Extraformational sediment recycling on Mars

Abstract

Extraformational sediment recycling (old sedimentary rock to new sedimentary rock) is a fundamental aspect of Earth's geological record; tectonism exposes sedimentary rock, whereupon it is weathered and eroded to form new sediment that later becomes lithified. On Mars, tectonism has been minor, but two decades of orbiter instrument-based studies show that some sedimentary rocks previously buried to depths of kilometers have been exposed, by erosion, at the surface. Four locations in Gale crater, explored using the National Aeronautics and Space Administration's Curiosity rover, exhibit sedimentary lithoclasts in sedimentary rock: At Marias Pass, they are mudstone fragments in sandstone derived from strata below an erosional unconformity; at Bimbe, they are pebble-sized sandstone and, possibly, laminated, intraclast-bearing, chemical (calcium sulfate) sediment fragments in conglomerates; at Cooperstown, they are pebble-sized fragments of sandstone within coarse sandstone; at Dingo Gap, they are cobble-sized, stratified sandstone fragments in conglomerate derived from an immediately underlying sandstone. Mars orbiter images show lithified sediment fans at the termini of canyons that incise sedimentary rock in Gale crater; these, too, consist of recycled, extraformational sediment. The recycled sediments in Gale crater are compositionally immature, indicating the dominance of physical weathering processes during the second known cycle. The observations at Marias Pass indicate that sediment eroded and removed from craters such as Gale crater during the Martian Hesperian Period could have been recycled to form new rock elsewhere. Our results permit prediction that lithified deltaic sediments at the Perseverance (landing in 2021) and Rosalind Franklin (landing in 2023) rover field sites could contain extraformational recycled sediment.

Figure 1. Rock cycle and sedimentary cycle on Mars.

(A) The Martian rock cycle includes igneous, sedimentary, and metamorphic rocks (Wilson et al., 1974; McSween, 2015; McSween et al., 2015). Impact events contribute through production of clastic debris, melt, and shock metamorphic products. Metamorphic rocks also include those formed at contacts between country rock and magmatic intrusions (e.g., Flahaut et al., 2011). (B) The Martian sedimentary cycle. As shown in A, this includes rock exposure, rock breakdown, and clast production, transport, deposition, and storage that result from impact cratering.

Figure 2. Curiosity rover field site context.

(A) Regional view of Gale crater; colors indicate elevation relative to Martian datum. Gale is located at 5.4°S, 222.2°W (aerographic latitude, west-positive longitude; 5.3°S, 137.9°E areocentric latitude, east-positive longitude). (B) Gale crater and its 5-km-high interior mound, Aeolis Mons; white trace indicates rover traverse. (C) Rover traverse (white trace) between sols 0 and 2650. The four study sites are indicated in yellow. (D) Rock units encountered along the Curiosity rover traverse through September 2019 as a function of elevation relative to the Martian datum. Blue annotations indicate the four study sites; Bimbe is an unconsolidated accumulation of boulders, cobbles, and pebbles that is younger than the Stimson formation and superposes outcrops of the Hartmann’s Valley member of the Murray formation (Wiens et al., 2020). The wavy line separating rocks of the Stimson and Murray formations, and between the Siccar Point group and Mount Sharp group, represents a subaerial paleo-erosion surface; the Stimson (Siccar Point group) rocks are younger (Watkins et al., 2016). MSL—Mars Science Laboratory. Note that, for this and subsequent figures, source images and source data products are listed in Supplement S2 (see text footnote 1).

Figure 3. Sedimentary rock fragments—and fragments from veins formed in fractures in sedimentary rocks—in modern Gale crater.

(A) Dark-gray, boulder-and cobble-sized sandstone fragments (e.g., arrows) shed from one of the Murray buttes. (B) Reddish mudstone pebbles; arrow indicates example mud-stone laminae. (C) Reddish mudstone fragments in an eolian bed form named Trumpet (largest examples at arrows); some include white veins within them. (D) Coarse sand-sized sandstone fragment (circled), consisting of very fine sand, among the eolian sands of the Namib dune in the Bagnold dune field. (E) Examples of sand-bearing concretions liberated from eolian sandstones of the Stimson formation. (F) Pebble-sized white clasts (examples at white arrows) interpreted as vein mineral fragments; yellow arrows indicate examples of white veins in angular mudstone boulders. Note pebble-sized, reddish mudstone fragments. (G) White, cuboid vein mineral fragment, interpreted to consist of calcium sulfate, exhibiting breakage along cleavage planes. (H) Conglomeratic boulder at Bimbe surrounded and partly covered by eolian sand; boulder in left foreground is a sandstone (Wiens et al., 2020). (I) Pebbles at the left and lower center were liberated from an outcrop of conglomerate (right); target name is Link (Williams et al., 2013).

Figure 4. Mudstone and vein mineral fragments in sandstone at Marias Pass.

(A) Regional view of the Marias Pass site; white trace indicates rover traverse; yellow arrows indicate traverse direction. (B) Clark-Missoula outcrop, showing a portion of a lens of coarse, dark-gray, eolian sandstone (Stimson formation) unconformably overlying light-gray and fractured (with white veins) Pahrump Hills member (Murray formation) mudstones. The light-toned granules and small pebbles in the Clark facies are interpreted to be mudstone and vein mineral fragments. The dashed blue trace below the Clark facies marks the basal Siccar Point unconformity. (C) Portion of Clark-Missoula outcrop showing that the unconformity between the sandstone and mudstone (dashed white trace) is uneven at millimeter scale and showing examples of mudstone fragments; whole fragments are indicated (white arrows); recessed (eroded) examples and sockets previously occupied by such grains are also indicated (yellow arrows). The clast indicated by a blue arrow displays bands interpreted to be depositional laminae. (D) Sketch showing interpretation of mudstone fragment weathering expressions. As the sandstone is undermined (because the subjacent mud-stone is less erosion resistant), pieces of sandstone might fracture and fall away, exposing protrusive mudstone fragments as well as broken fragments and sockets as some clasts also fall away. Wind abrasion (from dark-gray sand nearby; foreground in B) can further alter exposed, protrusive mudstone fragments. (E) Portion of the Clark-Missoula outcrop showing examples of angular, white (somewhat orange in this shadowed view) clasts interpreted as vein mineral fragments. White arrows indicate intact mudstone fragments; yellow arrows indicate eroded, recessive examples or sockets.

Figure 5. Vein mineral fragment in remnant of a Clark facies sandstone-filled fracture named Seeley.

(A) Context view, showing Seeley in relation to the Clark-Missoula outcrop of Figure 4. (B) Mastcam view of Seeley before it was broken by Curiosity’s wheels. Yellow dashed lines indicate rock protruding (as fins) from two sandstone-filled, former fractures that cut into the underlying Murray formation mudstone host rock. (C) Mastcam view of Seeley after it was broken. Location of D is indicated by the dashed white box. (D) Mars Hand Lens Imager (MAHLI) view of broken sandstone, which formerly filled a fracture in Murray formation mudstone. Yellow arrows indicate pieces of a white clast that was split along the break created by the rover wheels. (E) Stereo anaglyph (please view using red/blue glasses) showing white, broken vein mineral fragment incorporated into the sandstone; note that the cleavage pattern of intersecting planes exhibits a cubic or rhombic form.

Figure 6. Context views of the boulder- and cobble-strewn accumulation named Bimbe.

(A) Bimbe in regional context. Curiosity’s traverse is gray; yellow arrows indicate rover drive direction from northeast to south. Bedrock surrounding Bimbe consists of exposures of the Murray formation; Stimson formation eolian sandstones cap the Naukluft plateau and Murray buttes; the darkest gray features are modern eolian sand dunes. (B) Local context view of Bimbe showing the locations of boulders Tumba-Funda and Balombo; rover traverse is in yellow (arrowheads indicate drive direction). (C) Panoramic view of Bimbe, as viewed from the east; locations of Tumba-Funda and Balombo are indicated. (D) Rover navigation camera view of the Tumba-Funda and Balombo boulders; rover wheel for scale.

Figure 7. Tumba and layered clast in conglomeratic boulder named Tumba-Funda.

(A) Sandstone pebble named Tumba; locations of fracture and vein and one of its largest grains are indicated. Box shows location of C. (B) Context view of layered pebble in D and Tumba sandstone pebble in A. (C) Expanded view of a portion of Tumba; larger grain sizes are indicated. (D) Layered pebble in the Tumba-Funda boulder, likely a sedimentary lithoclast.

Figure 8. Recessed white features in Tumba-Funda boulder.

(A) Mars Hand Lens Imager (MAHLI) view looking downward on the boulder; arrows indicate pebble-sized, recessed white features. (B) Expanded view showing some of the recessive white features (arrows). (C) Close-up view of the recessive white feature named Funda.

Figure 9. Sandstone fragment in conglomeratic boulder named Balombo.

(A) Balombo boulder; outline indicates location of the ChemCam Remote Micro-Imager (RMI) view in B. (B) Sandstone pebble in the Balombo boulder, as viewed y ChemCam RMI. The location of the nearest ChemCam LIBS (laser-induced breakdown spectrometer) shot point, number 10, is indicated by ed crosshairs. The LIBS spectroscopy results ere published by Wiens et al. (2020).

Figure 10. Close-up views of Funda.

(A) White recessive feature named Funda, illuminated by sunlight from the top/upper right. (B) Stereo anaglyph view (please view using red/blue glasses). (C) Sketch of details evident in the Mars Hand Lens Imager (MAHLI) images of Funda, including rounded grains within Funda (green); features that could be clasts, crystals, pits, or facets (yellow); stepped, banded features (interpreted as fine laminae); and a truncation (interpreted as an unconformity in a layered sequence) between the banded, upper two thirds of Funda and the less-well-organized nature of the lower third. (D) Surface relief map determined using Structure-from-Motion (SfM) analysis (Garvin et al., 2017) of four overlapping Mars Hand Lens Imager (MAHLI) images; uncertainties are ±15%. The SfM point cloud product is in Supplement S6 (see text footnote 1). (E) Surface relief (±15% uncertainty) profile along line a–a′ in D, shown with interpreted lamination pattern and truncation surface.

Figure 11. Sedimentary lithoclasts in sandstone at Cooperstown.

(A) Regional view; tracks made by Curiosity’s wheels are visible; yellow arrows indicate rover drive direction. (B) Sandstones at Cooperstown; the dark, gray Rensselaer unit contains various angular granule- and pebble-sized clasts that protrude from the outcrop surface; also note sockets from which similar-sized clasts were removed. An unnamed, lighter-gray, recessive sandstone occurs beneath the Rensselaer unit. (C) Angular, platy granules and pebbles protruding from the Rensselaer unit sandstone (examples at arrows); also note empty sockets. (D) Mars Hand Lens Imager (MAHLI) view, under the overhanging Rensselaer unit, showing the recessive sandstone facies; it is crosscut by dark-toned, erosion-resistant veins. (E) Portion of dagger-shaped pebble, Deep Kill, which is too fine grained to distinguish whether it is igneous or sedimentary. (F) Example layered or fractured pebble in the Rensselaer unit.

Figure 12. Sedimentary rock fragments in a conglomerate at Dingo Gap.

(A) Regional view, including the Curiosity rover (left of center) and its dark-toned wheel tracks. Dashed polygons indicate the approximate locations of B and C; vertical yellow arrows indicate Dingo Gap; white trace indicates the subsequent rover path; yellow arrows at left and right indicate direction of rover travel. (B) Panoramic view of the south wall of Dingo Gap. The dashed outline approximately indicates the top and bottom of a conglomeratic channel body overlain by a thin, laminated facies (Edgar et al., 2016). (C) Outcrop on the north side of Moonlight Valley, showing contact (dashed white trace) between conglomerate and cross-bedded sandstone facies. (D–F) Examples of cobble-sized sedimentary lithoclasts in the conglomeratic facies, outlined in white.

Figure 13. Remnant lithified sediment fan at terminus of a canyon on the west side of Aeolis Mons.

(A) Gale crater with location of fan indicated. Yellow trace indicates the Curiosity rover traverse. (B) Remnant canyon and lithified fan; location of C is indicated; blue arrow indicates eroded impact structure superimposed on the fan. (C) Remnant lithified fan; location of D is indicated; blue, dashed circle indicates the impact structure. (D) Close-up view of two slopes (arrows), one of which is the wall of the remnant impact structure, cut into the lithified fan. The sizes of three large boulders, produced by erosion of the lithified fan sediment, are indicated.

Figure 14. Extraformational multicycling of sediment in Gale crater, Mars.

(A) Known cycles to which the sand grains inside the sandstone pebble Tumba were subjected. During the first known cycle, the sands were created, transported, deposited, lithified, and fractured (in which formed a vein), and then the resulting sandstone body became exposed at the Martian surface. The second cycle involved erosion of that sandstone, production, transport, and deposition of the Tumba pebble, followed by lithification within a conglomerate. The third cycle began with exposure of the conglomerate, followed by its weathering and erosion, which produced the Tumba-Funda boulder, deposited with the other boulders and cobbles at Bimbe. (B) Known cycles to which the tiny grains within the mudstone fragments in the Clark facies of the Stimson formation sandstone were subjected. The first known cycle involved clast creation, deposition, and diagenesis of the mud. The second cycle involved deep erosion of Aeolis Mons strata in Gale crater, forming the moat between the north wall of Gale crater and the north-facing slope of Aeolis Mons; exposure of the mudstone to erosion; transport and deposition of mudstone fragments with other sands; and lithification within a sandstone. The third cycle has more recently begun, with exposure and ongoing weathering of the sandstone at Marias Pass.

Figure 15. Gale crater target rock. Heavily cratered terrain, filled and partly filled impact structures, ejecta deposits, and eolian, fluvial, and alluvial sediments all likely occurred in the area obliterated by the Gale crater–forming impactor.

(A) Present-day configuration of the terrain in and around Gale crater. (B) Sketch showing current knowledge of terrain features present immediately before the Gale crater–forming impact occurred. Larger impact structures interpreted to have been superposed by Gale ejecta (including secondary craters) are indicated in green, valleys are in purple, and the north-south dichotomy boundary is in blue. Dashed valley trace and delta (purple) are inferred to have existed as a logical northeastward extension of the valley covered by Gale ejecta described by Irwin et al. (2005). The brown feature (v?) was proposed by Churchill (2018) to be a remnant volcanic edifice that might have predated the Gale crater–forming impact. For simplicity, the buttes and mesas immediately north of the dichotomy boundary (visible in A) are not sketched; some of these are also superposed by Gale ejecta.

Figure 16. Dark-toned wind streak emanating from the south side of Gale crater.

(A) Regional view from Mangalyaan shows that the streak extends ~400 km southward from Gale crater. (B) Close-up view of a portion of the southern rim of Gale, showing the dark-toned sediment that composes the wind streak. This material obscures much of the rugged topography normally found on the raised rim of an impact crater.

Figure 17. Mesa-capping rock composed of megaclastic sediment in western Candor Chasma, located at 6.58°S, 75.40°W (6.51°S, 284.69°E).

(A) Mesa (arrow) in context, surrounded by outcrops of light- and intermediate-toned, stratified bedrock. (B) Megaclastic nature of the capping material, with the location of C indicated. (C) Stereo anaglyph view of the angular, stratified megaclasts (please examine wearing red-left-eye, blue-right-eye three-dimensional glasses).

Figure 18. Potential for extraformational, recycled sediment at the planned Rosalind Franklin rover field site in Oxia Planum.

(A) Regional view of the Coogoon Vallis terminus and lithified deltaic sediment at Oxia Planum, near 18.1°N, 23.9°W (also 17.9°N, 336.2°E). Location of B is indicated by the blue arrow. (B) Light-toned, clay-bearing rock, interpreted as sedimentary (Quantin-Nataf et al., 2019), cut by Coogoon Vallis near the valley terminus. Compare the scale and pattern of fractures in the bedrock with image in C. (C) Example of sedimentary rock exposure in northern Gale crater (near but not visited by the Curiosity rover), shown at the same scale as the image in B; note the similar fracture pattern and impact structure retention.

Figure 19. Potential for extraformational, recycled sediment at the planned Perseverance rover site in Jezero crater, located at 18.6°N, 282.4°W (18.4°N, 77.7°E). C–G are presented at the same scale.

(A) Jezero crater and local context. The locations of B–E are indicated. Blue traces indicate inlet valleys (Neretva and Sava); purple trace indicates an outlet valley (Pliva). West of Jezero crater, olivine- and carbonate-bearing rock is cut by Neretva Vallis and Sava Vallis (Goudge et al., 2015). (B) Lithified, deltaic sediment of the “western delta” in Jezero crater. (C) Detailed view of the geomorphic expression of rock cut by Neretva Vallis. (D) Detailed view of the geomorphic expression of rock cut by Sava Vallis. (E) Detailed view of the geomorphic expression of rock cut by Neretva Vallis. (F) Compare with D; detailed view of an outcrop of the fine-grained (largely mudstone) Murray formation in Gale crater; dark-toned materials are windblown sands, and lighter-toned features are blocks of mudstone slightly displaced in outcrop expression. Yellow trace indicates the Curiosity rover traverse; yellow dots indicate locations where the rover parked after each drive; white numbers indicate the sols that bracketed this portion of the traverse. Color inset from Mars Hand Lens Imager (MAHLI) shows a dust-coated mudstone surface investigated along this traverse (arrow); the bands are a stair-stepped expression of fine laminae. (G) Compare with E; detailed view of a dust-coated outcrop of the Sheepbed (mudstone) member of the Yellowknife Bay formation in Gale crater (see Schieber et al., 2017). Yellow trace and dots indicate rover traverse through the area between sols 121 and 309. Color inset from MAHLI shows the very fine–grained nature of the mudstone; the larger, lighter-toned objects are dust clumps stirred by the rover’s wire brush tool; the finer, speckled surface illustrates silt-sized grains.