To what extent can intracrater layered deposits that lack clear sedimentary textures be used to infer depositional environments?

Icarus 248 (2015) 526–538

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To what extent can intracrater layered deposits that lack clear sedimentary textures be used to infer depositional environments? Sarah B. Cadieux ⇑, Linda C. Kah Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, TN 37996, United States

a r t i c l e

i n f o

Article history: Received 4 November 2013 Revised 1 November 2014 Accepted 4 November 2014 Available online 11 November 2014 Keyword: Mars Mars, surface Mars, climate

a b s t r a c t Craters within Arabia Terra, Mars, contain hundreds of meters of layered strata showing systematic alternation between slope- and cliff-forming units, suggesting either rhythmic deposition of distinct lithologies or similar lithologies that experienced differential cementation. On Earth, rhythmically deposited strata can be examined in terms of stratal packaging, wherein the interplay of tectonics, sediment deposition, and base level (i.e., the position above which sediment accumulation is expected to be temporary) result in changes in the amount of space available for sediment accumulation. These predictable patterns of sediment deposition can be used to infer changes in basin accommodation regardless of the mechanism of deposition (e.g. fluvial, lacustrine, or aeolian). Here, we analyze sedimentary deposits from three craters (Becquerel Crater, Danielson Crater, Crater A) in Arabia Terra. Each crater contains layered deposits that are clearly observed in orbital images. Although orbital images are insufficient to specifically determine the origin of sedimentary deposits, depositional couplets can be interpreted in terms of potential accommodation space available for deposition, and changes in the distribution of couplet thickness through stratigraphy can be interpreted in terms of changing base level and the production of new accommodation space. Differences in stratal packaging in these three craters suggest varying relationships between sedimentary influx, sedimentary base level, and concomitant changes in accommodation space. Previous groundwater upwelling models hypothesize that layered sedimentary deposits were deposited under warm climate conditions of early Mars. Here, we use observed stacking patterns to propose a model for deposition under cold climate conditions, wherein episodic melting of ground ice could raise local base level, stabilize sediment deposition, and result in differential cementation of accumulated strata. Such analysis demonstrates that a first-order understanding of sedimentary deposition and accumulation—despite a lack of textural information that inhibits interpretation of depositional mechanism— can provide insight into potentially changeable depositional conditions of early Mars. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Layered geologic units have been recognized on Mars since the 1971 Mariner 9 mission (Masursky, 1973). Since these early missions, images and data from both orbital satellites and planetary rovers have revealed evidence of layering across the martian surface, including within crater interiors (Anderson and Bell, 2010; Ansan et al., 2011; Edgett and Malin, 2002; Lewis et al., 2008; Wilson et al., 2007), in intercrater (Hynek and Phillips, 2008; Weitz et al., 2010; Zabrusky et al., 2012) and chaotic terrains (Sowe et al., 2008), and chasm interiors (Beyer and McEwen, 2005; Catling et al., 2006; Feuten et al., 2008; Murchie et al., ⇑ Corresponding author at: Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405, United States. E-mail address: [email protected] (S.B. Cadieux). http://dx.doi.org/10.1016/j.icarus.2014.11.004 0019-1035/Ó 2014 Elsevier Inc. All rights reserved.

2009; Okubo et al., 2008). Despite the abundance of layered units, the lack of identifiable source and depositional pathways has led to numerous hypotheses regarding lithology and depositional environment, including subaqueous deposition (Cabrol and Grin, 1999; Fassett and Head, 2005; Metz et al., 2009; Wharton et al., 1995; Wray et al., 2011), aeolian deposition (Hayes et al., 2011; Squyres and Knoll, 2005) possibly associated with a regional groundwater system (Andrews-Hanna and Lewis, 2011; Andrews-Hanna et al., 2010; Grotzinger et al., 2005), and deposition from pyroclastic ejecta (Fassett and Head, 2007; Moore, 1990) or impact-related surge deposits (Burt et al., 2008; Knauth et al., 2005). Regardless of origin, the presence of bedding preserved in stratigraphic successions indicates a sedimentological response to changes in depositional processes. Bedding is a fundamental characteristic of sedimentary rocks, representing layers of lithologic, textural, or structural unity that can be clearly distinguished from under- and

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overlying layers. Lower and upper surfaces of beds, or bedding planes, reflect changes in depositional conditions, such as sediment influx, energy flow, sediment cohesion/cementation, or sediment erosion (Christie-Blick and Driscoll, 1995). Similarly, the accumulation of beds reflects a combination of: (1) sediment input, (2) accommodation space (i.e., the vertical space available for sediment deposition at any point in time) and (3) base level (i.e., the level above which deposition is temporary and erosion can occur). Earth environments provide a basic model in which changes in accommodation space and sediment input largely reflect a combination of sea level and tectonic activity (Jervey, 1988). High sea level, for instance, provides subaqueous accommodation space in the marine realm and can control the degree to which surrounding regions are exposed and susceptible to erosion. Similarly, tectonic subsidence or uplift acts, respectively, to increase or decrease accommodation space, while at the same time acting as a secondary control on sediment supply. In non-marine environments on Earth, the most important concept for understanding sediment accumulation is base level. Base level represents a conceptual equilibrium surface beneath which sediment will accumulate and above which sediment will be susceptible to erosion (Boggs, 2006). In marine environments, base level is coincident with relative sea level (Fig. 1). However, in nonmarine subaqueous (i.e., lacustrine) environments, base level—here, the lake surface—is defined by the intersection of the ground water table with the topographic land surface (Fig. 1). In this scenario, an increase in accommodation space is attributed to an increase in the volume

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of the groundwater reservoir, which will drive the intersection with the land surface to higher elevations. Similarly, in a subaerial (i.e. aeolian) environment, the position of a subsurface water table, for example within an aeolian deposit, can also define base level and the maximum thickness of subaerially deposited sediment not susceptible to deflation and continued transport (Fig. 1; Havholm and Kocurek, 1994). Base level is useful in determining the extent to which sediment can accumulate, however, it does not provide unambiguous evidence of where sediment will be eroded. Rather, base level provides a measure of where sediment is susceptible to erosion. This distinction is particularly important in subaerial environments, where aeolian deposition and accumulation can also reflect regional and local wind patterns, pressure gradients, and topography (Kocurek, 1988). For example, in arid aeolian environments, when the water table does not intersect accumulated sediment, sediment accumulation occurs as a consequence of downwind deceleration in wind strength. Deflation of accumulated sediment will then reflect an exhaustion of local sediment supply (Mountney, 2006). In this case, local or regional base level does not effectively predict sediment accumulation and available accommodation space, and sediment accumulation is often stochastic. Observation of repetitive stacking of sedimentary units, however, suggests base level control of accumulation space, and provides a means by which to compare depositional environments. Information regarding sediment accumulation within depositional environments can be interpreted by examining the features

Fig. 1. Controls on accommodation space on Earth, not to scale. (A) Marine environment. Eustasy and tectonics are the first-order controls on accommodation space. The small, gray arrow represents initial accommodation space. An increase in either eustasic sea level or tectonic subsidence results in a rise in relative sea level and an increase in accommodation space, shown by the black arrow. (B) Subaqueous, terrestrial (i.e. lacustrine) environment. Position of the water table and its intersection with the substrate topography controls accommodation space. Small, gray arrow indicates initial accommodation space. An increase in the size of the groundwater reservoir increases accommodation space, shown by large, black arrow. (C) Terrestrial subaerial environment. Position of the water table determines potential for aeolian accumulation (modified from Havholm and Kocurek, 1994). Small gray arrow represents initial position of the water table. With an increase in the position of the water table, caused by subsidence or absolute water table rise, new accommodation space is created, shown by the large black arrow. All sediment above the new water table will be susceptible to erosional deflation.

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of stratal packaging preserved within layered sedimentary strata. If sediment influx is sufficient to fill available accommodation space, the thickness of stratal packages can be used directly to infer changes in base level and associated accommodation space, and can potentially show thickening upward, thinning upward, stochastic, or cyclic packaging if controlled by changes in base level. Assuming a constant period, thickening upward packaging reflects a net increase in the rate at which accommodation space is gained in associated with an increase in local or regional water volume. Similarly, thinning upward packages reflect a net decrease in the rate at which accommodation space is gained. Cyclic packaging therefore reflects a hierarchy of processes that results in repetition of conditions associated with base level, accommodation and sediment influx (Schwarzacher, 2000). By contrast, stochastic packaging is likely to represent random processes in sediment influx or accommodation space (Rothman et al., 1994). Analysis of stratal packaging on Mars may help us understand the relative roles of sediment influx and base level, thereby providing crucial constraints on martian sedimentary depositional models. The layered deposits in craters of Arabia Terra provide an exceptional opportunity to explore records of sediment deposition during early martian history in order to determine if stratal patterns on Mars can provide evidence for local or regional drivers of sedimentation and sediment preservation. Intracrater deposits in Arabia Terra contain hundreds of meters of light-toned, finely bedded layers (Andrews-Hanna et al., 2010; Edgett and Malin, 2002; Fergason and Christensen, 2008; Lewis et al., 2008; Zabrusky and Andrews-Hanna, 2010). These deposits are commonly eroded into mounds with stair-stepped morphologies that alternate between slope- and cliff-forming units, suggesting repetitive or rhythmic deposition of distinct lithologies or rhythmic differential cementation of a single lithology. In Arabia Terra, a significant fraction of layered deposits are contained within craters, although some craters preserve layered strata existing above crater rims (Anderson and Bell, 2010; Malin and Edgett, 2000). Although the presence of both intra- and inter-crater layered deposits suggests a possible relationship between local and regional depositional conditions (Andrews-Hanna et al., 2010; Zabrusky et al., 2012), or at least episodic base level conditions that effected both intracrater and intercrater regions, this paper focuses exclusively on intracrater layered deposits. Differences in tectonic regimes and aqueous surface conditions between Mars and Earth should have substantial influence on the generation of local to regional scale accommodation space. The region of Arabia Terra, Mars, has been tectonically inactive for most of its history (Barlow, 2008), therefore tectonic subsidence and uplift is expected to have played little to no role in controlling accommodation space in Arabia Terra. Without tectonics, aqueous fluctuations would have been the primary driver of both base level and sediment accumulation. The extent of aqueous conditions for Mars however, is highly debated. Geomorphologic, sedimentological and geochemical evidence indicate that both liquid and frozen water have been present on or near the martian surface at different times in the planet’s history (Andrews-Hanna et al., 2010; Bibring et al., 2005; Fastook et al., 2008; Parker et al., 1993; Poulet et al., 2005; Squyres and Knoll, 2005). Stratal packaging within Arabia Terra, in most cases, can therefore be interpreted to reflect the relationship between sediment supply and the generation of accommodation space via changes in local or regional aqueous base level by either position of the water table or near surface water availability. Although orbital images are typically insufficient to define discrete modes of sediment deposition (Edgett and Malin, 2002; Grotzinger et al., 2011; Zabrusky et al., 2012), many of these images are of sufficient resolution to determine thickness of stratal components and the range of variability in stratal packages. We

present an analysis of strata within Becquerel Crater (8.2°W, 21.5°N), Danielson Crater (7°W, 8°N) and unnamed crater at 1.2°W, 8.9°N (herein referred to as Crater A) (Fig. 2). Previously in Becquerel Crater, Lewis et al. (2008) analyzed 66 ‘‘beds’’ and found a mean thickness of 3.6 ± 1.0 m, with no apparent differences in stratigraphic thickness upsection. More recently, Stack et al. (2013) analyzed 158 ‘‘beds’’ within the same mound of layered strata, with a mean thickness of 2.02 ± 0.03 m and found a statistically significant thinning upsection trend. In each of these previous studies, only one stratigraphic succession from each crater was measured. Here, multiple, detailed stratigraphic columns from each crater are used to analyze bed thicknesses, identify stratigraphic patterns and to consider potential alternative hypotheses for sediment deposition. By building upon earlier analysis of stratal packaging of intracrater layered deposits we question if driving forces for sedimentation in these craters can be defined by stratal patterns, and explore if these forces may have been acting locally or regionally. In this paper, we suggest that analysis of stratal packaging can present constraints on plausible depositional conditions that can be tested in future ground-based missions to Mars. 2. Materials and methods 2.1. Identification of layered units The determination of stratal packaging on Mars requires some fundamental assumptions regarding the nature of the strata being measured. Observed from orbit, the primary geomorphic pattern reflected in layered strata is a stair-stepped pattern, with a recessively weathered interval commonly blanketed by windblown sediment and capped by a more resistant interval (Fig. 3). In this study, as well as earlier examinations of stratal packaging (Lewis et al., 2008; Stack et al., 2013), measurements of stratal thickness were taken from the top of a resistant unit (or ‘‘cap’’) to the top of the next successive resistant unit. We suggest, however, that these intervals should not be identified as ‘‘beds’’, as in Lewis et al. (2008) and Stack et al. (2013), but instead as recessive/resistant couplets which reflects the inherent ambiguity of orbital images. It is possible that recessive/resistant couplets represent two distinct sedimentary units of different composition, lithology or grain size. However, couplets may also represent a single lithology with differential cementation of lower and upper portions or a single depositional package composed of multiple beds of varying thickness, lithology, or cementation that is outside the resolution limits of the orbital images. As with earlier analyses of strata within Becquerel Crater, couplets grouped together in a systematic pattern are referred as bundles (cf. Lewis et al., 2008). Bundles are defined by visual systematic differences in topographic expression, wherein the resistant upper half-couplets were less pronounced in the lower portion of the bundles, and become more pronounced in the upper portion of the bundles (Fig. 3). 2.2. Construction of stratigraphic thickness High Resolution Imaging Science Experiment (HiRISE) images were used to identify details of intracrater deposits, and HiRISE stereo images (Table 1) generated by the U.S. Geological Survey using Socet Set according to methods of Kirk et al. (2008) were used to construct DEMs. Post-processed DEMs are archived at the HiRISE Operational Center at the University of Arizona. HiRISE digital elevation models (DEM) were then used to measure couplet thickness. The expected precision of the vertical elevation values extracted from digital terrain models (DTMs) is estimated to be

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Fig. 2. (A) Global map of Mars. Dashed white line shows the approximate position of the topographic dichotomy, which separates the northern lowlands from the southern highland terrains; white box shows area of Arabia Terra. (B) Regional extent of Arabia Terra with craters of interest marked: (1) Becquerel; (2) Danielson; (3) Crater A. Becquerel Crater is the largest and most northernmost crater in this study, with diameter of 165 km. Danielson Crater (64 km) and Crater A (30.2 km) are at similar longitudinal positions.

0.3 m (Kirk et al., 2008). Orientation and dip directions of recessive/resistant couplets were determined by selecting three points along the contact between a resistant interval and the overlying recessive interval and extracting three-dimensional coordinates from DEMs. The dip corresponding to the chosen points was calculated using traditional 3-point problem methodology (Davis et al., 1996). In each crater, multiple orientation and dip measurements were made throughout each section and averaged to obtain a single orientation and dip measurement for the entire section. Calculations of stratal dip show approximately sub-horizontal bedding in study regions from Becquerel Crater and Crater A. Dip measurements within Danielson Crater have a larger range than those observed in both Becquerel and Crater A, indicating that dips are not uniform through the study area. Dips calculated within adjacent fault blocks show the widest variation, suggesting that the variation in measured dip angle reflects rotation of fault blocks. In an attempt to minimize error resulting from local faulting, adjacent blocks where dips were highly variable were not analyzed. For each measured section, a topographic profile running perpendicular to the strike of the outcrop was extracted from the DEM. The distance between the upper and lower boundaries of each couplet was measured using the ENVI software package. From dip, elevation, and pathway distance, the stratigraphic thickness of

each identifiable couplet was calculated. In flat lying strata, reconstruction of true stratigraphic thickness corresponds directly to elevation. In cases of dipping strata, it was necessary to correct for dip, strike, topographic slope, and topographic position. All values presented were based on a single measurement of each couplet thickness. Composite stratigraphic successions were constructed for each crater where couplets could be readily traced between laterally adjacent successions. Where couplets were uniform in erosional expression and faults were present, distinct maker couplets were used to correlate successions. Where there was no lateral change in couplet thicknesses, correlative couplet thicknesses were averaged together to construct the composite stratigraphic succession for each crater. 2.3. Statistical methods Distribution of couplet thicknesses was assessed for normal distribution via the Kolmogorov–Smirnov test, and uniform distribution via a chi-square goodness-of-fit test. Visually traceable couplets were analyzed for lateral change in thickness using a two-sample paired t-test. Half-couplet thicknesses were analyzed for variation between upper and lower units using a two-sample

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be recognized in the northern portion of the mound before the appearance of layering was disrupted (Fig. 4B). In Danielson Crater, hundreds of meters of strata are exposed on the crater floor and covering the walls of the crater. In addition, strata are exposed within a series of NE–SW trending, elongate hills (Fig. 5A) revealing stacked couplets of strikingly uniform erosional expression (Fig. 5B). Substantial faulting, which trends NE–SW, largely parallel to hill orientation, displaces couplets up to 10 m vertically. Similar to Danielson Crater, layered strata in Crater A are found both covering the crater floor and superimposed on the crater walls (Fig. 6A), although well-exposed strata occur primarily in small, discrete mounds (Fig. 6B). Much of the crater floor in Crater A is covered by either dark-tined dunes or substantial thicknesses of dust, obscuring the exposures of strata outside of the discrete mounds. As with Danielson Crater, couplets within Crater A display a strikingly similar erosional expression (Fig. 6B).

4. Stratigraphy of intracrater deposits

Couplets

Bundles

Fig. 3. Portion of HiRISE image of layered deposits in Becquerel Crater rotated to display stratigraphic section page up. Couplets include an upper, high albedo, resistant ‘‘cap’’ and an underlying, low albedo, recessive lower unit. Bundles are defined by systematic differences in topographic expression, wherein the resistant upper half-couplet is less pronounced in the stratigraphically lower portion of the bundles and becomes more pronounced stratigraphically higher in the bundle. Here, 4 bundles are observed, each containing 11–15 couplets.

Table 1 Stereo-pairs of HiRISE images used to construct digital elevation models. Crater

Image 1

Image 2

Becquerel Crater Danielson Crater Crater A

PSP_001546_2015 PSP_002733_1880 PSP_002047_1890

PSP_001955_2015 PSP_002878_1880 PSP_001902_1890

t-test. Changes in couplet thickness throughout the succession were analyzed for trends in thinning or thickening using linear regression. The observed significance probability, p, from a twosided t-test was used to reject the null hypothesis for p < 0.05, which implies there is no statistically significant difference in couplet thicknesses. 3. Craters Three craters within Arabia Terra were examined in this study: Becquerel Crater, Danielson Crater, and Crater A (Fig. 2). Becquerel Crater is the northernmost and largest crater analyzed in this study, with diameter of 165 km. Danielson Crater (64 km diameter) is 775 km south of Becquerel Crater. Crater A, the smallest crater analyzed (30 km diameter) is to the east of Danielson Crater (Fig. 2). Layered deposits in Becquerel Crater occur in a single mound in the southern part of the crater (Fig. 4A). Irregular erosional expression and dust coverage, allowed only 10 bundles to

4.1. Stratigraphic measurements Thicknesses of all measured couplets were greater than the resolution and precision of DEMs. In Becquerel Crater, dips of couplets ranged from 0.3° to 5.5°, with a mean dip ± standard error of 1.5 ± 0.2°. A total of three stratigraphic sections within the layered mound were measured (Fig. 4B), with mean couplet thicknesses ± standard error ranging from 3.5 ± 0.2 m in succession 2 to 3.9 ± 0.2 m in succession 3 (Table 2). Over a lateral distance of 3 km, couplet thicknesses were unable to statistically be distinguished from each other (n = 129, p = 0.235 for succession 1–2 and n = 124, p = 0.133 for succession 2–3). At most, 8 bundles were identified visually in each stratigraphic section, containing 6–12 couplets each. All couplets were visually traced between laterally adjacent regions. The composite stratigraphic section totaled 319 m in thickness, with 89 couplets and mean couplet thickness ± standard error of 3.6 ± 0.1 m (Table 2). In Danielson Crater, dip of couplets ranged from 0.6° to 26.3°, with a mean ± standard error of 13.8 ± 1.1°. Four sections were analyzed for stratigraphic thickness (Fig. 5B), with mean thickness ± standard error of each depositional couplet ranging from 6.3 ± 0.5 m in succession 1 to 8.7 ± 0.7 m in section 4 (Table 2). Rotated blocks of strata were excluded from stratigraphic measurements because of the highly variable dips that were measured in these regions. Unlike in Becquerel Crater, not all couplets in Danielson Crater could be traced laterally, suggesting that couplets may reflect depositional mechanisms that can show lateral variation (e.g. fluvial deposition). A two-sample t-test, however, indicated that mean couplet thicknesses were similar in each of the individual measured successions (n = 85, p = 0.61 for succession 1–2; n = 102, p = 0.247 for succession 2–3; n = 72, p = 0.731 for succession 3–4). In addition, similarity in couplet thickness across the crater interior suggests that the stacking pattern of couplets was ultimately driven by basin wide changes in accumulation space. Regions of visible continuity between adjacent successions were therefore used to construct a composite section of 126 couplets, 968 m in thickness, and mean thickness ± standard error of 7.7 ± 0.3 m (Table 2). Dip of couplets in Crater A were similar to those in Becquerel Crater, ranging from sub-horizontal to 6.9°, with a mean of 1.6 ± 0.2°. Stratigraphic columns were constructed for 11-layered mounds (Fig. 6B), each containing 4–10 couplets (Table 2). A large range in stratigraphic thicknesses were observed from mound-to-mound, with mean thickness ± standard error ranging from 9.0 ± 1.3 m in mound 10 to 18.0 ± 1.3 m in mound 2. There were not enough couplets per mound to determine if thicknesses were statistically sim-

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N

Fig. 4. (A) Context map of geomorphic units within Becquerel Crater. Mapped units are overlain on composite CTX images (5 m/pixel resolution). Mapped units include crater rim and regional strata into which the crater impacted (yellow), topographically raised, light-toned layered deposits (red), lower elevation layered deposits (purple), low albedo debris and/or dunes (green), and unknown or dust-covered units which are suspected to include portions of the exposed crater floor (blue). Black box indicates the location of B. (B) Portion of HiRISE image PSP_001546_2015. Three sections were measured for stratigraphic thicknesses, marked by red lines. Filled circles represent topographic boundaries that define bundles. Despite regional faulting that occurs between the two eastern most successions, couplets can be traced between laterally adjacent successions.

Fig. 5. (A) Context map of geomorphic units within Danielson Crater. Mapped units are overlain on composite CTX images (5 m/pixel resolution). Mapped units include crater rim and regional material into which the crater impacted (yellow), light-toned layered deposits (red), and low albedo debris and/or dunes (green). Black box indicates the location of B. (B) Portion of HiRISE image PSP_002733_1880. Four sections were measured for stratigraphic thickness, marked by red lines. Examples of stratigraphic marker couplets between successions are shown in yellow. Abundant faulting and dust coverage obscures the lateral extent of strata, and many couplets cannot be traced between individual sections.

N

Fig. 6. (A) Context map of geomorphic units within Crater A. Mapped units are overlain on composite CTX images (5 m/pixel resolution). Mapped units include crater rim and regional strata into which the crater impacted (yellow), light-toned layered deposits (red), lower elevation layered deposits (purple), low albedo debris and/or dunes (green), and unknown or dust-covered units which are suspected to include portions of exposed crater floor (blue). Black box indicates the location of B. (B) Portion of HiRISE image PSP_001902_1890. Ten of the 11 mounds measured for stratigraphic sections are highlighted in red. One mound, not pictured here, occurs 7 km north of the pictured region. Low albedo sediment occurs between mounds, obscuring direct correlation of couplets between mounds.

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Table 2 Stratigraphic measurements for each crater and succession in Arabia Terra. Number of couplets

*

Total thickness (m)

Range of thickness (m)

Mean thickness ± SE (m)

Becquerel 1 51 2 74 3 51 C* 88

230 257 235 319

1.4–8.0 1.3–7.5 1.4–7.6 1.3–7.7

3.8 ± 0.2 3.5 ± 0.2 3.9 ± 0.2 3.6 ± 0.1

Danielson 1 35 2 53 3 52 4 32 * C 126

220 404 445 284 968

2.0–12.5 1.2–19.4 2.1–24.8 2.4–19.6 2.0–19.4

6.3 ± 0.5 7.6 ± 0.5 8.5 ± 0.6 8.7 ± 0.7 7.7 ± 0.3

Crater A 1 7 2 10 3 7 4 4 5a 9 5b 7 6 7 7 5 8 6 9 7 10 7 * C 18

105 179 78 64 102 95 83 82 76 76 63 223

10.5–17.5 11.8–25.5 2.5–14.7 9.5–15.6 4.6–17.7 9.3–18.6 3.8–18.9 12.5–22.7 6.9–16.5 6.6–13.7 4.7–15.0 5.6–16.5

15.0 ± 0.9 18.1 ± 1.3 11.2 ± 1.6 12.9 ± 1.0 11.4 ± 1.5 13.6 ± 1.2 11.9 ± 2.1 16.4 ± 2.0 12.6 ± 1.7 10.8 ± 0.9 9.0 ± 1.3 12.4 ± 0.7

Composite succession.

ilar. Substantial erosion of strata between mounds and the presence of wind blown sediment inhibited correlation of strata between individual mounds. Sub-horizontal bedding, however, permitted reconstruction of relative stratigraphic positions from couplet

µ

elevation. Using these elevation relationships, correlative couplets yielded a composite stratigraphic succession of 18 couplets, 223 m thick, with a mean couplet thickness of 12.4 ± 0.7 m (Table 2). 4.2. Stratigraphic trends Composite stratigraphic sections in each crater were used to analyze stratigraphic trends. Couplet thicknesses in the composite section for each crater were consistent with a Kolmogorov–Smirnov normal distribution (Becquerel n = 89, p = 0.15; Danielson n = 126, p = 0.15, Crater A n = 18, p = 0.95). A normal distribution indicated that couplets are not distributed exponentially or according to a power law, and therefore are most likely not stochastic in their expression. Couplets in Becquerel and Crater A were consistent with a chi-square goodness of fit test for uniform distribution (Becquerel n = 89, p = 0.99; Crater A n = 18, p = 0.78), suggesting similar couplet thickness throughout the succession. Couplets in Danielson Crater were not consistent with a chi-square goodness of fit test for uniform distribution (n = 126 p = 0.002), suggesting couplet thicknesses varied throughout the succession. Linear regression further indicated no statistically significant upsection trend for couplet thickness in Becquerel (n = 88, p = 0.08) and Crater A (n = 18, p = 0.18), using a critical value of 0.05. Within individual bundles of Becquerel Crater both thickening and thinning upsection trends were observed (Fig. 7). In several bundles (4, 5, 7, 8), couplet thicknesses were thickest at the base, whereas in other bundles couplets varied upsection. In Crater A, couplets thickened from the base of the section upward for 3 couplets, and then varied, clustered around the mean, before thinning upsection for the last 4 couplets (Fig. 7). In Danielson Crater, linear regression indicated a statistically significant thickening up the stratigraphic succession (n = 126, p = 0.003).

µ

µ

Fig. 7. Composite stratigraphic column for Becquerel Crater, Danielson Crater and Crater A showing stratigraphic thickness of individual couplets. Gray vertical line shows mean couplet thickness. Linear regression analysis results are listed in each panel. For p less than 0.05, the null hypothesis is rejected and the section displays a thinning or thickening upsection trend. In Becquerel Crater, dashed gray lines represent bundles, which are numbered in italics. Thickening upsection trends occur in bundles 1 and 6, and no thinning upsection trends are observed. Overall, p > 0.05 indicating no statistically significant thickening or thinning upsection trend is observed. In Danielson Crater, couplet thicknesses vary upsection, falling both above and below the mean value with a significant thickening upsection trend observed (p < 0.05). In Crater A, couplet thicknesses are primarily clustered around the mean, with no upsection trend (p > 0.05).

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5. Discussion 5.1. Potential depositional conditions Without a clear sediment source for the regional craters, it is unknown if sediment deposited in Becquerel Crater, Danielson Crater, and Crater A resulted from a common source, such as aeolian sedimentation, or whether deposition occurred independently within each basin. The lack of obvious sediment source has led some researchers to interpret layered deposits to have originated as a massive dust-ice deposit that once resembled the polar layer deposits (Bridges et al., 2008; Niles and Michalski, 2009; Tanaka, 2000). Volcanic processes with reoccurring episodes of activity can similarly form layered deposits (Barley, 1993; Fisher and Schmincke, 1984; Knauth et al., 2005). Fine tephra can result in air fall deposition that drapes over pre-existing topography, forming massive, kilometer-thick units with potentially layered beds (Knauth et al., 2005). The Medusae Fossae Formation and Electris deposits, for example, have been interpreted to have formed from volcanic ash deposition (Kerber and Head, 2010; Kerber et al., 2012). Such subaerial deposition hypotheses, with no apparent aqueous component, such as air fall deposition of volcanic or impact related ejecta have the potential to lead to couplets of variable thickness. Sediment accumulation in aeolian environments has different controlling mechanisms, such as wind patterns, pressure gradients, and topography (Kocurek, 1988), which could lead to stochastic packaging. However, couplet thicknesses in all three craters were not stochastic, and do not appear to be continuous draped units, making airfall deposition in the absence of a regional aqueous base level an unlikely model to describe the layered strata deposited in craters of Arabia Terra. Evidence of non-stochastic stacking patterns strongly suggests accumulation of sediment controlled by a local or regional aqueous base level. In the case of the three craters examined here, an absence of clearly defined channels within crater rim materials further suggests that the majority of sediment does not represent fluvial deposition, and more likely represents a combination of aeolian and lacustrine deposition. The discrete depositional mechanism, however, is more difficult to deduce because the difference between aeolian and lacustrine deposition commonly reflects only a small difference between the position of the local water table and the topographic position of the land surface. Furthermore, deposition that occurred above local base level, such as aeolian deposition above a local water table, would be susceptible to later erosion, resulting in deflation events that could easily mask evidence of original depositional mechanism. Similarly, when sediment does not completely fill accommodation space, such as in some lacustrine environments, it becomes impossible to discern original cycle thickness. In order to explore the potential origins of observed stacking patterns, we restrict interpretation to scenarios in which deposits do not exceed the rim of the crater. In this case, due to the absence of tectonic subsidence of uplift in the Arabia Terra region, the depth of a crater from its rim to floor represents the maximum accommodation space available. Strata within the crater could therefore represent either incomplete filling of accommodation space by sediment whose sedimentary flux varied in such a way to produce apparent couplets, or by a progressive increase in base level matched with sediment input that completely filled available accommodation space. In the former scenario, a constant aqueous level would define both base level and accommodation space, and sediment influx would be the primary control on stratal packaging. In order to produce couplets, deposition would need to vary rhythmically, as in pulsed run-off that provides differentiation in grain size. By contrast, if sediment input was constant, as more sediment

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was deposited, available accommodation space would decrease, and couplets would be expected to thin (Bohacs et al., 2000; Read and Goldhammer, 1988). The absence of a strong signal of thinning in the preserved couplets suggests that the aqueous interface must have fluctuated in concert with filling of accommodation space within the crater. In the latter scenario above, if we assume that the position of base level is associated with the position of aqueous interface, in order to produce stacked couplets, base level within the crater would have to progressively increase, although the rate of base level change may have varied. A fluctuating base level is consistent with previous models proposed by Andrews-Hanna et al. (2010) and Andrews-Hanna and Lewis (2011). In this scenario, accumulation of sediment must outpace water table rise to allow the formation of a sedimentary sequence (Havholm and Kocurek, 1994). A rise in the water table acts to cohesively retain sediment, and a deflation surface forms as sediment is removed down to the level of the capillary fringe of the water table. If the water table rises above the depositional surface lacustrine sedimentary deposits may accumulate between or atop associated aeolian deposits (Bohacs et al., 2000; Havholm and Kocurek, 1994) resulting in a series of wet interdunes and playas such as that observed by the MER Opportunity Rover (Grotzinger et al., 2005). In this scenario, evaporation at the aqueous interface may permit mineral precipitation, resulting in local cementation within the more resistant upper half of the sedimentary couplet. Repeated fluctuation of the water table, or continuous upwelling with periods of enhanced aridity and mineral precipitation, has the potential to create stacked couplets. In Becquerel Crater and Crater A, no statistical thickening or thinning trend was observed from bottom to top of the composite stratigraphic succession suggesting that the base level fluctuation was never substantially greater than sediment accumulation (Van Wagoner et al., 1988). To accomplish this, accommodation space defined by base level must have been filled as rapidly as it was created, or sedimentation must have exceeded accommodation space. Assuming base level and accommodation space are determined by water level, each couplet would represent the same base level increase if sedimentation were constant. Because there are no lateral changes in sediment thickness in Becquerel Crater, this indicates that sediment dispersal was uniform, such as settling out of suspension forming sheet deposits (Stack et al., 2013). If sedimentation was constant, repetition of depositional parameters (e.g. tidal, seasonal, solar, orbital cycles) would need to have occurred over extended periods of time to cause fluctuations in base level. In Danielson Crater, in order to produce a thickening upsection trend in couplet thicknesses, an increase in accommodation space by increasing base level would have been needed if accommodation space were filled with each couplet. It is possible that the sediment influx in Danielson could have been comparable to Becquerel Crater and Crater A if sedimentation outpaced base-level increase, however, the increase in base level would have resulted in sequential thickening of depositional couplets (Brown et al., 2006; Montanez and Olsleger, 1993). At this level of investigation, it is important to recall that both lacustrine and aeolian environments cannot readily be differentiated without observation of sedimentary structures, grain-size analysis and/or mineralogy. Both lacustrine and aeolian environments can experience change in the position of the aqueous interface, deriving from either ground- or surface-water sources. On Earth, mixed lacustrine/aeolian environments are common, wherein a basin can switch from over-filled to balanced-filled to under-filled depending on the amount of pooled water relative to sediment input (Bohacs et al., 2000). For example, the Green River Formation records deposition that is, at times, fluvial, over-filled, balanced, and under-filled (Bradley, 1964; Bohacs et al., 2000). In

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addition, different lake types can coexist in adjacent basins (Bradley, 1964). An over-filled basin will occur when the flux of sediment and water constantly exceeds accommodation. Changing climatic conditions could cause the basin to become under-filled, wherein accommodation exceeds water and sediment supply, resulting in closed basin hydrology with lakes interspersed with playa deposits. In a mixed lacustrine/aeolian depositional environment in a closed basin, if sediment deposition fills or exceeds available accommodation space, evaporation at the aqueous interface drives lithification of the upper sediment package, resulting in formation of a depositional couplet. If this couplet represents the complete filling of accommodation space, the deposition of a successive couplet requires production of additional accommodation space driven by base level. 5.2. Climatic implications of layered strata There has been much contention as to the extent to which early Mars was warm and wet (Andrews-Hanna et al., 2007; Baker et al., 1991; Fairén, 2010; Head, 2013; Phillips et al., 2001; Zabrusky et al., 2012). Evidence for liquid water across ancient terrains of Mars has perpetuated the argument that the early martian climate was substantially different than present, capable of sustained warm and wet periods, if only episodically, and with an Earth-like hydrological cycle (Andrews-Hanna et al., 2007; Baker et al., 1991; Phillips et al., 2001). Recent work has suggested that under this warm, wet climatic regime, a groundwater upwelling model could produce sedimentary deposits (Andrews-Hanna and Lewis, 2011; Andrews-Hanna et al., 2010). In this model, sediment infilling would occur either as partial fill of a crater/depression until the water table ceased to rise and the flux of groundwater to the surface terminated, or from complete infill of sediment where the rising water table intersected the surface throughout the surrounding area. Evaporation of groundwater at the sediment surface would precipitate salt and cement materials in place. A waning hydrologic cycle would cause precipitation and evaporation rates to decrease, dropping the water table below the surface. Sedimentary deposits left above the water table would be susceptible to deflation (Andrews-Hanna and Lewis, 2011; Andrews-Hanna et al., 2010). Alternatively, a significant increase in precipitation rate may result in fluvial erosion of deposits (Andrews-Hanna and Lewis, 2011). Although this warm, wet groundwater model explains the existence of sedimentary couplets showing both rhythmic thicknesses and upsection thickening by changes in the magnitude of groundwater upwelling, the potential for a cold, wet early Mars should also be considered when exploring the origin of sedimentary depositional packages. A water-enriched, but cold early Mars could have experienced a hydrological cycle dominated by cold, saline and/or acidic fluids (Fairén, 2010). Under these conditions, episodic or periodic increases in surface temperature, whether from impacts, volcanism or climatic variations (Head, 2013), could cause ice to melt and form confined aquifers of liquid water recharged by rain or snowmelt (Kite et al., 2013; Richardson and Mischna, 2005). Within a predominantly frozen groundwater reservoir, melting of ice could result in groundwater-driven accumulation of strata. Similar to warm conditions, unconsolidated sediment would temporarily accumulate in the crater where it would be susceptible to deflation. In this scenario, even minor warming and melting of ground ice near the ground surface (Hooper and Dinwiddie, 2014) would allow water to percolate into craters, pool and stabilize sediments (Fig. 8). It is unlikely, however, that melting would occur deep in the frozen crater strata because snow and sediment would insulate the crater floor from solar heat influx (Everest and Bradwell, 2003; Kjaer et al., 2006). During warm intervals, evaporation at the liquid interface could also drive the upward movement of liquid through capillary action, potentially providing a

mechanism of ion delivery necessary for differential cementation and formation of a depositional couplet. Any melt water exceeding the thickness of aeolian sediment would result in short term lacustrine deposition, and any sediment deposition above this position would be susceptible to deflation. Subsequent cooling climate would potentially re-freeze the active hydrological layer and reestablish conditions necessary to sustain deposition of another sequence of aeolian sediments. Further warming intervals would drive establishment of another active layer, and development and accumulation of another depositional couplet (Fig. 8). Similar processes to that proposed here are observed in Alaska, where cold-region dunes composed of interbedded sand, snow and ice and are activated by seasonal thaw resulting in initiation of alluvial processes (Hooper and Dinwiddie, 2014). A cold, wet environment with ground-ice/melt model for couplet formation has the ability to explain stratal patterns observed within the three studied craters. Couplet thicknesses differ among the craters, signifying distinct difference in depositional conditions. The most pronounced difference is the quasi-periodic bundling in Becquerel Crater, and thickening upsection trend observed in Danielson Crater. The bundling of couplets in Becquerel Crater has previously been interpreted to be resultant from orbital cyclicity of 120 ky cycles and 1.2 My modulations (Lewis et al., 2008). Couplets in Becquerel Crater have a mean thickness of 3.6 m, suggesting annual deposition to be unlikely, with orbital cyclicity providing a more reasonable deposition rate, estimated at 100 lm yr 1 (Lewis et al., 2008). The global climate of Mars is greatly influenced by obliquity, as the tilt of the planet’s spin axis ranges from 0° to 60° (Touma and Wisdom, 1993), influencing the mean annual insolation at low latitudes by 10% or more (Lewis et al., 2008). Increased heat influx would potentially melt ground ice (Richardson and Mischna, 2005), allowing liquid water to flow into craters. As a result, warming caused by orbital forces could allow for melting and stabilization of sediment in the crater. The bundling of depositional couplets observed in Becquerel Crater may represent the waxing and waning of climatic cycles through several obliquity modulations (Hinnov, 2013; Laskar et al., 2004). Although orbital forcing demands regional climate change, it does not necessarily imply that each crater records the same amount of liquid influx. Development of sedimentary couplets would depend on the crater size and depth, presence of accumulated strata within the crater, amount of new aeolian sediment at any given time, position of the sedimentary deposits with respect to the ground-water table and amount of liquid water that infiltrates the crater during periods of ground-ice melting. The amount of melt-water formation would be a function of the duration of warmer conditions, with greater ground-ice melting resulting in more extensive fluid infiltration. Ultimately, relative positions of both aqueous and sedimentary interfaces would affect both deposition and the degree to which evaporation at the aqueous interface affected cementation and sediment stabilization. Therefore, although bundling of couplets was not identified within Danielson Crater or Crater A, it is possible that couplet formation was also the result of orbital forcing driving melting ground-ice. If so, couplets in Danielson and Crater A could reflect forcing by 120 ky cycles without substantial influence by the 1.2 My modulation (Andrews-Hanna and Lewis, 2011). Assuming water is sourced only from melting ice within crater rims, if the amount of melt was constant throughout the region, the ratio of melt water to crater area would have been less for a large, deep crater like Becquerel Carter. The increase in area may have allowed for the recording of both modulations of the orbital climate record. Alternatively, Danielson Crater and Crater A may have received substantial meltwater influx during only the most intense or prolonged changes in orbital climate parameters, with remaining modulations reflected by missed beats. The potential for missed beats in

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(A)

(B)

(C)

Fig. 8. Ground-ice/melt model for intracrater sediment deposition. (A) Aeolian sediment temporarily accumulates in crater; subsurface pore and fracture networks are icefilled. Sediment is unconsolidated and susceptible to deflation. (B) During times of warmer climate (e.g. times of greater solar insolation) either locally or globally, melting of near-surface ground ice creates an active groundwater layer. Groundwater percolates into the crater, pooling above the subsurface sediment–ice interface. If the upper surface of the water table occurs at or near the sediment surface, evaporation and capillary flow can drives mineral precipitation and sediment lithification at the aqueous interface, forming the well-lithified ‘‘cap’’ of the sedimentary couplet; extraneous sediment above this zone of mineralization is susceptible to deflation. If the upper surface of the water table extends above the sediment surface, ponding may occur. (C) In this model, cooler climates drive freezing of the active layer, as well as potential sediment pore fluids, allowing initiation of a subsequent depositional couplet.

climate-sediment cycles is related to the Sadler effect: even as cyclicity within stratigraphic packages may indicate cyclicity in time, its absence in the stratigraphic record does not necessarily indicate the absence of cyclic forcing (Schwarzacher, 2000). Differences in depositional packages amongst the craters may have also resulted from relative age differences. At this level of investigation, we are unable to determine the timing of sediment deposition within each crater and its relation to other intracrater stratal packages. The Noachian–Hesperian boundary is when sedimentary deposits have been previously suggested to form, wherein the Noachian is interpreted as being both warmer and wetter, before transitioning to drier conditions in the Hesperian (Andrews-Hanna and Lewis, 2011; Fairén, 2010; Zabrusky et al., 2012). However, extensive erosion has likely obliterated a subset of craters used for age dating, inhibiting interpretation age for these deposits (Hartmann, 2005). Warmer conditions at the end of the Noachian may have resulted in more substantial groundice melting, causing increasing temporary accommodation space and resulting in the thickening of stratal deposits in Danielson Crater. During the Noachian–Hesperian transition, increasing aridity and decreasing temperatures may have reduced the ability for climate modulations to result in substantial ground-ice melting. Under increased aridity and decreased temperatures, a substantial

active layer would have developed only during the warmest periods of the orbital cycle, possibly resulting in missed beats or a ‘‘wet-pass’’ filter (Kite et al., 2013). The ‘‘wet-pass’’ filter, which reflects a combination of temperature and pressure conditions wherein snow melts and liquid surface water exists on Mars, would occur only when both obliquity and eccentricity are high and perihelion occurs near equinox (Kite et al., 2013). Finally, although we concentrate on intracrater layer deposits, an ice-melt model for accumulation of sediment may also be applicable to intercrater layered deposits or layered deposits that extend above the crater rim. Such a scenario would require either a regional base level above the level of local crater rims, in which case intercrater deposition would be expected to reflect deposition after complete infilling of local craters (Malin and Edgett, 2000), or a source of locally upwelling groundwater (Andrews-Hanna et al., 2007) which provided a regionally variable aqueous base level. In the latter scenario, warm climate conditions would result in active upwelling, with evaporation at the liquid–atmosphere interface driving cementation of sediment. During colder climate conditions, aeolian sediment might still accumulate, but would be susceptible to deflation unless warming permitted reactivation of the aqueous base level (cf. Kite et al., 2013).

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5.3. Applicability of stratal packages to interpret depositional environments If couplet thicknesses of sedimentary deposits on Mars are to be used to interpret possible depositional mechanisms and environments, it is necessary for there to be consistency between analyses. Stratigraphic thicknesses computed for Becquerel Crater and Crater A here are consistent with results reported by Lewis et al. (2008). Although couplet thicknesses computed for Danielson Crater (7.7 ± 3.3 m), varied from previous reports (9.7 ± 1.5 m and 10 ± 1.6 m; Lewis et al., 2008; Lewis and Aharonson, 2014) they are within one standard deviation. Variation is also observed with mean thickness in comparison with results of Stack et al. (2013). For both Becquerel and Danielson Craters, Stack et al. (2013) measured thinner couplets. In their analysis, high-resolution images (0.25 m/pixel) were overlaid on DTMs; these thinner couplets cannot be differentiated at the 1.0 m/pixel resolution used in our analysis. For Becquerel Crater, successions were taken at similar locations, however in Danielson Crater, Stack et al. (2013) constructed stratigraphic successions over uplifted blocks that showed large variation in dip, which were avoided in this study. Another difference between the results presented here and other analyses is the variation in stratigraphic trends. In our analysis, Becquerel Crater is found to have no statistically significant thickening or thinning upsection trend, and couplets are consistent with uniform distribution. Stack et al. (2013) report a statistically significant thinning upsection trend. If we assume each couplet represents complete infilling of accommodation space, as we do in this investigation, upwards thinning of couplets would reflect the successive decrease in the rate at which accommodation space is increased. By contrast, if we relax this assumption and allow sediment deposition that does not fill available accommodation space, a thinning trend would require a long-term change in sedimentary influx. It should be noted that the absence of statistically relevant upsection trends in Becquerel Crater reflects, in part, the determined alpha level (0.05) and if we were to have chosen a larger critical value (such as 0.1) a thinning upsection trend would be inferred for Becquerel Crater as well. The opposite is observed in Danielson Crater, where Stack et al. (2013) indicate no upsection trend in couplet thickness whereas results of this study indicate a thickening upsection trend. In terms of aqueous level, if each couplet represents complete filling of accommodation space and production of new accommodation space, both upsection trends can be explained by a changing base level over time. The variation in trends between this study and Stack et al. (2013) may reflect differences in location of measurement within stratigraphic succession, which would suggest that varying mechanisms occurred during deposition. The Stack et al. (2013) study analyzed a larger stratigraphic succession, which could be cause of the variation observed. Regardless, both a thickening upsection trend and no upsection trend could result from a mixed lacustrine/aeolian system, where a basin could switch from over-filled to under-filled due to changing climatic conditions (Bohacs et al., 2000). Despite variations in couplet thicknesses and stratal trends between studies, orbital images are able to provide first order information regarding depositional conditions. On Earth, layered stratal packages can form in a number of different depositional environments. A recessive/resistant couplet such as is observed in this study’s craters, may be analogous to a parasequence on Earth (Van Wagoner et al., 1988). In this scenario, individual beds that comprise the couplet reflect varying hydrodynamic energies, wherein the lower recessive interval may indicate low energy deposits that form characteristically fine-grained and thinly bedded layers whereas the upper resistant interval could represent highenergy deposits that are characteristically coarse-grained and more

thickly bedded. However, in order to create a fining upward or graded sequence, a decrease in energy is needed. Sedimentary couplets are also seen in glacial lakes in sedimentary varves, with fine grained layers representing low energy conditions and sand/silt layers representing higher energy conditions (Ridge and Larson, 1990). Additionally, in aeolian environments, where sedimentary deposition is characteristically more uniform in grain size, couplets likely result from preferential cementation of the sedimentary package via evaporation of ambient pore fluids and mineral precipitation at the air–water interface (Havholm and Kocurek, 1994). In all of these environments, a combination of similar couplet thickness, as well as thickening and thinning upsection are possible based on changes in accommodation space and base level. While we were unable to define a clear depositional environment for intracrater layered deposits of three craters in Arabia Terra, we have demonstrated that both lacustrine and aeolian environments have the ability to change the position of aqueous interface, driving changes in base level and sediment accumulation. These variations in aqueous interface can arise under a range of climatic conditions, from a warm early Mars (Andrews-Hanna and Lewis, 2011; AndrewsHanna et al., 2010), to a cold early Mars (Kite et al., 2013). At this level of investigation, without sedimentary structures, grain-size analysis and/or mineralogy these environments and climatic regimes cannot be differentiated. Therefore, while analyzing stratal deposits on Mars may help to rule out possible mechanisms of deposition and environments and hypothesize new plausible conditions, couplet thicknesses and trends alone cannot be used to decipher depositional environments and climatic regimes.

6. Conclusions The presence of sedimentary bedding in craters on Mars indicates a sedimentological response to changes in depositional processes, reflecting a combination of sediment input, base level and accommodation space. Here, we build upon earlier analyses of stratal packaging by measuring stratigraphic thicknesses in multiple sections within Becquerel Crater, Danielson Crater and Crater A in Arabia Terra. Measurements of stratal thickness were taken from the top of a resistant ‘‘cap’’ to the top of the next successive resistant ‘‘cap’’. Therefore, we suggest that in the future, these intervals should no longer be identified as ‘‘beds’’, but as depositional couplets. However, it is important to recognize that this does not indicate that each sedimentary unit is two beds, but rather could be a single bed with gradation in erosional resistance. For the three craters analyzed here, the lack of stochastic stratal patterns and fluvial networks entering craters lead us to suggest a mixed lacustrine/ aeolian environment with sediment accumulation driven by fluctuation of an air–water interface within a ground-or surface-water reservoir. We suggest that each couplet potentially represents complete filling of accommodation space, and stacking of depositional couplets reflects a change in local or regional base level and a concomitant production of new accommodation space. Rhythmic distribution of couplet thickness suggests a progressive increase in accommodation space overprinted by a fluctuation in the rate of change in the addition of accommodation space. Such variation likely reflects a climatic driver affecting a mixed lacustrine/aeolian environment. Such a depositional environment has been previously interpreted under a warm martian climate (Andrews-Hanna and Lewis, 2011; Andrews-Hanna et al., 2010). However, we suggest that groundwater-driven accumulation of strata could also occur within a predominantly frozen groundwater reservoir envisioned for a cold, early Mars (Fairén, 2010; Kite et al., 2013). Both of these climatic regimes are consistent with previous interpretations of astronomical forcing of sedimentary packages (Lewis et al., 2008), where absent bundling of couplets in Danielson

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Crater and Crater A is the result of missed beats or a ‘‘wet-pass’’ filter (Andrews-Hanna and Lewis, 2011; Kite et al., 2013). Variation in couplet thicknesses and stratal patterns observed here and in other studies demonstrates the difficulties in using stratal patterns in order to interpret depositional mechanisms. These variations in couplet thicknesses may be due to from sampling resolution and sample size, and can result in different mechanistic conclusions about the same crater. However, by utilizing orbital images to analyze stratal patterns we are able to propose plausible hypotheses for depositional conditions. Similarities in erosional expression of intracrater deposits in Arabia Terra suggests regional implications, however differences in couplet thicknesses and stratal patterns crater-to-crater demonstrates localized differences. Therefore, although analysis of stratal packaging can provide important insight into changes in accommodation space through time, limited textural information provided by orbital images of these sedimentary deposits inhibits our ability to decipher discrete mechanisms of sediment deposition. Acknowledgments We thank SEPM, Southeast GSA, and GSA Planetary Geology Division for funding through their student research grant programs; S.A. Young, M.M. Hage and C. Achilles for assistance in editing; M. Chojnacki and USGS for creation of HiRISE DEMs, and E. Kite and 2 anonymous reviewers for comments that improved this manuscript. References Anderson, R.B., Bell, J.F., 2010. Geologic mapping and characterization of Gale Crater and implications for its potential as a Mars Science Laboratory landing site. Mars 5, 76–128. Andrews-Hanna, J.C., Lewis, K.W., 2011. Early Mars hydrology: 2. Hydrological evolution in the Noachian and Hesparian epochs. J. Geophys. Res. 116 (E02007). http://dx.doi.org/10.1029/2010JE003709. Andrews-Hanna, J.C., Phillips, R.J., Zuber, M.T., 2007. Meridiani Planum and the global hydrology of Mars. Nature 446 (7132), 163–166. Andrews-Hanna, J.C., Zuber, M.T., Arvidson, R.E., Wiseman, S.M., 2010. Early Mars hydrology: Meridiani playa deposits and the sedimentary record of Arabia Terra. J. Geophys. Res. 115 (E06002). http://dx.doi.org/10.1029/2009JE003485. Ansan, V., Loizeau, D., Mangold, N., LeMouelic, S., Carter, J., Poulet, F., Dromart, G., Lucas, A., Bibring, J.P., Gendrin, A., Gondet, B., Langevin, Y., Masson, P., Murchie, S., Mustard, J., Neukum, G., 2011. Stratigraphy, mineralogy, and origin of layered eposits inside Terby crater, Mars. Icarus 211, 273–304. Baker, V.R., Strom, R.G., Gulick, V.C., Kargel, J.S., Komatsu, G., Kale, V.S., 1991. Ancient oceans, ice sheets and the hydrological cycle on Mars. Nature 352 (589), 589–594. Barley, M.E., 1993. Volcanic, sedimentary and tectonostratigraphic environments of the 3.46 Ga Warrawoona Megasequence: A review. Precambr. Res. 60, 47–67. Barlow, N.G., 2008. Mars: An Introduction to its Interior, Surface, and Atmosphere. Cambridge Planetary Science, 264pp. Beyer, R.A., McEwen, A.S., 2005. Layering stratigraphy of eastern Coprates and northern Capri Chasmata, Mars. Icarus 197, 1–23. Bibring, J.P. et al., 2005. Mars surface diversity as revealed by the OMEGA/Mars Express observations. Science 307 (5715), 1576–1581. Boggs, S.J., 2006. Principles of Sedimentology and Stratigraphy. Prentice Hall, 688pp. Bohacs, K.M., Carroll, A.R., Neal, J., Mankiewicz, P.J., 2000. Lake-basin type, source potential, and hydrocarbon character: An integrated-sequence-stratigraphicgeochemical framework. In: Kelts, K.R. (Ed.), Lake Basins through Space and Time. AAPG Studies in Geology, pp. 3–34. Bradley, W.H., 1964. Geology of Green River formation and associated Eocene rocks in southwestern Wyoming and adjacent parts of Colorado and Utah. In: U.S.G.S.P. Paper (Ed.), pp. A1–A86. Bridges, J.C., Kim, J.R., Tragheim, D.G., Muller, J.-P., Balme, M.R., Pullan, D., 2008. Sedimentary rocks in Becquerel Crater: Origins as polar layered deposits during high obliquity. Lunar Planet. Sci. Brown, H.E., Holbrook, W.S., Hornbach, M.J., Nealon, J., 2006. Slide structure and role of gas hydrate at the northern boundary of the Storegga Slide, offshore Norway. Mar. Geol. 229, 179–186. Burt, D.M., Knauth, L.P., Wohletz, K.H., 2008. Sedimentation by impact cratering on Mars. Lunar Planet. Sci. XXXIX. Cabrol, N.A., Grin, E.A., 1999. Distribution, classification, and ages of martian impact crater lakes. Icarus 142, 160–172. Catling, D., Wood, S., Leovy, C., Montgomery, D., Greenberg, H., Glein, C., Moore, J., 2006. Light-toned layered deposits in Juventae Chasma, Mars. Icarus 181 (1), 26–51.

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