Comparison of modern deposits in the Panamint Valley, western United States, to core and geophysical data from a Permian (Rotliegend, Germany) tight gas field allows for improved understanding of the interaction of tectonics and sedimentary processes during Rotliegend deposition. The Panamint Valley was selected for a modern analog of the subsurface Rotliegend Basin because both study sites are characterized by (1) elongated grabens with large-scale bounding fault zones resulting from synsedimentary transtensional tectonics; (2) fault-controlled paleotopography as key controlling parameter for the sediment facies distribution, including alluvial fans, dunes, wet and damp interdune sandflats, and ephemeral dry lake deposits; and (3) local sediment provenance from sedimentary and volcanic rocks. The analysis of satellite images and field data from the Panamint Valley enabled the development of a conceptual model involving topography, synsedimentary faulting, and wind activity as controlling factors for the sediment facies distribution. The application of the model to the reconstructed Rotliegend paleotopography of the German subsurface study site allows for prediction of the facies distribution prior to the Triassic–Cretaceous tectonic overprinting. As a consequence, we expect a sediment facies succession from (1) alluvial fan deposits along the hanging walls of the basin-bounding fault zones to (2) distributary fluvial channel deposits toward the basin center and (3) ephemeral lake deposits in the deepest basin area. (4) Eolian dune accumulation and preservation is mainly concentrated on hanging-wall locations. However, additional dune deposits are proposed above overlapping step faults and on footwalls of synsedimentary active faults. (5) Sandflats occur on the upwind and downwind margins of the dune field. These predictions are calibrated to core and geophysical well log data.
Studying the sedimentary and tectonic complexity of Permian (Rotliegend sandstone) tight gas fields in Central Europe requires integrated approaches from field-based analog studies, laboratory analysis, seismic and well data interpretation, and structural modeling. One of these tight gas fields in the focus of recent research is located in northwestern Germany, east of the Dutch Groningen gas field, on the eastern footwall of the Ems Graben at ∼4200 m depth (Figs. 1 and 2). The tight character of the reservoir is attributed to extensive formation of quartz overgrowth, pressure solution, and authigenic fibrous illite (Gaupp and Solms, 2005). The reservoir rocks, which are formed by heterogeneous fluvio-eolian facies, were deposited at the southwestern margin of the Southern Permian Basin (Fig. 1; Vackiner et al., 2011). These rocks are overlying the Carboniferous basement and patchy andesitic to basaltic Rotliegend volcanic rocks.
The prediction of location and distribution of sandstone reservoirs in the fluvio-eolian deposits is challenging, particularly due to the multiphase tectonic overprinting during the Triassic, Jurassic, and Cretaceous (Lohr et al., 2007). Much of the original Permian structural and stratigraphic grain, including the location of Permian depocenters and associated eolian reservoirs, was rearranged. Therefore, most Permian fault-controlled paleohighs do not match the present-day structural highs (Vackiner et al., 2011); however, their identification is crucial for the understanding of Upper Rotliegend II facies patterns and accommodation space generation. Well information in the study area is only available from present-day structural highs below the Zechstein salt, while present-day structural lows are commonly undrilled. Subsurface information of these sites is thus often limited to seismic data. The few cored wells alone do not allow a satisfactory regional interpretation and/or extrapolation of the sedimentary facies.
Because of the limitations of geophysical data, the modern analog study presented herein took place in the northern part of Panamint Valley (Lake Hill Basin) in Inyo County, eastern California, United States. It is located in a zone of active transtensive deformation along the North American plate boundary (Stewart, 1988), an area of complex intracontinental deformation (Lee et al., 2009). The north-south–trending Panamint Valley represents one of the active grabens of the Basin and Range Province (Smith, 1976; Fig. 3). The regional tectonic setting strongly influences the hydrothermal and structural characteristics of the region (Jayko et al., 2008). Large alluvial fans developed simultaneously with modern tectonics. The fans source a shallow ephemeral dry lake in the central basin. In the North Panamint Valley, an active dune field is located on alluvial material between the dry lake and the currently active fan. The study site further includes volcanic rocks at the base of the sedimentary succession. Consequently, the Panamint Valley provides a sedimentary facies, tectonic setting, and basement type similar to those of the Rotliegend subsurface study site. A particular focus of this analog study was to delineate the possible topographic control on the sedimentary facies distribution, and to study the reactive volcanic influence on the sedimentary system in the German study site. Even though the Panamint Valley’s present-day configuration represents only a snapshot in its development history, it provides valuable high-resolution insights into the facies distributions, the facies architecture in a regional context, and the interaction of contemporaneous sedimentary systems, unavailable from geophysical subsurface data and punctual information of wells and cores.
In the following, the geological settings of the study areas in northwestern Germany and in the Panamint Valley, western U.S., are described in two separate subsections. The first is a discussion of the subsurface study site, Ems Graben in northwestern Germany. The second subsection focuses on the modern analog study site in the Panamint Valley, western U.S.
Subsurface Study Site, Ems Graben, Northwestern Germany
The subsurface study area is located at the boundary of the Ems Graben at the southwestern margin of the Southern Permian Basin, and is characterized by a U-shaped, mostly north-south–trending Zechstein salt wall, situated above an asymmetrical Late Permian graben (Vackiner et al., 2011). During deposition of the Rotliegend, the Southern Permian Basin represented an intracontinental basin of ∼1700 km length and 300–600 km width, extending from the eastern UK to Poland and the Czech Republic (Plein, 1993; McCann, 1998; Fig. 1). During deposition of the Upper Rotliegend II, a perennial saline lake occupied the central part of the Southern Permian Basin (Gast and Gaupp, 1991).
The Ems Graben in the central Southern Permian Basin underwent sedimentary transtensional tectonics during deposition of the Upper Rotliegend II, while subsequent phases of tectonic activity, e.g., rifting in the North Sea during earliest Triassic until Late Jurassic to Early Cretaceous time (Ziegler, 1990), overprinted the Rotliegend structural highs (Vackiner et al., 2011). The reconstructed graben structure in the study area is characterized by two bounding, north-south–trending strike-slip to normal fault zones with offsets of as much as 250 m in the west (Fig. 2, FZ-1) and as much as 150 m in the east (Fig. 2, FZ-2). To the north, the eastern fault zone ceases, and the asymmetrical graben changes into a half-graben (Vackiner et al., 2011; Fig. 2). A third fault zone in the graben center exhibits an Upper Rotliegend II fault-controlled paleorelief of ∼100–150 m.
The deepest part of the graben most likely represents an area where ephemeral dry lakes occurred during the Upper Rotliegend II deposition (Vackiner et al., 2011). This assumption is supported by the identification of a polygonal pattern in three-dimensional (3D) seismic data, interpreted to represent faults and fractures that had their origin in giant desiccation polygons on ephemeral dry lakes (Antrett et al., 2012).
The main tight gas reservoir interval is 80–170 m thick and unconformably overlies the top of the Upper Carboniferous. It consists of fluvio-eolian sediments deposited ca. 259–260 Ma. The Ameland lake-level highstand (ca. 260 Ma; Gast, 1991; Legler and Schneider, 2008) coincided with the onset of sedimentation in the study area. Following deposition of the reservoir interval, the study area underwent several lake-level fluctuations (e.g., Legler and Schneider, 2008). Core data of the Upper Rotliegend II sediments in the study area revealed that these are of fluvio-eolian origin, including braided stream, sheetflood, eolian dune, and wet to dry interdune deposits of the Wustrow and Bahnsen Members (ca. 260–259 Ma, early Wuchiapingium; Vackiner et al., 2011). The majority of the eolian sediment was supplied from local sediment sources, such as Carboniferous highs and patchy andesitic to basaltic volcanic rocks, via eastern trade winds (Gast, 1988; McCann, 1998; Glennie, 1990a; Rieke et al., 2001). In contrast, the source of major fluvial sediment input was located in the Variscan hinterland toward the south (Glennie, 1990b; Plein, 1993). The preservation of eolian dunes was governed by tectonic subsidence (Kocurek, 2003).
Modern Analog Study Site, Panamint Valley, Western United States
The Panamint Valley is located in the central part of the Basin and Range Province and developed as isolated basin in a pull-apart system (Burchfiel et al., 1987). The general structure of the basins is a rhomb-shaped graben or half-graben bordered by strike-slip faults (Aydin and Nur, 1985; Price and Cosgrove, 1990). The Panamint Valley is bounded by three major fault systems that are still active (Figs. 3 and 4): the north-south–trending, slightly oblique right-lateral strike-slip–dominated Ash Hill fault in the west (Densmore and Anderson, 1997), the low-angle north-northwest–south-southeast–trending normal dip-slip detachment Panamint Valley fault zone in the east (Burchfiel et al., 1987), and the northwest-striking dextral Hunter Mountain fault zone in the north (Smith, 1976; Blakely and Ponce, 2001). In response to long-term fault activity, the northern Panamint Valley is bordered by mountains as high as 1800 m to the east along the Panamint Valley fault zone and by hills as high as 800 m to the west along the Ash Hill fault. The elevation difference between the ephemeral dry lake, located at ∼400 m above mean sea level, and the mountain peaks is as much as 1350 m.
The inner part of the Panamint Valley is a relatively shallow depression with an estimated maximum sedimentary basin fill of <1000 m at its northern end (Blakely et al., 1999; Blakely and Ponce, 2001). The Panamint Valley is one of a continuous chain of basins linked by the Owens River system during glacial and pluvial periods (Jayko et al., 2008).
The sedimentary facies of the Panamint Valley comprise different types of alluvial fans, a mud-dominated ephemeral dry lake surface, and eolian dune, interdune, and sandflat deposits (Fig. 4; Supplemental File1). The ephemeral dry lake located in the center of the valley is bounded by alluvial fans descending from the mountain ranges in the north, east, and west. Toward the north, the dry lake sediments are overlain by a transition of sandflat and eolian dune deposits that partially cover the northern alluvial fans at ∼700–830 m elevation. The present-day location of the dunes is due to a northward upfan migration from an original position closer to the dry lake at an average rate of 0.8 m/yr (Prestud Anderson and Anderson, 1990). The eastern and western alluvial fans interfinger with patchy andesitic to basaltic volcanic rocks.
DATA AND METHODS
The available data from the subsurface study area in northwestern Germany comprise 3D seismic reflection data in time and depth, wireline log data, and core material. Key stratigraphic horizons were interpreted on pre-stack depth-migrated 3D seismic data, covering an area of 293 km2 (Vackiner et al., 2011). At the target interval, the seismic data set has an average vertical resolution of ∼40 m. In addition, information from 14 wells, including digital wireline logs from 7 wells and core data from 4 wells, was used for the sedimentary facies reconstruction and for stratigraphic correlation. Dipmeter logs of three wells (wells 2, 3, and 3a) and one formation microimaging/scanning log (FMI/FMS at well 3a) were used for analyzing structural dips and dip directions of foresets in the eolian successions. Thin sections from core material were analyzed for the mineral content and cement mineralogy.
The large-scale tectonic setting of the Panamint Valley was studied by combining information from satellite images (Google Earth) (Fig. 4), light detection and ranging (Lidar) data provided by GeoEarthScope (Southern and Eastern California project, SoCal_Panamint target, http://opentopo.sdsc.edu/gridsphere/gridsphere?cid=datasets), and U.S. Geological Survey digital geological maps (Jennings, 1975; Jennings et al., 2010) (Fig. 4). Based on field work, the following sedimentary facies were identified and mapped: alluvial fans of different angles and braided stream systems, eolian sediment bodies such as dunes, interdune, and sandflat deposits, and mudflat sediments of the ephemeral dry lake. Fault interpretations were adapted from Jennings (1975) and Jennings et al. (2010) and compared to the measurements of high-resolution Lidar data, our own field observations (Fig. 5), and satellite image interpretations. The composition of sediment samples from dune sands and clays of the dry lake surface was analyzed by thin-section analyses and X-ray diffractometry (XRD).
We discuss here the results of the analysis of the subsurface study site in northwestern Germany and summarize the results of the field work in the Panamint Valley.
Subsurface Study Site Germany
The 3D seismic interpretation of the subsurface study site of the Ems Graben, northern Germany, reveals considerable differences in sediment thicknesses between hanging-wall and footwall settings interpreted to record tectonic activity of the graben during the deposition of the Upper Rotliegend II studied interval (Fig. 2). We observed step faults, pull-apart structures, and relay ramps that induced a complex fault-sediment interaction.
The lower parts of core data from wells 2 and 3 show coarse-grained, grain-supported structureless conglomerates with polymict components of various sizes, interbedded with fine- to medium-grained, small-scale cross-bedded sandstones (Fig. 6). This facies is interpreted to represent deposits from braid-dominated alluvial fans, with conglomerates being deposited in gravel bar deposits while sandstones accumulated as intrachannel fills. On top of these fans, sediments show no internal geometry and consist of breccias to conglomerates with angular to rounded grains of as much as 5 cm in diameter. These deposits are interpreted to have originated from hyperconcentrated gravitational mass flows (Fig. 6A). Frequently interbedded sandstones with bimodal fine and medium grain sizes and cross-stratification are either remains of eolian dune deposits or fluvially reworked eolian units.
On top of the alluvial fan intervals, the sedimentary facies changes toward a section dominated by eolian deposition (in cores of wells 2, 3, and 3a) with dry (dry sandflat, Fig. 7; eolian dune base and eolian dune, Fig. 7) to wet (interdune mudflat, Figs. 7, 8, and 9; interdune pond or lake, Fig. 10; fluvial and pond or lake margin, Fig. 8) sedimentary successions. The fluvio-eolian–dominated intervals reach thicknesses of as much as 150 m in wells at the footwall of FZ-2, and only 50 m at the footwall of FZ-1. The evaluation of dipmeter logs showed that the footwall of FZ-1 is mainly composed of low-dipping sandflat deposits and wet deposits producing an irregular log signature. FMI/FMS and dipmeter log analysis in sandstone–dominated successions of the FZ-2 footwall (Fig. 11) indicate upward-increasing dip angles (blue pattern) in dune-dominated areas with prevailing westward dips, partly superimposed by an irregular pattern of damp sandflat deposits. Measured west to west-southwest dip directions are in line with the prevailing wind direction from the east to east-northeast described previously (e.g., Gast, 1988; Rieke et al., 2001).
Eolian strata of dune origin form the principal reservoir rocks. The interpretation of core material reveals that individual dune sets show maximum preserved thicknesses of 3 m and a pervasive cross-bedding with a considerable spread in paleotransport indicators. Therefore, the dune bodies are interpreted to represent isolated barchans or barchanoid dunes with amalgamated dune ridges (aklé dunes; Fig. 9). Initial dune set thicknesses are difficult to determine. Prevailing barchanoid dune forms suggest limited sediment supply and/or restricted availability of accommodation, which prevented the evolution of larger, more stable dune forms such as transverse dunes (Mountney, 2006), or that the eolian system built above preservation space and had little potential for being incorporated into the rock record (Kocurek and Havholm, 1993). The preservation of dune deposits in paleofootwall positions (FZ-1) may be related to subtle paleorelief, the formation of subseismic-scale footwall collapse compartments, or increased moisture content at the sediment surface (Vackiner et al., 2011). We estimate the maximum initial dune heights to ±20 m, assuming that (1) ∼60% of the original dune heights were eroded during sedimentation (Allen and Allen, 1990) and (2) a depth compaction coefficient of 0.27 km−1 for fine- to medium-grained sandstones can be applied (Sclater and Christie, 1980). Typically, dip angles increase gradually from values of <5° (dry sandflat) over 5°–15° (dune base) to 15°–35° (dune). Dune tops are characterized by erosional truncation (Fig. 9) and overlain by the next dune, a deflation lag, or a wet deposit.
Interdune units exhibit a variety of facies types, including damp to wet sandflat, mudflat, lake margin, and pond deposits (Figs. 8 and 10). Eolian mudflats (Fig. 10) are mainly composed of clay (>50%; Amthor and Okkerman, 1998) with lensoid to aligned concentrations of siltstone and very fine to fine-grained sandstones. They are characterized by convolute bedding and ball-and-pillow structures, and are the most abundant interdune deposits. Their depositional environment is mainly shallow subaquatic with blown-in eolian sands (George and Berry, 1993). The pond or lake interdune deposits consist of 100% clay; they are structureless and red, due to the absence of the reducing effect of hydrocarbon migration (Chan et al., 2000). The sediments in the transition from dune to interdune pond and/or lake deposits (Fig. 6) are composed of fine- to medium-grained sand with intercalated clays. They are interpreted to originate from the progradation of the lee sides of eolian dunes into ponds or lakes. Ripple laminations along marginal pond and/or lake deposits are common. Wet sandflat deposits (Fig. 8) are very fine to fine-grained, poorly sorted sandstones and siltstones with clay contents of 20%–50% (Amthor and Okkerman, 1998). In contrast, damp sandflat deposits (Fig. 8) are slightly coarser grained, fine- to medium-grained sandstones with clay contents of <20% (Amthor and Okkerman, 1998). Both facies types are characterized by the occurrence of discontinuous, irregular to wavy argillaceous adhesion ripples and small-scale contortions (<0.2 m amplitude). They are often accompanied by irregular, lensoid to aligned concentrations of sandstone (Mountney and Jagger, 2004; George and Berry, 1993). Thin-section analysis (Fig. 12) of the eolian dune and interdune deposits showed quartz, feldspar, lithoclasts, and clay minerals comprising chlorite and illite as main components. Sandstone classification (after Pettijohn, 1963) reveals that the sandstone is a litharenite. The detrital quartz grains are partially coated by discontinuous illite and minor chlorite rims.
At the base of the sedimentary succession, vesicular textured andesitic to basaltic volcanic lava flows with extremely variable thicknesses of 2 m (well 3) to 120 m (well 7) were drilled on the footwalls of the western (FZ-1) and the eastern fault zone (FZ-2). Similar vesicular andesitic and basaltic Upper Rotliegend II lava flows were described by Geißler et al. (2008) from local grabens at the southern margin of the Southern Permian Basin. An example of a vesicular textured lava flow is shown in Figure 13. Lava tops are heavily brecciated due to reworking, e.g., by mass flows. Therefore, a high amount of volcaniclastic material is observed in the sediments directly overlying the volcanic rocks. Thin-section analysis of the eolian dune deposits above the volcanic rocks showed quartz grains discontinuously coated by early diagenetic chlorite and illite rims, which originate from weathered volcanic material (Fig. 12).
Due to the fact that the seismic resolution at the depth of the target interval in northwestern Germany only allows for basic interpretations and well information in the study area is only available from present-day structural highs, conclusions about lateral continuity or distribution of sedimentary facies or volcanic rocks can only be roughly interpolated and extrapolated. During the core interpretation and the work on isopach maps, uncertainties of the relative positions of sediment bodies in relation to paleotopography emerged. The alluvial fan deposits were observed in Upper Rotliegend II paleofootwall positions of basinward downstepping fault zones. In addition, the main gas-bearing reservoir rocks, which are eolian dune deposits, conformably overlie these alluvial fan deposits. Accommodation of eolian dunes is thus dependent on wind direction and topography, similar to the controls on accommodation in the modern Panamint Valley dune field. Lateral extent and internal architecture of the dune field cannot be determined solely on the basis of existing subsurface data.
The relative timing of fault activity with respect to sediment deposition as well as the fault-sediment interaction of the Panamint Valley (Figs. 4 and 5) can be determined based on field and satellite observations. Sediments of either alluvial fans or eolian dunes cover most faults of the Panamint Valley. Faults in the east further offset active channels of alluvial fans, indicating recent synsedimentary fault activity (Figs. 5D, 5E). Enhanced cementation associated with fault planes (Figs. 5A–5C) and obvious sulfur smells suggest modern fluid circulation in hydraulically active faults. Precipitation of euhedral calcite crystals of as much as 0.5 cm diameter can be observed along fault scarps, but they are also present along certain stratigraphic layers, like limestone and mudstone or dolomite intervals, in the closer vicinity of faults.
To the north, east, and west of the Panamint Valley, extensive alluvial fans descend from the footwall blocks of the outermost basin-bounding faults (Figs. 4 and 6). The alluvial fans cover the major faults, like the Ash Hill fault in the west and the Panamint Valley fault zone in the east, that, in turn, interact with the active channel network of the fan apron. In the central part of the basin, the alluvial fans partly cover the dry lake surface. The lack of desert varnish implies recent sediment transport along all alluvial fans. Two types of alluvial fan systems can be related to variable slope angles in the northeastern and the northwestern basin (Blair and McPherson, 1994). Alluvial fans dominated by sheetflood and braided distributary stream processes are mainly located in the northwest of Panamint Valley, e.g., along the northern Ash Hill fault zone, and have slopes of ∼4° (Fig. 14). The northwestern Panamint Valley is dominated by late Pleistocene to recent alluvial fan deposits characterized by gravel bar and swale microtopography that consist of silt, fine- to coarse-grained sand, gravels, cobbles, and rare boulders (Jayko, 2009). The braided channel network comprises coarse-grained, massive gravel bar deposits and fine- to medium-grained, cross-stratified intrachannel deposits. In the northeastern Panamint Valley, debris flow–dominated alluvial fans with enhanced slope angles of as much as ∼10° (Figs. 6 and 14) characterize the Panamint Valley fault zone. At the intersection point of the fan trench with the general alluvial fan surface, the confined flow along fan-head channels changes into a distributive flow along a network of sand-filled shallow channels (Fig. 6). The northeastern Panamint Valley is dominated by late Pleistocene to Holocene alluvial deposits that are essentially unconsolidated (Jayko, 2009). A comparison of the eastern and the western flanks of the Panamint Valley shows that (1) the occurrence of sheetflood and braid-dominated alluvial fans is associated with lower topography and higher erosion rates of Tertiary to Quaternary volcanic rocks and Paleozoic shales and carbonates, and (2) the occurrence of debris flow–dominated alluvial fans is associated with higher relief and reduced erosion of Paleozoic carbonates, Tertiary granitoids, and Tertiary to Quaternary volcanic rocks. Incision depths of >10 m of most channels imply that the present erosion rate of the fans is higher than their accumulation rate. The northern Panamint Valley is characterized by middle to Late Pleistocene (referred to as inactive) alluvial fan deposits (Jayko, 2009), which are cemented to variable degrees.
The Panamint Dunes are located on inactive alluvial deposits north of the dry lake centered between the three major fault zones (Figs. 4 and 8A). The dune field, including dry sandflats with wind ripples north and south of the dunes, covers an area of ∼2.1 km × 4.5 km. The prevailing wind direction is from the south and infrequently from the northeast; this coincides with the dominant orientation measured for dune and ripple crests. The dunes reach a maximum height of ∼30 m and are classified as star dunes to aklé dunes, while barchanoid and aklé dunes are dominating toward the margins of the dune field. The fault-induced relief of the Hunter Mountain fault zone that borders the Panamint Valley to the north acts as a trap for wind-blown sediments, which are then deposited as eolian dunes and sandflats. The existence of overlapping step faults and synsedimentary fault activity causes continuous sedimentation along fault zones, leading to an easier upwind migration of dune sands to footwall positions. Sandflats occur on the upwind (proximal) and downwind (distal) margins of the dune field. Such sandflats form during long-lasting strong winds and periods when sand supplies are limited (Fryberger and Dean, 1979, 1983; Kocurek, 1988). A gradual increase in dip angles from 0°–5° at the dry sandflat, to 10°–15° at the dune bases, to >15° at the dunes was observed. Deflation lags accompany or directly overlie the sandflats. Interdune deposits comprise muddy to fine-grained sandy damp to wet sandflats (Fig. 8), mudflats, lake margins (Fig. 7), and ponds (Figs. 7 and 10). Dune marginal deposits (Fig. 7) are composed of fine-grained sand with interfingering clay. In many cases, a progradation of the lee side of eolian dunes into the ponds or lakes was observed. Ripple lamination along marginal lake or pond deposits is common. The damp and wet sandflat deposits (Fig. 8) were identified as siltstones or very fine to fine-grained poorly sorted sandstones. It can be assumed that these sediments were deposited under the influence of a shallow groundwater table at ∼3 m depth but were also affected by ephemeral flooding events (Fryberger et al., 1988; Meadows and Beach, 1993). The eolian sedimentation is further influenced by the saline groundwater and by the adhesion of wind-transported eolian grains to an episodically damp sediment surface. Small current ripples or horizontal laminations are common.
The XRD measurements of the dune sand samples show quartz, feldspar, and calcite as main components. Accessories are illite/muscovite, chlorite, kaolinite, and amphibole. Thin-section analysis further revealed high percentages of lithoclasts (Fig. 12), a composition indicating that the local sediment sources of the sand dunes were the Quaternary alluvial fans, the Tertiary volcanic rocks, the Mesozoic to Tertiary granitoids, metasediments, and the Paleozoic dolomites to limestones. XRD measurements of samples from the lake surface revealed dolomite, quartz, illite/muscovite, calcite, and feldspar as the main components. Accessory minerals are chlorite, kaolinite, hematite, thernadite, and amphiboles. Swelling clays were identified by glycol dehydration of the clay fraction.
Patchy basaltic to andesitic volcanic rocks cover large parts of the mountain ranges surrounding the Panamint Valley (Fig. 13). This main volcanic unit (ca. 14 Ma) is introduced by basaltic to andesitic flows and associated debris-flow deposits, which include minor amounts of interlayered basaltic lava (Andrew and Walker, 2009). The relatively thin layers were formed by lava flows (Andrew and Walker, 2009) that followed a preexisting topography, and are therefore localized along the major channel systems on the hanging walls of faults. Lava flow deposits that were also observed in the alluvial fans were mechanically eroded and chemically weathered by a network of braided channels (Fig. 13). The high amount of volcaniclastic material found in the alluvial fans indicates that the volcanic rocks served as one of the major local sediment sources. Furthermore, quartz grains from the dune succession on top of the alluvial fans and in front of the volcanic rocks show discontinuous chlorite and illite coatings that probably originate from weathered volcanic material.
The described current sedimentary facies architecture and the tectonic setting of the Panamint Valley provide a 3D temporal snapshot that is in many aspects similar to the sedimentological and structural setting of the subsurface study site in Germany during the Upper Rotliegend II deposition. On the basis of the modern analog study in the Panamint Valley, a comparative geological model of the Upper Rotliegend II in the subsurface study area was established. In the following, this model and a detailed sedimentary facies analysis at macroscale and microscale and its relationship to a fault-controlled morphology are discussed for both study sites.
The geological observations of the field and the subsurface study site suggest that the Panamint Valley with its complex synsedimentary, transtensional fault zone activity and related sediment facies architecture can be used as a field analog for the subsurface study site in Germany. The transtensional tectonic regime that formed the Panamint Valley and that could be reconstructed for the subsurface study site in northwestern Germany during deposition of the Upper Rotliegend II is interpreted to have influenced sedimentary processes, ultimately causing a distinct sedimentological pattern. In the following, we discuss the Panamint Valley’s fault-controlled topography as an important controlling factor for the sedimentary facies distribution within the valley, and consider wind directions and sediment provenance. The line of arguments used for the surface example will then be applied similarly to the subsurface study site (Figs. 15 and 16). In the following, the most probable subsurface sedimentary facies distribution will be discussed.
On the basis of the fault-controlled paleotopography map (Fig. 15A), we reconstructed the alluvial fan deposits, which were observed at the base of the wind-blown sediments in the core material. In the Panamint Valley, alluvial fans sourced from the northwestern range are sheetflood and braid-dominated alluvial fans (Figs. 4 and 14). Volcanic rocks and carbonate-cemented metasediments form the main sediment sources. As the elevation between the ephemeral dry lake and the mountain peaks in the northern Panamint Valley is ∼350 m in the west and 1350 m in the east (Fig. 14), we focus on the western flank’s topography to compare with the reconstructed paleotopography of the subsurface study site. The extent and pattern of the facies in the subsurface study site in northwestern Germany were transferred from the facies map of the Panamint Valley (Fig. 15B). For the German subsurface study site, a minimum paleofootwall elevation of ∼250 m has been calculated by isopach analysis. On the basis of core interpretation and due to comparable topography, it can be interpreted that similar sheetflood and braid-dominated alluvial fans developed in the German study site. The paleotopographies, from which the alluvial fans originate, are Carboniferous carbonates and Rotliegend volcanic rocks. The general sediment source composition is thus also in accord with our observations from the Panamint Valley.
At both study sites, dunes are sitting on sheetflood and braid-dominated alluvial fans in middle slope position (Figs. 4 and 9). The prevailing wind direction and the topographic relief are of primary interest for the analysis of the distribution of wind-blown sediments. In the Panamint Valley, the main wind direction is from the south. Hunter Mountain, which rises 370 m above the dunes, represents a windward trap for eolian sediment (Fig. 16). The well log, report, and core data set of the study area in northwestern Germany were used as basic information to verify the locations of the reconstructed sedimentary facies distribution; the most probable locations for dune and sandflat accumulations are depicted in Figure 15C. Comparable to the modern analog study, dunes on the footwall of FZ-2 in the subsurface study area developed as small barchanoid to aklé dunes under a unimodal east–east-northeast wind direction in a lee side trap (Figs. 15 and 16). The dunes of the Panamint Valley and the subsurface site show a high degree of similarity in terms of dune heights (i.e., 20–30 m), the amount of interdune deposits, and the dune sediment composition analyzed from thin sections. Furthermore, at both study sites dune bases often interfinger with aquatic interdune deposits, associated with hydroplastic deformation structures (Fig. 7). Well records from the footwall of FZ-1 reveal the occurrence of sandflats, deposited on top of fluvial sandstones and conglomerates with underlying monomict volcaniclastic breccias ranging to conglomerates and volcanic rocks. Footwalls only exhibit sediment thicknesses of ∼200 m, whereas hanging-wall sediments are as thick as ∼450 m. The fault-induced paleorelief is estimated as ∼250 m during deposition of the Upper Rotliegend II. Based on the distinct facies distribution observed in the Panamint Valley, the isopach maps that reveal a paleorelief of ∼250 m, and the prevailing east–east-northeast wind direction, it was inferred that the studied subsurface fault zone might have formed a windward trap with a higher amount of conglomerates and sand accumulations in the hanging-wall position (Fig. 15C). The increased Upper Rotliegend II sandstone thickness in the hanging wall is also governed by the preservation space that cannot be determined from the comparison to the modern analog, and thus represents the major uncertainty in the comparison. The preservation of dune deposits in hanging-wall positions (FZ-1) may be related to the situation in a relative depression, which is less affected by erosion, and to increased moisture content at the dune bases. Considering accommodation and preservation, the most probable realization of dune and sandflat deposits is depicted in Figures 15C and 16. Eolian sandstones might also have reached and been preserved in the reconstructed basin center.
The ephemeral dry lake at the Panamint Valley (Fig. 4; Supplemental File [see footnote 1]) is characterized by the occurrence of huge desiccation polygons and is centered in the deepest part of the basin. The deepest part of the synsedimentary Rotliegend asymmetric graben to half-graben basin is located on the hanging wall of FZ-1 (Figs. 2 and 15A). We assume that this might have become preferentially occupied by ephemeral lakes, or was at least exposed to the influence of a near-surface water table. This is supported by the distinct polygonal pattern in this part of the basin, very similar in terms of shape and size to the polygonal pattern on the dry lake surface in the Panamint Valley (Antrett et al., 2012). Due to the postulated high moisture content of the sediment surface, eolian dunes might have accumulated and might be preserved on the hanging wall of FZ-1.
At both study sites, patchy andesitic to basaltic lava flow units with vesicular texture occur in the footwalls (Fig. 13). In the Panamint Valley, these volcanic rocks are also intercalated with the alluvial fans in hanging-wall positions. Consequently, the hanging-wall volcanic rocks are eroded by the alluvial fan braided channel system and serve as active sediment sources (Fig. 13). In the German subsurface study site, the occurrence of basaltic to andesitic lava flows is only proven for footwall positions. Due to the fact that volcanic lava flows are on the footwall close to the fault zone, we propose that they continue into graben positions. The input of altered volcanic material from the footwall into the deeper part of the basin provides a source of reactive Al3+ and Si4+ and supports excessive and early diagenetic development of aluminosilicate minerals (Jeans et al., 2000). Swelling clays (e.g., smectite) most likely originate from the weathering of the volcanic rocks (Roen and Hosterman, 1982).
Thin-section analysis of both Panamint dune samples and the Upper Rotliegend II dune deposits reveal discontinuous coating of quartz and feldspar grains by detrital or synsedimentary early diagenetic chlorite and illite (Fig. 12), which is also provided by the weathering of volcanic material and transported via the alluvial fan channel system. Furthermore, mechanical abrasion of clay coatings around quartz grains is observed in samples from both study sites. In stabilized dunes, subject to seasonal rainfall or intermittently flooded dunes, coatings can be very continuous (Winspear and Pye, 1995). In contrast to this, the coats are abraded by eolian transport if dune sands are subsequently deflated and remobilized into active dunes (Walker, 1979; Ajdukiewicz et al., 2010). We therefore propose that active dune sediment transport is responsible for the abrasion of the clay coatings around quartz grains at both study sites.
The main uncertainties in comparing the sediment distribution of the Panamint Valley to the German subsurface study site are represented by the localization of the mobile eolian sands (Fig. 15C). Over time, the German study site might be affected by a different rate of sediment supply, higher or lower water table, and small variations in topography. However, the model (Fig. 16) comparing the Panamint Valley sedimentary facies distribution to the situation in the subsurface study site in Germany offers the most probable possible correlation of sediment dynamics.
The Panamint Valley’s heterogeneous sedimentary facies comprise different types of alluvial fans, sand dunes, mudflats, and sandflats. The distribution of these facies is controlled by synsedimentary faults and topography, the local sediment sources, and the prevailing wind direction. The abrasion of quartz grain coatings, dune types and sizes, the presence or absence of desert varnish, and the incision depth of alluvial fan channels can be used as proxies for estimating the sediment dynamics.
Core data analysis of the sedimentary facies from the subsurface tight gas reservoir in Germany compared to the Panamint Valley reveals sheetflood and braid-dominated alluvial fan deposits, partly stacked sand dune, sandflat, and interdune deposits. Cored sediment thicknesses and facies reveal pronounced changes across synsedimentary fault zones.
At both study sites, the presence of patchy basaltic to andesitic volcanic lava flow units strongly influenced the sediment composition and supplied clays to the sedimentary systems.
We developed a model comprising topography, synsedimentary faults, and wind directions as key controlling factors of the sediment facies distribution at the analog study site and compared this to the subsurface area reconstructed to an Upper Rotliegend II setting prior to multiphase tectonic overprinting. The location of eolian sandstones comprising dune and sandflat deposits controlled by fault-induced topography acting as an effective trap for eolian sand is particularly similar.
The field analog observations concerning the abrasion of quartz grain coatings, dune types and sizes, and the incision depth of alluvial fan channels can be used to reconstruct the sediment dynamics of the tight gas reservoir during deposition of the Upper Rotliegend II. At both study sites, the early postsedimentary chlorite and illite coatings around the detrital grains are abraded, indicating active transport of dune sands; this is supported at both study sites by the occurrence of nonstable dune form deposits, such as barchanoid and aklé dune deposits, in the cores from the subsurface study site and in the Panamint Valley. The Upper Rotliegend II paleotopography in the subsurface study site in Germany is not as pronounced as the current topography in the Panamint Valley; therefore, the different sedimentary facies provide different basinward extensions. Due to shallower paleogradients at the subsurface study site, the assumed ephemeral dry lake, for example, may have flooded a much larger area.
Our study shows that a thorough surface-subsurface analog study enables the detailed interpretation, interpolation, and prediction of sedimentary facies in areas characterized by limited subsurface information. The study further shows that paleosediment dynamics can be reconstructed using a paleotopographic restoration.
The study is part of the Wintershall and RWTH Aachen University Tight Gas Initiative. We thank Wintershall Holding GmbH and GDF Suez E&P Deutschland GmbH for providing the data and supporting this project. The paper benefitted in particular from fruitful discussions with Claudia Bärle, Harald Karg, Bernhard Siethoff, Wolfram Unverhaun, Petra Unverhaun, Wolf-Dieter Karnin, and Anton Irmen. We also thank Norbert Klitzsch and the E.ON Energy Research Center, who gave us the opportunity to conduct ground resistivity measurements. We also thank the United States National Park Services, especially Richard Friese, for issuing a research and collecting permit for Death Valley National Park. We are grateful to Midland Valley Ltd. for providing academic software licenses for use at RWTH Aachen University.