Subsurface Seismic Record of Salt Glaciers in an Extensional Intracontinental Setting (Late Triassic of Northwestern Germany)
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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).