Subsurface seismic record of salt glaciers in an extensional intracontinental setting (Late Triassic of northwestern Germany)

Markus Mohr* Geological Institute, RWTH Aachen University, Wüllnerstrasse 2, 52056 Aachen, Germany John K. Warren Shell Chair in Carbonate Studies, Sultan Qaboos University, Muscat, Oman Peter A. Kukla Janos L. Urai Geological Institute, RWTH Aachen University, Wüllnerstrasse 2, 52056 Aachen, Germany Anton Irmen Gaz de France Produktion Exploration Deutschland GmbH, Lingen (Ems), Germany

ABSTRACT In the Northwest German Basin of Central Europe, Late Triassic interaction of normal faulting and salt diapirism during regional extension in subsalt basement locally initiated lat- eral fl ow of surface-piercing salt in namakiers (salt glaciers). Using seismic sections and vari- ance attribute maps derived from high-resolution three-dimensional seismic data, we show that when a syndepositional fault cuts a near-emergent diapir crest, the caprock carapace was breached, opening a pathway for salt extrusion. The fault escarpment and the adjacent fault-induced depression allowed focused gravity-driven downward fl ow of salt across the land surface (a namakier) and its subsequent preservation and encasement in continental (arid redbed) sediments. Geodynamically there is an apparent distinction between the com- pressional setting of modern namakiers in the arid deserts of Iran and the fault-intersected extensional setting of stacked Keuper namakiers. Stacked namakiers preserved in thicknesses that are seismically resolvable are interpreted to indicate hyperarid conditions in Keuper time. The climate was typical of the highly continental Late Triassic Pangaean supercontinent as it rifted and sagged to form the incipient Atlantic Ocean.

Keywords: salt glacier, diapirs, Northwest German Basin, continental redbeds, extension.

INTRODUCTION seismic data, we present the fi rst (to our knowl- The salt structures are arranged in a series of Extrusions of diapiric salt onto continental edge) documentation of a subsurface example of elongated NNW-trending diapirs, spaced at and marine basins have been described by a a tiered set of namakiers in an extensional ter- intervals of ~10 km (Fig. 1B). number of authors (see GSA Data Repository restrial (redbed) setting. It is the preserved rem- Late Paleozoic sedimentation in the east Frisia Table DR11). Active salt glaciers (namakiers) are nant of a number of ancient salt glaciers extruded area (Fig. 1C) deposited upper Rotliegend sedi- spectacularly exposed on the fl anks salt moun- onto a Late Triassic redbed landscape within the ments in a series of playa mudfl at–sandfl at set- tains as high as 300 m in the Zagros fold belt subsiding Triassic Northwest German Basin. tings in an extensional regime. Approximately of onshore Iran in a system dominated by tec- The structural regime in the Late Trias- 800 m of bedded sulfate and halite were depos- tonic shortening. Today, outside of the Dead Sea sic of the intracontinental Northwest German ited in this area during the hydrographic isolation depression, there are no terrestrial namakiers in Basin of Central and northern Europe refl ects and drawdown of the Late Permian Zechstein regions of extensional tectonics (see the footnote incipient North Atlantic rifting and extension evaporite basin (Jaritz, 1973; Warren, 2006). of Table DR1). Even in the Dead Sea depression, that locally formed graben structures (Ziegler, Triassic sedimentation changed from continental the tectonic setting is not regional extension; 1990). Rifting in the study area triggered salt clastics during the Early Triassic Buntsandstein rather, it is a local-scale rapidly subsiding tran- movement and near-surface responses, includ- to shallow-marine sequences of carbonates and stensional pull-apart basin situated on a regional ing reactive, active, and passive diapirism evaporites in the Middle Triassic Muschelkalk, transform fault (Al-Zoubi and Ten Brink, 2001). (Mohr et al., 2005). The depositional surface to continental clastics with intercalated saline- In this paper, using cross sections and variance in Late Triassic (Keuper) time was made up pan evaporites during Keuper time. attribute analysis from three-dimensional (3D) of a series of salt-fi lled low-relief playas and Several phases of Triassic rifting infl uenced pans surrounded by desert redbeds, salt-fi lled the area and triggered its multiphase salt tec- *Current address: RWE Dea AG, Überseering 40, domes, and occasional namakiers. tonics (Figs. 1D, 1E). Kinematic restoration 22297 Hamburg, Germany. modeling shows that decoupled extension GEOLOGICAL EVOLUTION OF in the late Early (middle Buntsandstein) and 1GSA Data Repository item 2007240, Table DR1, THE EAST FRISIA AREA Middle Triassic (middle Muschelkalk) initi- summary table of selected worldwide salt extrusion The investigated salt structure is one of a ated salt diapirism and lateral salt fl ow with occurrences, is available online at www.geosociety. org/pubs/ft2007.htm, or on request from editing@ number of similar structures in the east Frisia growth of a pillow-like salt structure (Mohr geosociety.org or Documents Secretary, GSA, P.O. area of northwestern Germany, located near et al., 2005). Subsequent formation of a base- Box 9140, Boulder, CO 80301, USA. the Germany-Netherlands border (Fig. 1A). ment graben in the early Late Triassic triggered

© 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, November November 2007; 2007 v. 35; no. 11; p. 963–966; doi: 10.1130/G23378A.1; 3 fi gures; Data Repository item 2007240. 963 Figure 1. A: Location of A C Stratigraphy Map / section units D intersection 1km WET/Q line with E study area (inset B) in 1km 56° N N TERTIARY- T/Q scale for east Frisia, northwestern Baltic Sea QUATERNARY D and E K Germany, near Germany- North Sea Fig. 3C Netherlands border. B: CRETACEOUS K koE-J kmA Distribution and spacing Hamburg kmS-kmW of salt diapirs and salt B JURASSIC J kmS-kmW 53° N kmG pillows and/or anticlines - Berlin Up. Exter-Fm., koE GERMANY kuE DS M in investigated area. C: Z Arnstadt-Fm., kmA SU/SM

Hanover e SO Simplifi ed stratigraphy of LAN

NETHER

per Lat Weser- & Stuttgart- Z study area. D: Geological 0 100 km Md. 7° E 11° E Fm., kmS-kmW R interpretation of seismic Keu B W-E section through the 7° E Grabfeld-Fm., kmG E intersection N ASSIC N line with D T/Q S salt diapir and the periph- North Sea Lo. Erfurt-Fm. , kuE

TRI

eral sink. Rectangle indi- dle Muschel- Fig. 3D K Mid M cates location of enlarged kalk seismic sections (see East koE-J Bunt- SO Z kmA Frisia y Figs. 3C, 3D). E: Inter- ry sandstein SU/SM kmS-kmW pretation of N-S seismic tua Earl es kmG s Zech- section perpendicular to NL GERMANY Z M Em stein M kuE SO

MIAN D showing important Late SU/SM extensional fault and = Salt diapir Fig. 3A & B Rot- SO PER R = Salt pillows / 01020 km liegend Z well A linked salt structures in anticlines E. R its northern part. Vertical dotted line indicates intersection of the two profi les. reactive diapirism and the collapse of the salt creep to solution-precipitation creep (Urai et al., of the namakiers is a function of the rates of pillow followed by passive diapirism, during 1986; Schléder and Urai, 2007). extrusion, lateral fl ow, and solution by rain water which salt remained at or near the surface, Salt extrusion to the surface can occur once or (Kent, 1958; Talbot and Alavi, 1996). Many feeding peripheral salt glaciers. The extrusion many times in the history of a sedimentary basin fl ows are sourced on anticlinal fl anks rather than process stopped in the latest Late Triassic and and can be driven by extension, sediment load- anticline crests (Table DR1). Jurassic as the falling diapir crest was covered ing, or compression. The loci of most intense In extensional salt provinces, salt allochthons by marine sediments (Mohr et al., 2005). salt fl ow can thus migrate across a basin over and namakiers may form during the active to A major unconformity at the base of the Cre- time. The study area (Fig. 1A) represents one of passive diapir phases, where erosion and nor- taceous (Figs. 1D, 1E) marks a subsequent epi- the cases where salt extrusions can be tied to a mal faulting thin the overburden at the diapir’s sode of regional uplift and tilting in the Middle local extensional faulting regime combined with culmination. Gravity shedding and faulting of Jurassic–Early Cretaceous, accompanied by erosional thinning at or near the crest of a grow- thinned overburden then allow caprock frag- erosion and salt dissolution. Late Cretaceous– ing salt-fi lled structure. Neoproterozoic salt con- ments and salt to fl ow onto the land surface. early Tertiary compressional tectonics renewed stitutes the salt source in the namakiers of the Outer edges of namakiers expand and con- vertical salt movement under substantial sedi- Dashti and Hormazgan provinces in the Zagros tract via an interplay between rates of salt sup- mentary cover (Mohr et al., 2005). fold belt of Iran (Fig. 2), while Miocene salt cre- ply, external sediment supply (loading), and ates active namakiers in a redbed landscape near salt dissolution (Wenkert, 1979). While a salt NAMAKIERS: ANALOGS SUPPLY THE Qom, central Iran. Flow rates are much faster tongue is at the surface it is continually subject INTERPRETIVE FRAMEWORK during periods of rainfall, and the morphology to damming behind outcrop ridges, dissolution, The geometry, evolution, structural framework, and depositional environment of the Triassic salt glacier show similarities to modern continental Iranian namakiers (extrusive salt glaciers) on the fl anks of salt-cored mountains in an overthrust and wrench-faulted setting and to extension- driven Neoproterozoic “Christmas tree” diapirs that now crop out as salt ablation (allochthon) breccia and rauhwacke in the Flinders Ranges of South Australia (Dyson, 2004). Namakiers, or salt glaciers, are surface halo- kinetic features that can range in size from hundreds of meters to tens of kilometers across and in shape from local rugose wing-like expan- sions of a diapir stem, to mushroom-shaped lateral extensions of diapirs, to extensive sheets that coalesce into stratiform salt canopies, tiers, or layers. Namakiers creep downslope under very low shear stress after dramatic weakening Figure 2. Surface geology (left) and Landsat image (right) of Kuk-e-Namak (Dashti), Iran, 28°16′N, 51°42′E. Faults intersecting the anticlinal dome facilitated salt breakout and forma- of the salt by the processes of recrystallization tion of surface salt glacier, or namakier. Black arrows indicate bedding dip and azimuth in and grain refi nement, and associated changes anticlinal beds (geology after Kent, 1958; Landsat image courtesy of National Aeronautics in deformation mechanisms, from dislocation and Space Administration).

964 GEOLOGY, November 2007 and reworking, and it can supply highly saline fan-like salt apron that covers older sedimentary dissolution carapace. The lenticular cross sec- groundwater to salt pans in playa and pans atop layers (Fig. 3A). The tongue of the buried extru- tion of the salt glacier is clearly traceable through adjacent rim synclines (zones of salt defl ation). sion has a jagged outline and an irregular rugged the entire 3D seismic volume and can be distin- Such groundwater-driven reworking of halite surface with a divergent lineation from east to guished from the fl anking sedimentary layers. from the crests of growing salt structures into west. This contrasts with the smooth surfaces of Three previous salt extrusions of middle peripheral halite pans is seen in the distribu- the adjacent clastic sedimentary strata. The extru- Keuper age are identifi ed in seismic data from tion of several evaporite generations in the sion is on the western fl ank of a NNE-trending the Weser and Arnstadt Formations, and indicate Ravar basin of eastern Iran (Stocklin, 1961, in salt diapir, which has high variance contrast. classic “Christmas-tree” morphologies (Figs. 1D, Jackson et al., 1990) and in the distribution of For detailed interpretation of the variance 1E, 3C, and 3D). The seismic profi les show salt potash ores in ancient pans about the periphery map (Fig. 3A) and for stratigraphic calibration, wings connected to the western diapir and char- of Cretaceous salt structures near Khon Kaen, we used additional information from the 3D acterized by irregular low-contrast refl ectors that northeastern Thailand (Warren, 2006). Correla- seismic data volume and wireline data from four are similar in character to those of the salt mass of tions of wireline logs of saline-pan halite beds nearby exploration wells (Fig. 3B). Borehole A the diapir, and thus clearly distinguishable from intersected in wells A–C in the study area sug- is 500 m southwest of the diapir (see Figs. 1E, the adjacent sedimentary layering. For example, gest a similar dissolution-precipitation interplay 3B, and 3D), which has a notable bulge in its the lowermost salt glacier (IV) (Fig. 3D) forms in Northwest German Basin withdrawal sinks. westernmost part. West of the bulge a surround- a wedge of ~500-m-thick allochthonous salt The process of diapir and/or namakier dissolu- ing rim syncline is not developed, whereas else- between sedimentary strata. This oldest extru- tion and contemporaneous precipitation to form where it is typically fi lled with upper Keuper to sion is only visible in the N-S section (Fig. 3D) stacked saline pan beds, as well as surface out- Jurassic sediments. The salt glacier, now prob- because it spreads to the north, in contrast to salt fl ow of namakier salt, resulted in the preservation ably the residue of a former larger salt extrusion, glaciers II and III, which extend to the northwest, of recycled Permian salt in younger stratigraphic widens into the direction of fl ow to the south- and the youngest salt glacier (I; Fig. 3C), which levels (i.e., Triassic). This supports Trusheim’s west. It covers an area of 2.4 km2, is 2.7 km shows a southwestward fl ow direction. (1971) model of salt cyclicity in continental long, and is 0.5–2.2 km wide. The southern end A major normal fault cuts the sedimentary basins and is consistent with the notion that lay- of the structure trends southeast and is enclosed layers in the eastern part of the seismic sections ered Triassic salt originated entirely (in the case by a small graben structure. The topographic (Figs. 3C, 3D). On the variance map (Fig. 3A) of Keuper salt) or at least partially (in the case of low of the graben most likely formed the pre- the fault is indicated by a horizontal offset of Early Triassic Buntsandstein and Muschelkalk ferred fl ow path for the namakier. There is also a the sedimentary strata on both sides of the salt salt) from the near surface and surface reworking NE-SW–trending low north of the salt extrusion. glacier. As can be seen in Figures 1D and 1E of the main Permian salt source. This depression, formed by a normal fault (Figs. (red line), the listric extensional fault with a NW 1D, 1E), is mostly covered by the namakier. dip was active throughout the Mesozoic. The LATE TRIASSIC SALT GLACIERS A seismic cross section (Fig. 3C) of the fault cuts the salt diapir only during late-middle In the study area, seismic variance analysis mapped namakier (labeled as salt glaciers I–IV Keuper to late Keuper time (kmW-ko; Fig. 1C). was carried out on a high-resolution 3D seismic in Fig. 3) displays a 50–100-m-thick unit of len- data set. This clearly images (buried) salt extru- ticular shape defi ned by strong seismic refl ectors. DISCUSSION sions ~50 m below the basal Cretaceous uncon- The strong seismic refl ectivity of the glaciers can The investigated area is in a marginal posi- formity, at ~2 km depth. The variance map shows either be caused by a thin salt layer or represent tion of the Permian and Triassic basins in Cen- the morphology of the buried salt glacier to be a a residual clay and anhydrite layer related to a tral Europe and is close to major faults (Ziegler,

A B koE - J kmW C Section 3D Salt glacier I

3D W E N tion well C N

Salt glacier I Sec Salt glacier II SALT DIAPIR (Permian salt)

Section 3C

00 m

Section 3C 5 Salt glacier III Intersection well D line with D 1000 m well A D Salt diapir

Salt glacier N S

Salt glacier II

koE - J well B Salt glacier III Variance

500 m kmW kmA kmA Intersection line with C 012km 012km Salt glacier IV 0 1000 m well A

Figure 3. A: Variance map 50 m below unconformity at base of the Cretaceous displays uppermost salt extrusion as fan-like structure inside the circle. B: Geological interpretation of A showing geometry of Late Triassic salt glacier, position of the two cross sections of Figure 1, and location of the wells. C, D: Enlarged seismic sections (2.5 ×) showing salt glacier generations and salt diapir, and accompanying sedimentary layers (location in A; see Figs. 1D, 1E). Vertical dotted lines mark intersection of the two seismic profi les. Trace of well A is shown by dotted line in southern part of section D.

GEOLOGY, November 2007 965 1990). Supra-salt extensional faulting in the sive continental margins. Gravity gliding into REFERENCES CITED Late Triassic overburden adjacent to the diapir the basin at the submarine continental margin, Al-Zoubi, A., and Ten Brink, U.S., 2001, Salt diapirs fi ts the regional tectonic setting of the Triassic with upslope extension and downslope com- in the Dead Sea Basin and their relationship to Quaternary extensional tectonics: Marine and of the Northwest German Basin (Frisch and pression, controls the formation of diapirs and Petroleum Geology, v. 18, p. 779–797, doi: Kockel, 1999; Kockel, 2002). Several pulses their allochthon wings and tiers. 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Accommodation are seismically resolvable argues that hyper- Australia, in Post, P., et al., eds., Salt-sediment space for sediment fi ll was maintained by the aridity dominated in these isolated extensional interactions and hydrocarbon prospectivity: ongoing interplay between regional extension, depressions across substantial time frames. Concepts, applications and case studies for the salt fl ow, and depositional loading in depres- Other wise rainfall and groundwater fl ushing 21st Century: 24th Annual Gulf Coast Section SEPM Foundation Bob F. Perkins Research sions adjacent to the growing diapir. would have prevented the preservation and Conference, Houston, Texas, p. 79–96. At times when a syndepositional fault inter- stacking of namakier levels, as it possibly does Frisch, U., and Kockel, F., 1999, Quantifi cation of sected and cut a near-emergent diapir crest, the in Iran and the Dead Sea depression today. The early Cimmerian movements in NW-Germany, caprock was breached and a pathway opened distinction supports the notion that hyperarid in Bachmann, G., and Lerches, I., eds., Epi- for salt extrusion coupled to renewed diapir- conditions were typical in the interior of the continental Triassic, Volume 1: Zentralblatt für Geologie und Paläontologie, p. 571–600. ism. The fault escarpment and the fault-induced Pangaean supercontinent, even as it was rifting Jackson, M.P.A., Cornelius, R.R., Craig, C.H., depression allowed gravity-driven downward to become the incipient Atlantic Ocean. Gansser, A., Stocklin, J., and Talbot, C.J., fl ow of salt across the land surface (a namakier) No borehole has cored any of the Triassic 1990, Salt diapirs of the Great Kavir, central and its subsequent preservation atop continental namakiers documented in this study. The Late Iran: Geological Society of America Memoir 177, 139 p. (arid redbed) sediments that were accumulating Triassic section is considered noneconomic and Jaritz, W., 1973, The origin of the salt structure of in fault-defi ned collapse depressions adjacent is only intersected during the drilling of boreholes northwestern Germany: Geologisches Jahr- to the growing salt diapir. Somewhat similar designed to test for much deeper sub-Zechstein buch, Reihe A, v. 10, 136 p. fault intersections of overburden atop salt-fi lled hydrocarbon traps. We suggest that much of the Kent, P.E., 1958, Recent studies of South Persian bulges also control the onset of the namakier Triassic section in the Northwest German Basin salt plugs: American Association of Petroleum Geologists Bulletin, v. 42, p. 2951–2972. stage and active diapirism on or near the crests retains evidence of multiple former namakier Kockel, F., 2002, Rifting processes in NW-Germany of compressional growth anticlines in southeast- levels that were subject to pervasive synforma- and the German North Sea sector: Geologie en ern Iran (Fig. 2; Warren, 2006). tional leaching and are too thin or possess too Mijnbouw, v. 81, p. 149–158. Although both are redbed settings and little acoustic contrast to be resolved seismically. Mohr, M., Kukla, P., Urai, J.L., and Bresser, G., 2005, Multiphase salt tectonic evolution in NW namakiers occupy local fault-defi ned depres- Karstifi cation and dissolution of a namakier, even Germany: Seismic interpretation and retro- sions in the landscape, there is an obvious tec- as it is fl owing, always creates a carapace brec- deformation: International Journal of Earth tonic distinction at the regional scale between cia and a circum-namakier dissolution breccia Sciences, v. 94, p. 917–940, doi: 10.1007/ modern namakiers in Iran and the fault- (allochthon breccias of Warren, 2006). Some- s00531–005–0039–5. intersected extensional setting of the Keuper what more humid climatic episodes in the Trias- Schléder, Z., and Urai, J.L., 2007, Deformation and recrystallization mechanisms in mylonitic namakiers: the former is characterized by tec- sic would have caused all namakiers to shrink shear zones in naturally deformed extrusive tonic shortening, the latter by extension. With from their maximum extent, even after they were Eocene-Oligocene rocksalt from Eyvanekey the possible exception of the Pleistocene in the buried. Based on analogy to allochthon outcrops plateau and Garmsar hills (central Iran): Dead Sea depression, active terrestrial namakiers and breccia chains preserved in the “Christmas Journal of Structural Geology, doi: 10.1016/ j.jsg.2006.08.014 (in press). occur in compressional terrestrial (not marine) Tree” diapirs of the Flinders Ranges, we consider Talbot, C.J., and Alavi, M., 1996, The past of a future settings, as in the Iranian, Atlas, and Andean it highly likely that there are numerous levels of syntaxis across the Zagros, in Alsop, G.I., compressional belts (Table DR1). There are no former namakier (subseismic resolution) in the et al., eds., Salt tectonics: Geological Society large diapir-fed namakiers at the surface in any circum-diapir Triassic of the Northwest German [London] Special Publication 100, p. 89–110. modern terrestrial setting characterized by broad Basin. In many such levels the namakier halite Trusheim, F., 1971, Zur Bildung der Salzlager im Rotliegenden und Mesozoikum Mitteleuropas: regional extension (rifting). This is not to say was completely dissolved, and all that remains is Geologisches Jahrbuch, Beihefte, v. 112, 51 p. that namakiers cannot form during extension in a thin stratiform layer composed of an allochthon Urai, J.L., Spiers, C.J., Zwart, H.J., and Lister, G.S., a terrestrial continental setting, only that they do breccia comprising anhydrite residuals, clays, 1986, Weakening of rocksalt by water during not do so today under contemporary tectonics and salt-transported sedimentary blocks. long term creep: Nature, v. 324, p. 554–557. Warren, J.K., 2006, Evaporites: Sediments, resources and climate. Namakiers occur in older exten- and hydrocarbons: Heidelberg, Springer, 1036 p. sional terrestrial settings; remnants of Miocene ACKNOWLEDGMENTS Wenkert, D.D., 1979, The fl ow of salt glaciers: Geo- salt at the Pleistocene continental land surface We thank the German Research Foundation (DFG) physical Research Letters, v. 6, p. 523–526. and the SPP 1135 (Sedimentary Basin Dynamics) for Ziegler, P.A., 1990, Geological atlas of Western and are seen in seismic data from the Dead Sea dia- fi nancial support (KU 1476/1) and Gaz de France pirs (Al-Zoubi and Ten Brink, 2001) and Mio- Central Europe: The Hague, Netherlands, Shell Produktion Exploration Deutschland, ExxonMobil Internationale Petroleum Maatschappij, 239 p. cene diapiric salt relicts are at the Miocene land Production Deutschland GmbH, and EWE AG for surface along the Yemeni Red Sea coast (e.g., providing high-quality data sets. We also thank Manuscript received 20 September 2006 C. Krämer and H. Merkel for their drafting support Davison et al., 1996). Today, salt allochthons in Revised manuscript received 8 June 2007 and Martin Jackson (University of Texas at Austin) Manuscript accepted 14 June 2007 extensional settings are largely restricted to sub- and an anonymous reviewer for valuable improve- marine continental slope and rise settings in pas- ments to the original manuscript. Printed in USA

966 GEOLOGY, November 2007