Ptygmatic folds in evaporite strata

New Brunswick, eastern Canada

EV-1 [132 kb]

Fig. 1: A large sample displaying small-scale plastic deformation in Carboniferous evaporite sedimentary layers from southeastern New Brunswick. This spectacular sawn slice measures 13 x 47 x 7 cm. In this gypsum-rich evaporite, the mineralogy and water content are factors in the development of spectacular ptygmatic folds in the beautifully delineated strata. Royal Ontario Museum sample ROMES1720.

"Rock of the Month # 262, posted for April 2023" ---


Regional geology

The sample is from Smith's Quarry, Hillsborough, Albert county, south of Moncton in southeast New Brunswick, Canada. Here we will look at the local geology of this region, the mineralogy of the evaporite sediments, and then try to explain the spectacular fold styles! Evaporites are sedimentary deposits that literally form by the evaporation of sea water, in shallow tidal or supratidal settings, leaving behind sea salt (sodium chloride, halite) and other salts of potassium, magnesium, calcium, boron and other elements. This makes for an unusual "rock of the month", so some definitions and other background information will be provided as we look into this.

The complex geology of Maritime Canada includes its own share of industrial mineral deposits, such as the gypsum deposits of the Hillsborough area of New Brunswick (Atlantic Geoscience Society, 1985). Carboniferous rocks in southeast New Brunswick host deposits of oil shale, natural gas and petroleum, rock salt and coal (Wright, 1922). New Brunswick of course has seen other mines and quarries, from grindstones and granite to the massive sulphide base-metal deposits targeted by the 1953 Bathurst staking rush (Martin, 2003). Mineral specimens of diverse species have long been collected in the province, such as gypsum and its fibrous variety selenite, calcite and dolomite, agate and zeolites (Sabina, 1972).

Key industrial minerals, used especially for fertilizer production, are mined in southern New Brunswick. Repeated evaporation of seawater in areas of restricted circulation led to the buildup of impressive cycles of sediments in the ideal sequence limestone- gypsum- salt- potash (Atlantic Geoscience Society, 2001, pp.105-108, 158; and 2022, pp.127-130, 138-142). Potash is produced from several mines in the region: potash is an ore found in evaporite sediments, containing potassium in water-soluble form. Canada is the largest producer of potash in the world. Sylvinite is a sedimentary rock which is a key form of potash, a mixture of the principal minerals of potash, namely the chlorides sylvite (KCl, tastes bitter, ewww) and halite (NaCl, common salt, tastes like salt!). A third is carnallite, KMgCl3.6H2O

The Carboniferous (Visean, middle Mississippian, circa 340 Ma) Windsor Group of the Moncton basin was laid down within the much larger Maritime Basin of the Appalachians. The strata comprise a thick sequence of limestone, mudstone and conglomerate, anhydrite, halite and sylvinite (Wilson et al., 2006). The marine evaporite deposits, seen in detail, contain a wealth of unusual mineral species, summarized below, though the rarities are not sufficiently abundant to have a bearing on the physical properties of the rock, and thus the folding examined here.

Structural aspects

The region has long been noted for the unique tectonic styles associated with rocks that behave as slow-moving fluids under moderate temperatures and pressures, exemplified by salt domes (van de Poll, 1972). The local strata underwent deformation in the late Bashkirian (lower Pennsylvanian, circa 320 Ma). The Penobsquis salt structure, northeast of Sussex, began as a salt anticline which deformed into a "salt wall", "separated from underlying rocks by a mylonitic detachment horizon" (Wilson et al., 2006). Faults and shear zones also developed above the salt wall.

Roberts and Williams (1993) describe a high grade (28% K2O) sylvinite orebody in the Clover Hill area near Sussex that has undergone intense internal folding. The folds vary in size from cm to hundreds of m and are mostly tight to isoclinal. The ductile deformation can be compared to the intense shearing of stronger rocks in mylonite zones. Much of the area has poor exposure, but Wilson et al. (2006) used drill core and geophysical surveys to study the regional structures. The Moncton basin has numerous northeast- to east-trending faults, terrane boundaries during the assembly of the Appalachian orogen.

Our display sample..... is a beauty, but what is going on? Let's take a quick guess, then look for an informed opinion! Looking at the close-ups (Figs. 2 and 3) it is clear that the thickest (circa 2 cm) white layer has undergone minor brittle fracture at some time (perhaps as recently as its extraction from the quarry), but has stayed intact. It has barely bent or folded at all: massive gypsum or anhydrite (?). In contrast, white layers that are scant mm thick are interbedded with equally thin bands of darker (?) clay and quartz -rich silt, and these have undergone spectacular folding. The large structures developed in the evaporite-bearing basin attest to deformation typical of hard-rock basement rocks under more intense metamorphic and tectonic conditions. The close-up views of the small-scale folds are perhaps the result of dextral shear acting upon alternating layers of soft evaporite minerals (like gypsum) and the darker layers of silty, perhaps water-saturated clastic sediment. A close look at the apices (culminations) of the little folds suggests that the white and dark layers have deformed in subtly different ways. By all means, don't believe a word!

I have yet to find an exact analogy in a structural geology work, though I'm sure they exist. Roberts and Williams (1993), working on that rich sylvinite body (Potacan) near Sussex, noted that it had undergone intense internal folding, with ductile drag folds in both sylvinite and halite lithologies, and with development of penetrative slickensides. Folds similar to our specimen have been produced in lab experiments, and occur in harder rocks, such as metasediments (Ramsay and Huber, 1983, pp.8,29). The little folds resemble drag folds (secondary or parasitic folds, developed about larger folds, like the pleated surface of a roll of corrugated cardboard), "produced in the weaker beds by the differential movements of the more resistant rocks above and below them" (Gilbert Wilson, 1982, pp.80-85). Most probably, I suspect, the structures we see were formed some millions of years after the chemical sediments were precipitated, in response to the loading of overlying sediments and subsequent tectonic movements.

A final thought. The intricate little folds are also reminiscent of soft sediment deformation, which can often be witnessed along beaches and river banks, where the sandy or muddy recent sediment is as yet unconsolidated. One wonders whether any of the folding could have occurred prior to the lithifaction of the evaporites and their host strata. Would possible volume changes as the sulphates (de)hydrate, perhaps in response to seasonal changes in temperature and moisture, make a difference, and leave any aspect of the sample we see today?

EV-2 [251 kb]

Fig. 2: A close-up view, showing tight folds just left of centre of the large slab.


Miller Museum, Queen's University, Kingston, Ontario, October 2017

The Miller Museum features a lovely mixture of displays old and new (as in, suited to the early 20th and early 21st centuries, respectively!). In the first category is a case of gypsum, showing some vials filled with evaporites of different colours. The samples are from the Osman quarry, Hillsborough, Albert county. Gypsum is hydrated calcium sulphate, noted as being very soft (Mohs hardness 2, meaning it can be scratched with a finger nail). Heating of finely ground gypsum at 65°C drives off 75% of the bound H2O in gypsum to form plaster of Paris. When water is added to the latter it rehydrates and becomes pliable, allowing the plaster to be moulded before it sets.

A closer look at calcium sulphate. Gypsum is monoclinic CaSO4.2H2O, specific gravity 2.32. Varietal names are alabaster (massive), selenite (clear crystals) and satin spar (fibrous). Dehydration renders the sulphate slightly harder and notably denser. Seldom reported, bassanite is monoclinic CaSO4.0.5H2O, specific gravity 2.73.Complete dehydration yields anhydrite, orthorhombic CaSO4, specific gravity 2.98.

EV-3 [305 kb]

Fig. 3: Another close-up view, showing more open folds at the right-hand end of the slab. Are these then ptygmatic folds, in which the folding layer behaves in a viscous manner between two slabs of more competent rock (?). In the "hard rock" world, such folds occur when a relatively fluid material, such as a granitic melt formed in metamorphism (anatexis) intrudes a solid host such as a metasediment.


The evaporites are preserved in a region trending WSW from Hillsborough past Sussex and Penobsquis to Salt Springs (south of a line from Moncton westward to Fredericton, actually closer to a line running southwest from Moncton to Saint John on the north shore of the Bay of Fundy). The evaporites contain many unusual minerals, besides the predominant species such as gypsum, anhydrite, halite and sylvite. Prominent amongst these minerals are borates. as described in the Moncton sub basin (e.g., Burns et al., 1992). The Penobsquis potash mine offers other minerals, from everyday to rare (Roberts et al., 1993; Grice et al., 2002, 2005). The marine strata host a veritable zoo of odd minerals: hydroboracite, hilgardite, volkovskite, trembathite, kurgantaite, pringleite and ruitenbergite, strontioginorite, boracite, congolite, colemanite, chambersite, walkerite, veatchite, szaibelyite, danburite and penobsquisite, to name but seventeen! Each of these minerals is a borate, many but not all of which have some essential H2O in their formulae. Each borate contains one to three essential metals, such as Ca, Mg or K and/or Fe, Sr, Mn and Li. Penobsquis, Salt Springs, Sussex: all New Brunswick names associated with potash mining, and type localities for some of the borates. Curiously, there is a borate called sussexite, but it is named for a Sussex county in New Jersey! The chemistry of the marine brines from which all these salts were deposited in Carboniferous time can be examined in the captive fluid found in bubbles in the halite, fluid inclusions (Petrychenko et al., 2002).


Thanks to Katherine Dunnell for digging up the scant record of this remarkable specimen, and to Fran Manns for playing devil's advocate on soft sediments.


Atlantic Geoscience Society (1985) Geological Highway Map of New Brunswick and Prince Edward Island. Atlantic Geoscience Society Spec.Publ. 2 (1:638,000.

Atlantic Geoscience Society (2001) The Last Billion Years. A Geological History of the Maritime Provinces of Canada. Edited by R.A. Fensome and G.L. Williams. Atlantic Geoscience Society / Nimbus Publishing Limited, Halifax, 212pp.

Atlantic Geoscience Society (2022) The Last Billion Years. A Geological History of the Maritime Provinces of Canada. 2nd edition. Edited by R.A. Fensome and G.L. Williams. Atlantic Geoscience Society / Nimbus Publishing Limited, Halifax, 260pp.

Burns,PC, Hawthorne,FC and Stirling,JAR (1992) Trembathite, (Mg,Fe)3B7O13Cl, a new borate mineral from the Salt Springs potash deposit, Sussex, New Brunswick. Can.Mineral. 30, 445-448.

Grice,JD, Gault,RA and Van Velthuizen,J (2005) Borate minerals of the Penobsquis and Millstream deposits, southern New Brunswick, Canada. Can.Mineral. 43, 1469-1487.

Grice,JD, Gault,RA, Van Velthuizen,J and Pratt,A (2002) Walkerite, a new borate mineral species in an evaporitic sequence from Sussex, New Brunswick, Canada. Can.Mineral. 40, 1675-1686.

Martin,GL (2003) Gesner's Dream: The Trials and Triumphs of Early Mining in New Brunswick. CIMM, Brunswick branch, 328pp.

Petrychenko,O, Peryt,TM and Roulston,B (2002) Seawater composition during deposition of Visean evaporites in the Moncton subbasin of New Brunswick as inferred from the fluid inclusion study of halite. CJES 39, 157-167.

Ramsay,JG and Huber,MI (1983) The Techniques of Modern Structural Geology. Volume 1: Strain Analysis. Academic Press, Inc., 307pp.

Roberts,AC, Stirling,JAR, Grice,JD, Burns,PC, Roulston,BC, Curtis,JD and Jambor,JL (1993) Pringleite and ruitenbergite, polymorphs of Ca9B26O34(OH)24Cl4.13H2O, two new mineral species from Sussex, New Brunswick. Can.Mineral. 31, 795-800.

Roberts,W and Williams,PF (1993) Evidence for early Mesozoic extensional faulting in Carboniferous rocks, southern New Brunswick, Canada. CJES 30, 1324-1331.

Sabina,AP (1972) Rock and Mineral Collecting in Canada, Part III, New Brunswick, Nova Scotia, Prince Edward Island and Newfoundland. GSC Misc.Rep. 8 (revised version of 1964 edition), 106pp.

van de Poll,HW (1972) Stratigraphy and economic geology of Carboniferous basins in the Maritime provinces. IGC 24, Montreal, Excursion Guidebook A60, 96pp.

Wilson,P, White,JC and Roulston,BV (2006) Structural geology of the Penobsquis salt structure: late Bashkirian inversion tectonics in the Moncton basin, New Brunswick, eastern Canada. CJES 43, 405-419.

Wilson,G (1982) Introduction to Small-Scale Geological Structures. George Allen & Unwin, London, 128pp.

Wright,WJ (1922) Geology of the Moncton map area. GSC Memoir 129, 69pp. plus 4 maps.

Graham Wilson, draft, 11,16-17 February 2023, updates 15,20,30 March 2023

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