1 OMITTED]

DOBREN 1

Grey, coarse-grained, horizontally stratified quartzose sandstones on a N-facing cliff; cliff face inclined N at 85[degrees]. A dense network of arcuate honeycomb pits, mutually interconnected, extending 15 cm deep beneath rock-crust surface. Spherical pits are formed at places where the rock crust has fallen off. Cliff base 358 m a.s.l., 4 m above the bottom of a dry gorge. Sst: sandstone 0.5 m above cliff base; PRC: rock crust with honeycombs 2 m above cliff base; S: salt efflorescences under a rock-shelter 1 m above cliff base

DOBREN 2

Yellowish, medium-grained, horizontally stratified quartzose sandstones with admixture of coarse grains and granules on a N-facing cliff; cliff face inclined S at 85[degrees]. Individual large arcuate honeycomb pits developed beneath a locally developed armouring 1 cm thick, with tortoise-shell cracks. Shallow spherical pits are formed elsewhere. Cliff base 373 m a.s.l., 2 m above the bottom of a dry gorge. Sst: sandstone 0.5 m above cliff base; PRC, ARC: rock crust with lighter armouring 2 m above cliff base; S: salt efflorescences from a notch 0.5 m above cliff base

PLANYDUL

Yellow, coarse-grained, bioturbated quartzose sandstones on a ESE-facing cliff; cliff face inclined WNW at 86[degrees]. A dense network of arcuate honeycomb pits extending 15 cm deep beneath rock-crust surface. Cliff base 298 m a.s.l., 28 m above the bottom of a dry valley. Sst: sandstone 1 m above cliff base, depth 20 cm; PRC: rock crust with honeycombs 2 m above cliff base; S: white salt efflorescences 1-1.4 m above cliff base

[FIGURE 2 OMITTED]

KOBYLKA

Yellow, fine-grained, bioturbated clayey sandstones on a WSW-facing cliff; cliff face inclined ENE at 82[degrees]. A flat armouring with fine sculptation on the surface, hiding primary sedimentary structures. Small spherical honeycomb pits surrounded by white salt efflorescences are developed at places where the armouring has been destructed. Cliff base 339 m a.s.l., 9 m above the bottom of a dry gorge. Sst: sandstone 0.5 m above cliff base; ARC: 0.5 m above cliff base; S: salt efflorescences above a notch 0.6 m above cliff base

HRADCANY

Yellow, fine-grained clayey sandstones, horizontally stratified with vertical burrows, on an E facing cliff; cliff face inclined W at 82[degrees]. A flat but perforated armouring 3-10 mm thick is locally present, hiding primary sedimentary structures, and often preserved in the interiors of tafoni and older-generation honeycomb pits. Younger-generation spherical honeycomb pits of variable size and patches of white salt efflorescences are developed where the armouring has been destructed. Cliff base 319 m a.s.l., 16 m above the bottom of a dry gorge and 51 m above a stream (600 m to the N). Sst: 0.5 m above cliff base; PRC, ARC, S: 1.5 m above cliff base

ROHLINY

Yellow, fine-grained, thinly bedded quartzose sandstones with primary stratification dipping S at 13[degrees] (clinoforms) on a ESE-facing cliff; cliff face inclined ESE at 78[degrees], passing to a shallow rock-shelter. Armoured rock crusts ca. 15 mm thick are locally developed on strata-parallel ledges. Small spherical honeycomb pits are rare, salt efflorescences concentrate to the interior of the rock-shelter. Cliff base 393 m a.s.l., 70 m above the valley bottom with a small stream. Sst: sandstone 0.5 m above cliff base; ARC: armoured rock crust from top surface of a ledge1 m above cliff base; S: salt efflorescences from a rock-shelter 1 m above cliff base

ZABORCI

Grey, medium-grained quartzose sandstones with horizontal stratification on a WSW-facing cliff; cliff face inclined ENE at 78-89[degrees]. The rock crust (sample PRC 1, depth 5-7 cm) is light grey, ca. 10 cm thick, with a network of spherical to arcuate honeycomb pits arranged in lines parallel to bedding. Walls and bottoms of honeycombs are lined by a somewhat darker (rusty) layer down to the depth of 3-5 mm (sample PRC 2). This layer does not seem to function as armouring; instead, it shows some degree of degradation. Elsewhere, the surface of the rock crust is flat, with honeycomb pits restricted to cracks arranged in a tortoise-shell pattern. Salt efflorescences are present at places where the rock crust has fallen off, at the best protected (the most overhanging) sites of the face. Cliff base 426 m a.s.l., 27 m above the bottom of a dry valley. Sst, PRC: sandstone and the overlying rock crust 2 m above cliff base; S: 1 m above cliff base

CERNUC

Light yellowish, fine-grained quartzose sandstone with horizontal stratification and large-scale cross bedding on a SW-facing quarry face, inclined SW at 88[degrees]. It bears fine vertical rillenkarren in places but no honeycombs. Armoured rock crust 2-4 cm thick is slightly darker, with irregular wandkarren and small pits, totally obscuring primary sedimentary structures. A 1-mm rusty layer of clayey appearance is present on the surface of the armoured rock crust. Exposure base 201 m a.s.l. Sst, ARC: 1 m above the base of a quarry face

ROCK CRUSTS

The term "rock crust" is herein reserved for a surface layer of sandstone, usually 2-15 cm thick, which occurs almost exclusively on surfaces protected from a direct rainfall--i.e., on subvertical, vertical and overhanging rock surfaces or inside rock shelters. The exact limits of the rock crusts are difficult to define in the field, both in the direction parallel to and perpendicular to the cliff face. The effects of salt weathering vs. case hardening are of variable intensity, both geographically and within the outcrop scale. In the sandstones of the BCB, rock crusts often bear small-scale weathering forms, most notably honeycombs and wandkarren, and may be associated with the formation of tafoni. Elsewhere in the basin, however, honeycomb formation is very limited, and the cliffs rather show signs of surface crusting (e.g., Gory StoLowe Mts. in Poland).

Due to the coalescence of individual honeycomb pits, the outer parts of the rock crust are irregularly but unavoidably detached and destructed, thereby becoming a part of scree accumulations at the foot of the cliff face. The evaporation front usually changes in time; the change is typically connected with the recess of the rock surface. An uneven distribution of moisture along a cliff face, controlled by various intrinsic factors (lithology, fracturing) and extrinsic factors (height above the soil cover, shading), implies different rates of rock surface recess. As a result, different stages of rock crust development can be usually observed on each outcrop. Second-generation (less frequently third- or fourth-generation) rock crusts are a common phenomenon.

The presence of honeycombs does not necessarily imply the presence of a rock crust. Especially on weakly litihfied sandstones or on fresh outcrops (younger than ca. 200 years; cf. Mikulas, 2009), initial honeycombing can occur without any discernible hardening anywhere on the surface.

Some rock crusts in the BCB are notable for their hardened, "armoured" superficial layers. The contrastingly sharp morphologies and reduced porosities of these layers motivated the present authors to propose the term armoured rock crust (ARC) for the portions of the rock crust where continuous armouring is present. Armoured layers on medium-to coarse-grained quartzose sandstones are typically a few millimetres thick and mostly lighter than the underlying sandstone, unless iron and manganese minerals are involved, and sometimes bear a tortoise-shell pattern of shallow cracks (Dobren 2, Zaborci). Armoured layers on fine-grained clayey sandstones are thicker (1-3 cm), often contrasting in their greyish colour with the yellow sandstone underneath. Thick armoured rock crusts obscure primary sedimentary structures; their surfaces are smooth even in outcrops where bioturbation structures or cross bedding are otherwise highlighted by the action of salt weathering. Rock crusts with discontinuous or no armouring, bearing small weathering forms like honeycombs or wandkarren, are herein called patterned rock crusts (PRC) as a counterpart to ARC.

Figure 3 illustrates the contrasting features of PRC and ARC observed on medium- to coarse-grained quartzose sandstones (A) and those on fine grained clayey sandstones (B). Figure 4 shows examples of sharp boundaries between non-hardened and hardened sandstone surface at particular stages of the weathering cycle.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

X-RAY DIFFRACTION

Minerals of the rock crusts in the BCB were primarily identified using XRD, which provides an unequivocal identification of mineral phases. The low sensitivity of the method, however, necessitated a combination with optical microscopy and SEM, especially in the case of subflorescences (Table 1). Principal minerals of rock crusts in the studied region are gypsum (CaS[O.sub.4]-2[H.sub.2]O) and locally developed goethite ([alpha]FeOOH), but various water-soluble salts were also encountered in subflorescences: potassium alum [[KAl(S[O.sub.4]).sub.2]x12[H.sub.2]O], tschermigite [N[H.sub.4]Al(S[O.sub.4] )2"12[H.sub.2]O] and brushite (CaHP[O.sub.4]x2[H.sub.2]O).

Salt efflorescences from all sites contain gypsum. Alums were commonly identified, being represented by potassium alum at Dobren 1 and Hradcany and by potassium-ammonium alum [(K, N[H.sub.4])Al(S[O.sub.4])2-12[H.sub.2]O] at Dobren 2, Plany dul, Rohliny and Zaborci.

Armoured layers on medium- to coarse-grained sandstones were found to be sintered by gypsum with a contribution of brushite (Kobylka) or a thin coating of brushite (Rohliny--a site 200 m away from the described one). Gypsum was also found to be the main constituent of armoured rock crusts on fine-grained sandstones, being accompanied by clay minerals. Kaolinite (Hradcany, Cernuc), montmorillonite and halloysite (Kobylka) were identified.

OPTICAL MICROSCOPY AND SEM

Photomicrographs of the minerals and salts identified using the optical microscope and SEM are shown in Figure 5.

Gypsum is the main mineral in pore spaces identified by optical microscopy and SEM. It forms either individual large crystals growing between quartz grains (max. length 0.2 mm), or crystal aggregates filling voids. Gypsum crystals typically grow with their c-axes perpendicular to grain surfaces. Good examples were found in rock crusts both on fine-grained sandstones (Hradcany, Rohliny) and on medium-grained quartzose sandstones (Kobylka, Zaborri). Small gypsum crystals 20 Lim in length were also found dispersed in spongy clay matrix in SEM images from Plany dul (Fig. 5F).

Clay minerals form linings of quartz grains in many fine-grained clayey sandstones. Their content increases within the rock crust where they tend to form aggreagetes completely filling the voids. A sudden increase in thein proportion within a ca. 1 mm thick layer on the rock surface is also visible in medium- to coarse-grained quartzose sandstones. Clay linings on quartz grains and clay mineral aggregates are common substrates for the growth of gypsum crystals.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

SEM revealed the presence of thin (~100 Lim), amorphous-looking coatings of potassium alum with desiccation cracks in several samples from the rock surface (Plany dul, Kobylka, Zaborri). These coatings tend to flood, partly or completely, columnar gypsum crystals on the rock surface and often contain pollen grains and other impurities trapped by atmospheric deposition. Cubic pyrite crystals identified at cernuc by SEM typically overgrow detrital grains of altered K-feldspar.

RESULTS OF POROSIMETRY STUDIES

Results of porosity measurements are shown in Table 2 and Figures 6 and 7.

Patterned rock crusts on medium- to coarse-grained sandstones in the Kokorin area show, relative to unweathered sandstone, an increase in the size of macropores, which is manifested by a shift of the median pore diameter towards higher values. This may be either due to the creation of new interconnections among pores, or due to the detachment of closely packed grains. Although the volume of macropores in the rock crusts may relatively decrease, the increase in their size usually results in an increase in total effective porosity. At the Zaborci site in the Bohemian Paradise, on the contrary, a prominent decrease in the proportion of macropores was encountered, resulting in a reduction in total effective porosity (from 27 % to 22 %) in the inner part of the rock crust (PRC 1). Microscopic observations revealed a tighter grain packing in this region, with frequent linear contacts between grains (Fig. 8). In the walls and bottoms of the honeycomb pits (PRC 2), however, this trend becomes reversed: the proportion of macropores increases and so does the total porosity. The very low median pore diameter in honeycomb walls at Zaborri results from partial filling of large pores with clay minerals, gypsum, brushite and calcite (Table 1 and Fig. 5A, E).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Porosity changes in an armoured rock crust at Dobren 2, relative to unweathered sandstone, can be characterized by a decrease in the volume of macropores, a decrease in their size and a reduction of total effective porosity. No massive crystallization of salts was found in the armoured layer, so this reduction must be ascribed to the tight packing of sand grains with mostly linear contacts, similarly as in rock crust PRC 1 at Zaborci.

In fine-grained sandstones, armoured rock crusts share many common porosity characteristics. Total effective porosities in such crusts are always lower compared to the relatively sound sandstone below: they are reduced from 27 % to 24 % at Kobylka, from 21 % to 20 % at Hradany, from 32 % to 27 % at Rohliny, and from 31 % to 28 % at Cernuc. This reduction is associated with prominent shifts of the median pore diameter towards lower values (see Table 2). Notable is the almost disappearance of pores >40 [micro]m in diameter at Rohliny. As indicated by microscopic observations and XRD analyses, these changes can be most readily explained by partial sealing of large pores by clay minerals and by the growth of gypsum crystals. The reduction of macropores is partly compensated by a prominent increase in the volume of micropores, which was encountered at all four sites (Table 2, Figs. 6-7). At Hradcany, for example, the proportion of micropores in the total intrusion volume increased by the factor of 3. As the populations of macropores and micropores are always well separated, such an increase cannot be explained by a reduction of effective pore radii of the macropores. SEM observations suggest that it rather reflects microporosity in clay mineral aggregates whose proportion rapidly increases in the topmost few millimetres of sandstone outcrops. Microfracturing of detrital grains induced by the growth of gypsum crystals cannot be excluded; however, no convincing examples were found on SEM images (cf. Figs. 5A, E, G). Some of the microfractures may also represent desiccation cracks in thin coatings of potassium alum observed by SEM.

DISCUSSION

Mineral phases of salt efflorescences identified by the present authors on sandstones in the central part of the BCB generally correspond with those reported from this region by previous authors (Breiter, 1976; Cilek et al., 1998; Soukupova et al., 2002; Schweigstillova et al., 2009). Brushite, which has been reported from Ralska pahorkatina as a rare phase by Cilek and Melka (2000), was found also at Zaborci in the Bohemian Paradise as a common constituent of salt subflorescences together with gypsum. On the other hand, syngenite [[K.sub.2]Ca(S[O.sub.4])2-[H.sub.2] O] found at several sites in the Ralska pahorkatina and the Bohemian Paradise (Schweigstillova et al., 2009) was not encountered by the present study. Neither was confirmed the presence of alunogen [A[l.sub.2](S[O.sub.4])3-16[H.sub.2]O] or alunite [KAL,(S[O.sub.4])2[(OH).sub.6] ], which seem to be much more common in the NW part of the BCB (Lentschig-Sommer, 1960, 1961; Williams and Robinson, 1998; Prikryl et al., 2007). However, gypsum and alums (Beyer, 1912) are clearly the most common mineral phases in all regions of the BCB, while gypsum, clay minerals and cristobalite dominate the coarse-grained sandstones and conglomerates of the Polish Carpathians (Alexandrowicz and Pawlikowski, 1982).

The exact mechanism of action of saline fluids producing sandstone disintegration can be manifold: 1, an increase in intergranular pressure due to salt crystal growth, resulting in crack widening and loss of contact between grains (Hume, 1925; Winkler and Singer, 1972; Goudie and Viles, 1997), 2. an increase in intergranular pressure due to salt hydration (Winkler, 1975), 3. enhanced dissolution of detrital quartz and clay matrix by saline fluids (Young, 1987; Young et al., 2009), 4. reaction with expandable minerals in sandstone matrix (Pye and Mottershead, 1995). The prevailing mechanism may vary depending on sandstone lithology and climate (cf. Turkington and Paradise, 2005), although steady flow rates and high rates of evaporation seem to be critical for honeycomb and tafoni formation (cf. the comparison of Turkington, 1998).

Beyer (1912) proposed that crystallization of salts is a process producing the greatest effect on near-surface sandstone deterioration. Alum salts have been demonstrated the most destructive in sandstone weathering experiments, especially at the presence of gypsum (Williams and Robinson, 1998; Robinson and Williams, 2000). In the study area, the presence of gypsum and water-soluble sulphates or phosphates in amounts detectable by XRD was proved in most sandstones showing honeycomb weathering, and gypsum crystals were often observed to completely fill the pore spaces, although in certain areas of the rock crust only. Effects of salt hydration/dehydration should not be overestimated in our case because sandstone outcrops in forested areas are not exposed to large swings in moisture content, especially within the reach of soil humidity. An increase in the size of macropores, which was observed in sandstones within the patterned rock crusts, should be therefore attributed to the growth of salt and gypsum crystals and the resulting granular disintegration. The same conclusion was made by Prikryl et al. (2007) who found a positive correlation between bulk porosity and the concentration of water-soluble salts in rock crusts in the NW part of the BCB.

No opal coatings were found on the studied samples, and etching traces on quartz grains were observed only rarely (Zaborci). It can be, however, presumed that some silica was released from clay matrix and feldspar grains, which show frequent signs of corrosion. The presence of opal coatings and impregnations on the surface of sandstone outcrops has been reported from the NW part of the BCB by Cilek and Langrova (1994) and by Prikryl et al. (2007). In the central part of the basin, however, any substantial silica redistribution seems to be prevented by relatively lower precipitation and a higher evaporation rate of precipitation in the recharge area. Other factors, like a lower ionic strength of the fluids, may be also involved.

At the site of Zaborci, the changes in pore-size distribution described above as characteristic for patterned rock crusts are limited to a layer reaching 3 to 5 mm beneath the walls and bottoms of honeycomb pits. Changes in pore spaces deeper in the rock (down to 7 cm) rather parallel those observed in the armoured layers on medium- to coarse-grained sandstones elsewhere (Dobren 2 site), and are manifested by tight grain packing in thin sections at the absence of gypsum or salt subflorescences. This situation can be explained by repeated, periodical freezing of water in the near-surface layer, which would result in a temporary forced crystallization of the dissolved salts in the rock interior immediately beyond the frozen fringe (e.g., Williams and Robinson, 2001). Such recurrent process would eventually lead to re-orientation of grains and their tighter packing in a relatively narrow zone defined by the freezing front. The observed concentration of clay minerals to the topmost 1 mm beneath the surface of medium- to coarse-grained sandstones (Fig. 5B) may be also an effect of frost weathering. In fact, it cannot be excluded that tighter grain packing and clay mineral concentration, presumably induced by frost action, are the main causes of the growth of salt subflorescences in the BCB, since reduction in pore size provides a larger surface area for evaporation and, at the same time, reduces the flow rate of pore waters, thereby promoting super saturation deeper under the surface (cf. Rodriguez-Navarro and Doehne, 1999).

Little attention has been given by previous authors to salt weathering of fine-grained clayey sandstones in the BCB, possibly due to their poor exposure in the classical area of the Bohemian/Saxonian Switzerland in the NW part of the BCB and their infrequent use as building stone. Yet, the armoured layers several centimetres thick (Figs. 2B, C, F; Fig. 4) formed on this sandstone type are the most eye-catching examples of salt weathering/case hardening in the field. Reduction of macropores and the prominent increase in micropores are a rule, probably reflecting a combined process of growth of salt crystals and volumetric changes of clay minerals at contact with the saline fluids. The new observations are in general agreement with the degradation processes of clay-rich sandstones described by Warke and Smith (2000) from an urban environment in Northern Ireland: the zone of sandstone alteration, measured by the contents of calcium and sulphates, has been found to extend to a depth of 60 mm, and swelling and secondary microporosity of clays has been suggested as a factor augmenting the destructive effect of salt crystallization.

Our field observations documented various stages of destruction of armoured layers, similar to the process of contour scaling by Smith and McGreevy (1988). With the continued crystal growth and reduction of macropores penetrable for water in the armoured layer, pore waters and capillary waters restrict their circulation to the underlying sandstone. This implies a shift of the evaporation front deeper into the rock and the absence of salt efflorescences and honeycomb pits on the armoured rock surface. Then, the armoured layer becomes separated from the underlying rock crust by a thin fissure parallel to the rock surface. This separation itself should be explained rather by different volumetric response of the armoured layer to temperature changes than by salt weathering. A subsequent evaporation of saline pore waters within the fissure results in salt precipitation and finally in gravity-induced detachment of separate segments of the armoured layer.

New information on the distribution of secondary minerals and porosity in rock crusts can be matched with the present knowledge of factors influencing the origin of pitted/cavernous surfaces (cf. Mikulas, 2001, 2007). Besides "pure" patterned rock crusts (PRC) and "pure" armoured rock crusts (ARC), a variety of transitional forms can be observed in the field (Fig. 4). Further, it is beyond any doubt that PRC and ARC may form next to each other regardless of the homogeneous sandstone lithology. A succession of events may result in the formation of several generations of rock crusts, combining PRC and ARC types, or formed by PRC laterally passing into ARC. For new, fresh sandstone surfaces, such as those formed by rockfall, the course towards the development of PRC vs. ARC is decided about within the first few centuries or even decades (cf. Mikulas, 2009). If no honeycomb pits develop within this relatively short interval, a smooth rock crust will probably form and will almost irrevocably evolve into an ARC. In contrast, surfaces with early-formed pits tend to be conserved for a very long time, evolving into a specific distribution of destructive elements (evaporation front, growth of salt subflorescences, frost) reaching deeper into the rock. In rocks with homogeneous lithologies, the primary origin of honeycomb pits is controlled by the influx of pore waters, composition of solutes, and evaporation. For a given site, these factors are affected by local vegetation or configuration of instable blocks. Short-term climatic oscillations may also play a role. Such factors are usually difficult to decipher in the geological record available on sandstone cliffs or in their close vicinity (scree slopes or talus accumulations). As the controlling factors can be only rarely traced back to the past, the patterns of the resulting weathering forms may give an impression of random origin.

CONCLUSION

The presented differentiation between patterned and armoured rock crusts on natural sandstone surfaces in the BCB seems to be fully justified by differences in the pore-size distribution, mineral redistribution and the resulting weathering forms. Samples of PRC taken from the walls and the deepest parts of honeycomb pits, although having a more compact appearance than the unweathered sandstone below, show an increase in the size of macropores. In most cases, this increase implies a higher total porosity. Microscopic observations confirmed the presence of salt subflorescences in this zone, and suggest that this increase in pore size is due to the crystallization force of salts (notably gypsum) and the consequent loss of contact between grains. Dissolution of silica and hydration force of salts are believed to play a minor role in this process. Nevertheless, the common corrosion of feldspar grains and clay minerals in the matrix could have provided free silica and alumina for the secondary salts, notably potassium alum. A zone of tighter grain packing and pore-size reduction found in medium-grained sandstone just beyond honeycomb pits (depth 5 to 7 cm) could have originated by grain reorientation during repeated freezing of pore waters.

At macro-scale, the observed changes in pore spaces in the PRC contribute to the preservation and further evolution of topographically fixed cavernosity. An increase in pore size and pore connectivity in a near-surface zone is a factor which not only supports the influx of saline fluids from the rock massif but also enhances, at a favourable exposure of the cliff face, evaporation of these fluids near the rock surface. Such process leads to further salt crystallization and pore enlargement. In this respect, pore spaces function as a memory record of the weathering forms at a given place on the cliff face, and keep on generating the same forms unless the supply of saline fluids from the rock massif gets altered or depleted.

Tighter packing of grains in ARC on medium- to coarse-grained sandstones and concentration of clay minerals near their surface are rather a result of frost action. Although gypsum is present in some of the ARC, salt crystallization does not seem to be necessary for their formation. The reduced volume and size of macropores in the ARC are compatible with the abundant presence of clay mineral aggregates and gypsum crystals in these pores. Secondary porosity in kaolinite and the observed corrosion of feldspar grains are responsible for the prominent increase in the volume of micropores relative to that in unweathered sandstone. The mechanism of clay concentration into a several centimeters thick surface layer is a subject to discussion, and should be seen either in frost action or in chemical re-distribution of alumina and silica in the weathering zone, or a combination of both.

The relatively small volume of macro pores in the ARC does not allow fluid transport from the rock massif to the surface, and the evaporation front is shifted deeper. Crystallization of salts at a certain depth eventually leads to the destruction of the armoured layer, similar to contour scaling in arid environments. This is the principal difference from PRC where the evaporation front lies at, or near, the rock surface and allows a variety of weathering forms to develop.

ACKNOWLEDGMENTS

This research was supported by the Grant Agency of the Academy of Sciences CR, Project No. IAA300130806, and falls within the Research Plan AV0Z30130516 of the Institute of Geology of the Academy of Sciences CR, v.v.i., and AV0Z304605519 of the Institute of Rock Structure and Mechanics of the Academy of Sciences CR, v.v.i. X-ray diffraction patterns of salt efflorescences were collected and analysed by Petr Bezdicka (Institute of Inorganic Chemistry AS CR, v.v.i.), identification of some mineral phases of salt subflorescences was made by Martin Scastny (Institute of Geology AS CR, v.v.i.). The authors wish to thank Karel zak for friendly consultation, and Jan Sramek and an unknown reviewer for valuable comments on the manuscript.

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Jin ADAMOVI (1) *, Radek MIKULAS (1), Jana SCHWEIGSTILLOVA (2) and Vlasta BOHMOVA (1)

(1) Institute of Geology of the Academy of Sciences of the Czech Republic,, v.v.i., Rozvojova 269, 165 02 Praha 6, Czech Republic

(2) Institute of Rock Structure and Mechanics of the Academy of Sciences of the Czech Republic, v.v.i., V Holesovickach 41, 182 09 Prague, Czech Republic

* Corresponding author's e-mail: adamovic@gli.cas.cz

(Received December 2010, accepted March 2011)
Table 1 A review of the sites sampled within this study, with a
summary of minerals identified at each site by optical microscopy (*),
X-ray diffraction (**) and scanning electron microscopy with EDX
(***). Material: Sst--sound sandstone, RC--rock crust, ARC--armoured
rock crust (a harder surface part of the rock crust), PRC--patterned
rock crust (honeycombed), S--salt efflorescences and other surface
coatings Mineral composition: B--brushite, C--calcite, F--detrital
feldspar (Or orthoclase, Mi microcline, Plg plagioclase),G--gypsum,
Gl-glauconite, Goe--goethite, H--halloysite, K--kaolinite,
KA--K-alum, KNA--potassium-ammonium alum, M--montmorillonite,
Mi--mica, Py--pyrite, Q--detrital quartz, T--tschermigite

Sampling site             Coordinates          Sample No.   Material
                          WGS84

Dobren 1                  50.47836[degrees]N   23           Sst
                          14.54833[degrees]E   22           PRC
Ralska pahorkatina,                            24           S
Kokorin area

Dobren 2                  50.47799[degrees]N   27           Sst
                          14.54904[degrees]E   26A          PRC
(Ralska pahorkatina,                           26B          ARC
Kokorin area)                                  28           S

Plany dul                 50.47136[degrees]N   5            Sst
                          14.60079[degrees]E   6            PRC
(Ralska pahorkatina,                           7            S
Kokorin area)

Kobylka                   50.45481[degrees]N   15           Sst
                          14.61906[degrees]E   11           RC
(Ralska pahorkatina,                           14           ARC
Kokorin area)                                  13           S

Hradcany                  50.61213[degrees]N   81           Sst
                          14.70521[degrees]E   49           PRC
(Ralska pahorkatina,                           80           ARC
Doksy area)                                    51           S

Rohliny                   50.60693[degrees]N   37           Sst
                          15.19700[degrees]E   36           ARC
(Bohemian Paradise,                            38           S
Turnov area)

Zaborci                   50.63274[degrees]N   47           Sst
                          15.17474[degrees]E   45B          PRC 1
(Bohemian Paradise,                            45A          PRC 2
Turnov area)
                                               44           ARC
                                               46           S

Cernuc                    50.30075[degrees]N   79           Sst
                          14.23519[degrees]E   78           ARC
(central Bohemia,
Velvary area)

Sampling site             Mineral composition

Dobren 1                  Q *
                          Q *, F-Mi **
Ralska pahorkatina,       G **, KA **
Kokorin area

Dobren 2                  Q *
                          Q *, F-Or **, Goe *
(Ralska pahorkatina,      Q *, F-Mi **, F-Plg **
Kokorin area)             G **, KNA **,
                          phosphate ***

Plany dul                 Q *, clay min. *
                          Q *, G **, clay min. ***
(Ralska pahorkatina,      G **, KNA **
Kokorin area)

Kobylka                   Q *, clay min. *
                          Q *, G **, KA **, T **
(Ralska pahorkatina,      Q *, G **, B **, M **, H **
Kokorin area)             G **, KA ***

Hradcany                  Q *, Gl *
                          Q *, G **, K **
(Ralska pahorkatina,      Q *, G **, K **
Doksy area)               G **, KA **

Rohliny                   Q *, Gl *
                          Q *, G *, clay min. ***
(Bohemian Paradise,       G **, KNA **, K **
Turnov area)

Zaborci                   Q *
                          Q *, F-Or **
(Bohemian Paradise,       Q *, G **, B **, C **, clay
Turnov area)              min. *
                          Q *, G *
                          G **, KNA **

Cernuc                    Q *, Mi *
                          Q *, F-Mi **, K **, Mi **,
(central Bohemia,         clinochlore in the rusty
Velvary area)             layer ** Py ***

Table 2 A summary of results from mercury intrusion porosimetry.
Values of total effective porosity were calculated from skeletal
density obtained from helium pycnometry and bulk density obtained
from mercury intrusion porosimetry. The micropore/macropore diameter
transitions equal to the least-frequency pore diameters in the
differential pore size distribution diagrams, and are therefore
sample-specific.

Material: Sst--sound sandstone, PRC--patterned rock crust (PRC 1
inner crust, PRC 2 outer crust), ARC--armoured rock crust

Sampling site     Sample   Material   Total
                  No.                 intrusion
                                      volume
                                      (ml [g.sup.-1])

Dobren 1          23       Sst             0.144
coarse-grained    22       PRC             0.136

Dobren 2          27       Sst             0.114
medium-           26A      PRC             0.118
grained           26B      ARC             0.097

Plany dul         5        Sst             0.090
coarse-grained    6        PRC             0.098

Kobylka           15       Sst             0.147
fine-grained      14       ARC             0.099

Hradcany          81       Sst             0.124
fine-grained      80       ARC             0.112

Rohliny           37       Sst             0.167
fine-grained      36       ARC             0.138

Zaborci           47       Sst             0.136
medium-           45B      PRC 1           0.123
grained           45A      PRC 2           0.113

Cernuc            79       Sst             0.181
fine-grained      78       ARC             0.176

Sampling site     Macropore/        Proportion of      Median
                  micropore         micropores (% of   pore
                  diameter          total intrusion    diameter
                  transition (nm)   volume)            (nm)

Dobren 1               4000                2.6          43140
coarse-grained         4800                3.8          46070

Dobren 2               5200                4.3          45330
medium-                4200                7.1          52220
grained                5400                7.8          43690

Plany dul              4400                6.7          56750
coarse-grained         5300                5.0          56930

Kobylka                5400                6.9          28270
fine-grained           4300               17.6          24730

Hradcany               5000                4.3          31770
fine-grained           4800               14.3          23130

Rohliny                4200                4.4          27750
fine-grained           5500                5.9          20490

Zaborci                5200                7.6          30150
medium-                4100                9.2          23120
grained                4500                6.4          21420

Cernuc                 4200                3.3          25020
fine-grained           4200                7.1          23520

Sampling site     Total
                  effective
                  porosity
                  (%)

Dobren 1             23
coarse-grained       26

Dobren 2             26
medium-              24
grained              23

Plany dul            19
coarse-grained       22

Kobylka              27
fine-grained         24

Hradcany             21
fine-grained         20

Rohliny              32
fine-grained         27

Zaborci              27
medium-              22
grained              25

Cernuc               31
fine-grained         28
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Author:Adamovic, Jiri; Mikulas, Radek; Schweigstillova, Jana; Bohmova, Vlasta
Publication:Acta Geodynamica et Geromaterialia
Article Type:Report
Geographic Code:4EXCZ
Date:Jan 1, 2011
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