Desiccation Polygons Research Articles

Field investigation of dried lakes in western United States as an analogue to desiccation fractures on Mars

AbstractPotential Desiccation Polygons (PDPs), tens to hundreds of meters in size, have been observed in numerous regions on Mars, particularly in ancient (>3 Gyr old) terrains of inferred paleolacustrine/playa geologic setting, and in association with hydrous minerals such as smectites. Therefore, a better understanding of the conditions in which large desiccation polygons form could yield unique insight into the ancient climate on Mars. Many dried lakebeds/playas in western United States display large (>50 m wide) desiccation polygons, which we consider to be analogues for PDPs on Mars. Therefore, we have carried out fieldwork in seven of these dried lakes in San Bernardino and the Death Valley National Park regions complemented with laboratory and spectral analysis of collected samples. Our study shows that the investigated lacustrine/playa sediments have (a) a soil matrix containing ~40–75% clays and fine silt (by volume) where the clay minerals are dominated by illite/muscovite followed by smectite, (b) carbonaceous mineralogy with variable amounts of chloride and sulfate salts, and significantly, (c) roughly similar spectral signatures in the visible‐near‐infrared (VIS‐NIR) range. We conclude that the development of large desiccation fractures is consistent with water table retreat. In addition, the comparison of the mineralogical to the spectral observations further suggests that remote sensing VIS‐NIR spectroscopy has its limitations for detailed characterization of lacustrine/playa deposits. Finally, our results imply that the widespread distribution of PDPs on Mars indicates global or regional climatic transitions from wet conditions to more arid ones making them important candidate sites for future in situ missions.

Surface evolution of salt‐encrusted playas under extreme and continued dryness

AbstractMiocene continental saltpans are scattered in the Central Valley of the Atacama Desert, one of the driest regions on Earth. These evaporitic deposits are hydrologically inactive, and are detached from groundwater brines or aquifers. The surface of the saltpans, also known as salars, comprises desiccation polygons, commonly with nodular salt structures along their sides. The morphology and bulk mineralogy of salt polygons differs between and within salars, and the shape and internal structure of salt nodules varies between different polygon types. Based on field observation, and mineralogy and crystallography data, we generated a conceptual model for the genesis and evolution of these surface features, whereby rare rainfall events are responsible for the transformation of desiccation salt polygons and the initial formation of salt nodules along polygon borders. In addition, frequent, but less intense, deliquescence events further drive the evolution of salt nodules, resulting in a characteristic internal structure that includes laminations, and changes in porosity and crystal morphologies. As a result, and despite the extreme dryness, the surfaces of fossil salars are dynamic on timescales of several years to decades, in response to daily cycles in atmospheric moisture, and also to rare and meager rainfall events. We propose that fossil salars in the Atacama Desert represent an end stage in the evolution of evaporitic deposits under extreme and prolonged dryness. Copyright © 2015 John Wiley & Sons, Ltd.

Potential desiccation cracks on Mars: A synthesis from modeling, analogue-field studies, and global observations

Potential desiccation polygons (PDPs) are polygonal surface patterns that are a common feature in Noachian-to-Hesperian-aged phyllosilicate- and chloride-bearing terrains and have been observed with size scales that range from cm-wide (by current rovers) to 10s of meters-wide. The global distribution of PDPs shows that they share certain traits in terms of morphology and geologic setting that can aid identification and distinction from fracturing patterns caused by other processes. They are mostly associated with sedimentary deposits that display spectral evidence for the presence of Fe/Mg smectites, Al-rich smectites or less commonly kaolinites, carbonates, and sulfates. In addition, PDPs may indicate paleolacustrine environments, which are of high interest for planetary exploration, and their presence implies that the fractured units are rich in smectite minerals that may have been deposited in a standing body of water. A collective synthesis with new data, particularly from the HiRISE camera suggests that desiccation cracks may be more common on the surface of Mars than previously thought. A review of terrestrial research on desiccation processes with emphasis on the theoretical background, field studies, and modeling constraints is presented here as well and shown to be consistent with and relevant to certain polygonal patterns on Mars.

Relative sea-level change, climate, and sequence boundaries: insights from the Kimmeridgian to Berriasian platform carbonates of Mount Salève (E France)

The present study analyses the stratal architecture of the Late Jurassic (Kimmeridgian) to Early Cretaceous (Berriasian) sedimentary succession of Mount Saleve (E France), and four Berriasian stratigraphic intervals containing four sequence-boundary zones reflecting lowering trends of the relative sea-level evolution. Massive Kimmeridgian limestones characterized by the presence of colonial corals appear to be stacked in an aggrading pattern. These non-bedded thick deposits, which are interpreted to have formed in balance between relative sea-level rise and carbonate accumulation, suggest a keep-up transgressive system. Above, well-bedded Tithonian-to-Berriasian peritidal carbonates reflect a general loss of accommodation. These strata are interpreted as a highstand normal-regressive unit. During the early phase of this major normal regression, the vertical repetition of upper intertidal/lower supratidal lithofacies indicates an aggrading depositional system. This is in agreement with an early stage of a highstand phase of relative sea level. The Berriasian sequence-boundary zones investigated (up to 4 m thick) developed under different climatic conditions and correspond to higher-frequency, forced- and normal-regressive stages of relative sea-level changes. According to the classical sequence-stratigraphic principles, these sequence-boundary zones comprise more than one candidate surface for a sequence boundary. Three sequence-boundary zones studied in Early Berriasian rocks lack coarse siliciclastic grains, contain a calcrete crust, as well as marly levels with higher abundances of illite with respect to kaolinite, and exhibit fossilized algal-microbial laminites with desiccation polygons. These sedimentary features are consistent with more arid conditions. A sequence-boundary zone interpreted for the Late Berriasian corresponds to a coal horizon. The strata above and below this coal contain abundant quartz and marly intervals with a higher kaolinite content when compared to the illite content. Accordingly, this Late Berriasian sequence-boundary zone was formed under a more humid climate. The major transgressive–regressive cycle of relative sea level identified and the climate change from more arid to more humid conditions recognized during the Late Berriasian have been reported also from other European basins. Therefore, the Kimmeridgian to Berriasian carbonate succession of Mount Saleve reflects major oceanographic and climatic changes affecting the northern margin of the Alpine Tethys ocean and thus constitutes a reliable comparative example for the analysis of other coeval sedimentary records. In addition, the stratigraphic intervals including sequence-boundary zones characterized in this study constitute potential outcrop analogues for sequence-boundary reflectors mapped on seismic profiles of subsurface peritidal carbonate successions. The detailed sedimentological analyses provided here highlight that on occasions the classical principles of sequence stratigraphy developed on seismic data are difficult to apply in outcrop. A sequence-boundary reflector when seen in outcrop may present successive subaerial exposure surfaces, which formed due to high-frequency sea-level changes that were superimposed on the longer-term trend of relative sea-level fall.

Desiccation mechanism for formation of giant polygons on Earth and intermediate-sized polygons on Mars: Results from a pre-fracture model

We present a pre-fracture incrementally non-linear elastic–hydric model to constrain the models of formation of giant (spacing of 20–300m) desiccation cracks on Earth, and possibly intermediate sized (80 to 350m) polygonal networks of cracks located in many impact craters on Mars which have been interpreted to form by desiccation as opposed to thermal contraction (El Maarry et al., 2010). The results of the model show that tensile stresses rise monotonically with desiccation. However, in soils with diffusivities below 10−4m2/s, it is not possible to generate high enough stresses to cause fracturing. On the other hand, intermediate values between 10−2 and 10−4m2/s create optimum conditions for the formation of cracks at the time scales suggested for the formation of giant desiccation polygons on Earth. Our model clearly shows that for typical diffusivity values of clayey soils with a considerable amount of smectites enough stress can build up to stimulate cracking of various spatial scales. These results corroborate earlier assumptions that giant desiccation polygons on Earth occur through lowering of the water table rather than surface evaporation. Extending the model to Mars shows that soils would crack within similar diffusivity limits but in slightly longer periods of time owing to the lower gravity. Finally, a model for formation of desiccation cracks on Mars is presented that shows that two main conditions need to be fulfilled for desiccation cracks to occur under current martian climatic conditions, namely, that the thermal and soil diffusivity conditions 1) allow for the formation of an unsaturated zone with a considerable thickness while at the same time 2) limiting the growth of the permafrost downward which would disrupt the cracking process.

Tide-dominated Middle Devonian sequence from the northern part of the Holy Cross Mountains (Central Poland)

The dolomitic Wojciechowice Formation distinctly differs from the remaining, mainly shaly Middle Devonian succession in northern part of the Holy Cross Mountains (Central Poland). The upper Member of the Formation (Crystalline Dolostone Mb.), in greater part dolomitized but also containing limestone beds, is composed of shallowing-upward cyclothems well exposed in Skaly quarry in the Bodzentyn syncline. The lower parts of the cyclothems, interpreted as subtidal facies, contain fossils characteristic for restricted environments. They are grouped into two assemblages. The first, with brachiopods (largeBornhardtina andEmanuella), massive stromatoporoids, and subordinate gastropods and amphiporoids is related to a deeper subtidal environment, while the second (mainly amphiporoids, gastropods, ostracodes and calcareous algae) is shallower subtidal. Towards the top of succession the fossil content radically decreases. The upper parts of cyclothems are composed mainly of different types of laminites. In these parts of the section, interpreted as intertidal/supratidal units, stromatolites, desiccation polygons, intraformational breccias, and common bioturbations are present. The whole succession was deposited in a low-energy environment, only intermittently affected by high-energy events. For their most spectacular example of this, aBornhardtina-coquinite, a tempestitic origin is proposed. The interval with cyclic sedimentation studied correlates with the dolomitized lower “Unit I” of the Stromatoporoid-Coral Kowala Formation from the southern part of the Holy Cross Mountains, which exhibits sabkha-type cyclicity. The differences in development of cyclothems in both regions resemble outer and inner part of an extensive platform, and correspond well with basic trends of the Lower-Middle Devonian transgression in the Holy Cross Mountains. The general succession of formations deposited during this process coincides with transgressive events on Johnson's eustatic curve for the Devonian.

Devonian depositional environments in the Darwin Mountains: Marine or non-marine?

The depositional environment of the Devonian Taylor Group has been subject to considerable debate for over 30 years. The debate stems largely from a belief that the abundant and diverse trace fossils represent a marine ichnofauna, whereas sedimentary features, including palaeosols, desiccation polygons and red beds, are more typical of a non-marine setting. The debate is reconciled by a reinterpretation of the trace fossil assemblage which shows that the trace fossils comprise a typical fresh water (Scoyenia ichnofacies) assemblage, and their occurrence in the Taylor Group in the Darwin Glacier area is entirely consistent with deposition in a mixed fluvial-lacustrine-subaerial environment.

The Mediterranean salinity crisis: Alternative hypotheses

Many ancient salt deposits share characteristics quite different from those observed in salts deposited in Recent coastal salinas and sabkahs: their vast areal extent (up to 5 million km2) and great thickness of contained salt (exceeding 2,000 meters in some cases) in particular. They may exhibit features developed in littoral and supra-tidal saline environments which have been cited as evidence of a shallow origin for the saline giants: laterally-persistent, thin (annual?) laminae, poikiolitic and displacive (nodular) gypsum and anhydrite, chevron crystals, “satin-spar” veinlets, desiccation polygons. However, conditions which aresufficient may not benecessary for the development of a sedimentary feature; to be rigorously interpreted the conditions must be both. Other features of the saline giants suggest a depositional environment unlike any known today. The salts appear to have accumulated very rapidly (1 mm to 1 decimeter/year) in sediment-starved rift or intra-cratonal basins several hundred meters deep. Water depth, at least during part of the depositional phase, approximated the depth of the basin, and surface water was of normal salinity or brackish during intervals. Basin margins are typically ornamented by bioherms and thin platform sediments, stratigraphically high above time-equivalent salts of the basin center.

Subaqueous shrinkage cracks in the Devonian of Scotland reinterpreted

Abundant lenticular, straight to curving cracks on bedding planes are commonly interpreted as subaqueously formed shrinkage cracks (syneresis cracks), the two most influential examples being those in the Green River Formation (Picard 1966) and the Devonian Orcadian Basin of Scotland (Donovan and Foster 1972). Cracked surfaces in the Orcadian Basin deposits show all gradations between lenticular cracks and well developed polygons of subaerial origin. Some lenticular cracks are inferred to have formed during subaerial desiccation. Many of the simplest lenticular cracks also have the shapes of gypsum crystals and originated as pseudomorphs. The gypsum crystals were often important in providing nuclei for later desiccation cracks. More complex crack patterns of incipient to fully formed desiccation polygons have arisen from desiccation of surfaces containing gypsum crystals or pseudomorphs resulting from gypsum crystal dissolution. Cracks of all types are preserved by sand infills introduced by wind transport across dried-up lake floors. Reassessment of the literature reporting subaqueous cracks in rocks shows that many of these cracks are likely to have originated in a similar way, partly as gypsum crystals, and partly by limited subaerial desiccation. The remaining examples have either already been reinterpreted in other ways, or are insufficiently documented to rule out a subaerial origin. There is no clear evidence for subaqueously formed cracks being preserved in rocks.

Middle Ordovician (Whiterockian) trilobites from the Sunblood Formation, District of Mackenzie, Canada

In the South Nahanni River area of the District of Mackenzie, the Middle Ordovician Sunblood Formation comprises mainly limestones and dolostones of intertidal and shallow-subtidal origin, as indicated by the presence of desiccation polygons, fenestral fabric, and oncolites. Faunas of well-preserved, silicified trilobites from a low-diversity, nearshore, Bathyurus-dominated biofacies are compositionally distinct from faunas in correlative strata around North America that represent different shelf to upperslope biofacies. A temporal biostratigraphy applicable to nearshore biofacies through much of the Whiterockian Series consists of five zones, in ascending order: Bathyurus mackenziensis, B. sunbloodensis, B. margareti, B. nevadensis, and B. granulosus. Twenty-six species are assigned to 18 genera, of which Ludvigsenella is new. Three new species of Bathyurus are B. mackenziensis, B. sunbloodensis, and B. margareti.

Trace fossils as paleoenvironmental indicators in the Taylor group (Devonian) of Antarctica

The Taylor Group of southern Victoria Land, Antarctica contains varied and abundant trace fossils, which have been interpreted by palaeontologists as indicating marine deposition. However, sedimentologists have largely favoured non-marine deposition on the basis of desiccation polygons, fining upward cycles, unidirectional paleocurrents, small channels, red beds and paleosols. The debate is reconciled by a reinterpretation of the trace fossil assemblage. It is shown that widespread and well preserved Cruziana, Diplichnites, Rusophycus and Skolithos which have previously been used as marine indicators comprise an ichnofacies ( Scoyenia) which is entirely typical of fresh water.

Giant desiccation fissures filled with calcareous eolian sand, Hermosa Formation (Pennsylvanian), southeastern Utah

At two stratigraphic intervals within the upper member of the Upper Pennsylvanian Hermosa Formation, calcareous eolian sand fills downward-tapering fissures that are as much as 18 cm wide and 5.7 m deep. Fissure fillings define orthogonal polygons 10 m or more in diameter. One of the host beds is primarily composed of subtidally deposited limestone, the other is a thinly laminated, nonmarine red siltstone. Both systems of fissure fillings are directly overlain and underlain by large-scale cross-stratified, calcareous eolianites. The limestone host bed contains chert pseudomorphs after gypsum. Compaction of host rocks contorted fissure fillings and caused doming of eolian strata over each fissure. Platy mineral grains in fissure fillings are aligned subparallel to bedding in the host rocks, supporting the view that the fissures were passively filled rather than forcefully injected. These ancient fissure systems are similar in scale and pattern to those that define giant desiccation polygons in numerous Great Basin playas. The Pennsylvanian fissures, like their Holocene counterparts, probably formed when groundwater tables dropped from shallow levels within fine-grained, impermeable deposits into underlying aquifers, greatly decreasing the extent of the capillary fringe. Our study of the fissures and host rocks supports the hypothesis that carbonate grains within the eolianites were deflated from uncemented marine sediments that were broadly exposed during regressive intervals.

Nearest-Neighbour Analysis: A Technique for Quantitative Evaluation of Polygonal Ground Patterns

Nearest-neighbour analysis can be applied to polygonal ground patterns to give a quantitative evaluation of the pattern. The nearest-neighbour statistic (R) indicates the degree to which an observation departs from an expected random pattern. The R-statistic is particularly useful in comparing patterns by highlighting differences that are not subjectively obvious. Because the nearest-neighbour technique eliminates the effect of scale, it can be used to compare a variety of patterns, including ice-wedge and desiccation polygons, lava cooling cracks, and tectonic tractures. Nearest-neighbour analysis can also be applied to the large-scale polygonal ground on Mars and other planetary bodies. points to evaluate the randomness or regularity of the pattern. The nearest-neighbour procedure has never been used to describe polygonal ground. Besides describing the patterned ground, nearestneighbour analysis may also be useful in understanding the climate, material, and processes that formed the pattern.

On the curl of desiccation polygons

Two qualitative mechanisms, differential water loss and differential contraction because of a textural gradient (normal or inverse), have been advanced in the literature to explain the curvature of desiccation crack polygons. A geometrical model is described which shows how these mechanisms operate to produce either concave-up or convex-up polygons, depending on the direction of the gradient of water loss or shrinkage capacity. The model shows that the angle subtended by the edges of a dried polygon increases with the ratio of the width to the thickness of the polygon, thus explaining the thin, tightly enrolled mud curls widely reported from fluvial environments. It also reveals that the radius of curvature of a curled polygon increases with the initial thickness of the dried layer, but declines with the differential shortening across it. Calculations based on the model are compatible with field and laboratory observations.

Fine structure of the stratified microbial community at Laguna Figueroa, Baja California, Mexico. I. Methods of in situ study of the laminated sediments

Abstract The microbial community of the flat laminated sediments in the hypersaline lagoon at Laguna Figueroa has undergone dramatic changes due to prolonged periods of submergence under fresh water. This paper describes the techniques used in the first comprehensive in situ ultrastructural study of a stratified microbial community formerly dominated by Microcoleus sp. and introduces the changes caused by flooding and burial. Flat mat desiccation polygons fixed in 2.5% buffered glutaraldehyde were studied by light and transmission electron microscopy. Typical pre-flood Microcoleus -dominated laminated mat from 1977 was compared to post-flood laminated mat from 1979. The technique faithfully preserves the various laminae with their associated organisms and sediment. Morphological identification of several genera of photosynthetic prokaryotes including cyanobacteria, purple, and green bacteria was made by analysis of cell membrane and wall structure. Amoebic cysts could also be recognized in situ. Bacteria of unknown generic affinity, but with distinctive ultrastructure were discovered. A filamentous purple photosynthetic bacterium was found to be a common component of the 1979 mat and two bacteria within a bacterium associations were seen in sections from the 1977 mat.

Strain Measurements in Eastern Marginal Shear Zone of Mount Sedom Salt Diapir, Israel

The Mount Sedom salt diapir, of Pliocene-Pleistocene age, is a vertically layered salt wall more than 2,000 m thick trending north-south parallel with the strike of the layers. The diapir head is cut by a dissolution salt table overlain by a residual caprock. Deformation features include, at greater depths, rotated boudins, stretched desiccation polygons, and fracture cleavage formed through flowage of the salt, and, at shallow depths, folded rock salt beds, folded dolomite-shale beds, and bedding-plane slip. Measurements in the eastern marginal shear zone indicate an extensional-shear distortion of 2.1--1.7 of ductile shear (probably at depth, in an early stage) recorded in the rock salt beds, plus 0.4 of slip shear recorded as displacements of the salt table at a shallow, later stage. The strain in this marginal area is considerably larger than the average for the main, inner part of the diapir. The average rate of rise for the whole diapir is 3 to 4 mm/year since the late(?) Pleistocene.

The geology of the Viking Lander 2 site

Viking Lander 2 landed on a flat plain of fine-grained sediment overlain by dispersed, evenly distributed boulders. The fine-grained material is probably part of a high-latitude mantle comprising material swept south from the polar regions. The boulders, which have distinctive deep pits, or vesicles, may be the residue of an ejecta deposit from the crater Mie. Alternatively, they may be the remnants of lava flows which formerly covered the region. Polygonal sediment-filled cracks may have been formed by ice wedging, similar to the process that occurs in terrestrial permafrost regions. Alternatively, they may be desiccation polygons.

Environmental Significance of Certain Yellow-Brown Dolomites, Great Basin, California and Nevada: ABSTRACT

Within the dominantly gray limestone and dolomite sequences of the Great Basin, sporadic yellow-brown dolomite is present as discrete beds, lentils, or wisps--the latter imparting a mottled or pseudobrecciated appearance. These distinct deposits, studied in Ordovician, Devonian, Pennsylvanian, and Triassic formations, are fine grained, are usually unfossiliferous, and posses a less than 5 percent terrigenous fraction of well-rounded quartz and sparse feldspar. Laminations, desiccation polygons, and possible relict bird's-eye structures also are apparent. Stratigraphically, these deposits commonly overlie dolomite sequences several hundreds of feet thick and are in turn overlain by limestone sequences of varied thicknesses. The persistent occurrence of these yellow-brown dolomites at the top of dolomitized sequences possibly indicates that the yellow-brown lithology represents supratidal or penecontemporaneous dolomitization from which seepage-refluxion brines ultimately descend. The underlying rocks commonly are coarser grained, contain normal marine fossils, and reveal gravity-controlled dolomitization structures. In some cases where yellow-brown deposits are incorporated in a completely dolomitized interval, the beds below the lowest yellow-brown horizon reveal multiple dolomitization. End_of_Article - Last_Page 770------------

Stratigraphy of Chaffee Group (Upper Devonian), West-Central Colorado

During the Late Devonian, sediments deposited in shallow marine, brackish, and tidal mud-flat environments accumulated in west-central Colorado. This sedimentary sequence comprises the Chaffee Group, which is divided into two formations; the lower third, the Parting Formation, consists predominantly of detrital rocks; the upper two thirds of the group, the Dyer Formation, is composed of carbonate rocks. The Parting Formation is divided into three stratigraphic units, designated A, B, and C, in ascending order. Unit A is chiefly quartzose sandstone. Unit B is composed of four persistent beds which, in ascending order, are shale, thin dolomite, shale, and sandy dolomite or dolomitic sandstone that contains dolomite pebbles. Unit C consists of lenses and beds of quartzose sandstone, sandy shale, and sandy dolomite. On the east and northeast all three units change in facies to quartzose sandstone. The Parting Formation is principally a transgressive sequence, but the middle dolomite bed and the top bed of unit B represent regressive phases. The source for the detrital sediments was a crystalline highland flanked by sedimentary rocks, located east and northeast of the study area. This highland persisted throughout Chaffee deposition, but yielded more detritus to the Parting Formation. The maximum advance of the Devonian sea occurred during upper Parting and lower Dyer deposition. The Dyer Formation is a regressive carbonate sequence that can be divided into two carbonate members. The lower member consists of dark-gray, dolomitic, fossiliferous limestone, and herein is named the Broken Rib Member. The upper member consists of light-gray dense, stromatolitic dolomite, and herein is named the Coffee Pot Member. The Broken Rib Member apparently reflects deposition in a sublittoral environment. The stromatolites of the Coffee Pot Member are interpreted as having formed by the trapping and binding action of blue-green algae, which grew in the littoral zone. Intraformational breccias formed by the disruptive action of waves on desiccation polygons are associated with the stromatolites.

Inorganic bedded cherts from the Magadi area, Kenya

The High Magadi beds can be divided into a lower and an upper sequence, separated by a bed rich in Tilapia nilotica remains. The lower sequence contains one or more magadiite horizons. Through digging of trenches it has been possible to establish that the cherts of the “Chert Series” are stratigraphically equivalent to the magadiite horizons. The transition magadiite-chert has been observed in many places and may occur over a distance of less than 50 cm. It is accomplished by removal of sodium by percolating waters. The resulting bedded chert may preserve the fine lamination present in magadiite. The conversion is accompanied by many textural and structural features such as large desiccation polygons, buckling, reticulation, extrusion, casts of mud-cracks and calcite.