Geologic Mapping in Southern Margaritifer Terra on Mars: Constraining the Timing of Fluvial Activity in Nirgal Vallis MTM quadrangles -20037, -25037, -30037 and -30032 Sharon Wilson & John Grant (Smithsonian) Debra Buczkowski (JHU) Cathy Weitz (PSI) Robin Fergason (USGS) USGS Planetary Geologic Mappers Meeting, Flagstaff, Arizona June 12-15, 2017 Map Quads in Margaritifer Terra 2 Background SW Margaritifer Terra provides a long and complex record of aqueous processes •N-H age ULM outflow system • Segmented, incises/fills between/across Holden and Ladon impact basins •Evidence for LH fluvial and (or) lacustrine modification (Holden and Uzboi Vallis) • LH to EA alluvial fan deposits (e.g., Grant and Wilson, 2011) •EA-MA Hale impact mobilized water creating fluidized debris flows and channels (Jones et al., 2011) Motivation • Water rich history; place deposits in broader framework • Constrain timing, source, duration and relative importance of aqueous and other geomorphic processes • Climate history and associated habitability 5 new IAU names 4 DMU Label Unit name, Age, Description Additional Characteristics Interpretation Plateau and Highland Units Phyllosilicate-bearing layer exposed in Map Status Noachian geomorphic surface resulting Terra unit (Early Hesperian to Late walls of Nirgal Vallis. Overlies from impact cratering, ejecta Noachian) – Widespread, smooth to mountainous unit (Nm) and highland emplacement, HNt rolling, cratered, and variably unit (Nh). Underlies all other units. prolonged weathering, and erosion dissected surface between degraded Equivalent to smooth undifferentiated primarily by water and wind. impact craters. plans (Spu) unit (Grant, 1987); wrinkle ridges. Highland unit (Early Hesperian – Noachian geomorphic surface resulting Late Noachian?) Heavily cratered, Overlies mountainous unit. Underlies from impact cratering, ejecta Nh differentially mantled. Contains older all other units. No evidence of emplacement, valley networks, grabens, few phyllosilicate layer. prolonged weathering, and erosion wrinkle ridges. primarily by water and wind. Mountainous unit (Middle Noachian) – Bedrock promontories including Deeper Noachian bedrock outcrops impact crater central peaks and Nm Oldest unit in map area exposed during impacts and overlain by mountain chains along impact basin terra unit rings. Mountains typically steep and variably eroded depending on age. Crater and Channel Fill Units Smooth unit 2 (Middle to Early Amazonian) – smooth (at scales of Deposits post-date the initial formation 10s to 100s of meters), dark-toned of Hale. Local aeolian erosion of the deposit. Occurs at margins of As1 unit Bright in THEMIS day IR. Formed by As distal margins of the lobes implies a fine- 2 in pre-existing valleys, topographic de-watering of the material in unit As . 1 grained component, perhaps produced by depressions, craters, and embays weathering. Variable thickness. secondary craters from Hale. Thins with increasing distance from Hale. Smooth unit 1 (Middle to Early Bright in THEMIS day IR. Locally Amazonian) – smooth (at scales of forms lobes w/ distinct margins. Some 10s to 100s of meters), dark-toned flow lobes are characterized by roughly Deposits post-date the initial formation deposit. Channels and streamlined parallel ridges, oriented perpendicular of Hale. Local aeolian erosion of the deposits common close to Hale to the presumed direction of flow. Light- As distal margins of the lobes implies a fine- 1 crater. Occurs in pre-existing valleys, toned, meter-scale boulders, aeolian grained component, perhaps produced by topographic depressions, craters, and bedforms and extensive cracks (some weathering. Variable thickness. embays secondary craters from Hale. that cross-cut ridges) are common on Thins with increasing distance from lobe surfaces, and layering is not Hale. evident. Alluvial deposits composed primarily of • Fan unit (Early Amazonian to Late Cone-shaped deposits derived from Linework, CMU, gravel and fines, emplaced by fluvial Hesperian) – sloping or cone-shaped deeply dissected impact crater walls sediment transport with little to no AHf deposits. Distributary paleochannel coalesced into fans in craters Luba, evident contribution from debris flows. DMU largely networks preserved in negative or Roddy, Gringauz and Holden. Bright in Low abundance of boulders at HiRISE (more commonly) positive relief. THEMIS nighttime IR. scale. complete Fan unit (Late Hesperian) – Base of deposit is offset to the north Fluvial deposit compose primarily of Degraded material on the floor of Hf toward Holden. Upper fan-shaped light-toned material. Some boulders Uzboi Valles at the mouth of Nirgal deposit is symmetric to mouth of Nirgal. incorporated but upper section is layered. Vallis. Light-toned material, layered. Deflated crater-fill deposits overlying • Efforts focused on Etched unit (Early to Late Hesperian Holden crater ejecta. Includes Hesperian) – Erosionally resistant Phyllosilicate-bearing, layered deposits fine-grained, phyllosilicate-bearing, He southern map area material in Uzboi Vallis. Light-toned and knobs. lacustrine and (or) distal alluvial knobs. deposits overlain by coarse-grained alluvial sediments. Channel unit (Early Hesperian to • Evolution of Nirgal Surfaces eroded by Late Noachian to Late Noachian) – Eroded surfaces Early Hesperian catastrophic flooding NHch related to early incision of Uzboi Overlies HNt. and things veneered by coarse fluvial Vallis. Streamlined outcrops of HNt • Extent of Hale sediments during waning flow. and Nm common. Surficial Deposits deposits Dune unit (Late Amazonian) – Recent aeolian dunes, likely composed Ad concentration of typically dark-toned Dark in THEMIS nighttime IR. of chemically unaltered basaltic sand. bedforms. Crater Units Crater 3 unit (Late Amazonian to • Talk will present the science that was Late Hesperian) – floor, rim and Impact material, fractured rim and continuous ejecta of morphologically Locally overlies HNt, Nh. Underlies Ac continuous ejecta of Hesperian and 3 fresh impact craters little modified by As and As . 1 2 Amazonian impact craters. rim erosion and (or) infilling. Hale published/in preparation for publication crater. Crater 2 unit (Late to Early Moderately degraded crater rim and Hesperian) – rims of Holden and Hc Overlies HNt. Underlies unit AHf. impact ejecta from Hesperian Holden 2 Luba and adjacent areas mantled by that will be incorporated into the final and Luba (and other unnamed craters) their continuous ejecta. Crater 1 unit (Late Noachian) – 5 Remnant eroded rims from heavily Highly degraded crater rims from the Nc Little to no ejecta blanket preserved. 1 modified craters such as Vinogradov, Noachian. map, figures and pamphlet Roddy and Gringauz. Geologic History in the Southern Map • Uzboi Vallis: • ~400 km long, somewhat sinuous valley, southernmost Blunck segment of ULM • Bond (D=111km) and Hale (D=125km) destroyed probable source outlet from Argyre • Mid to Late Hesp. Holden Paleolake in Uzboi blocks northern end • Paleolake in Uzboi basin Martynov (4000km3) in Late Hesperian (Grant et al., 2011) paleolake • Lake in Uzboi breached Holden’s rim and drained • Early to Middle Amazonian Hale impact 6 • Uzboi’s largest tributary • ~700 km long Noachian Nirgal Vallis longitudinal valley • Unique morphology • Streamlined features and incised channel at the margin of the Uzboi lake • ~10 m-thick, laterally continuous Fe/Mg-smectite phyllosilicate layer a few m below surface (Buczkowski, et al., 2010, 2013; LeDeit et al.) Outstanding Questions • What is the age, timing and duration of fluvial activity in Nirgal? • Did discharge from Nirgal contribute to Lake Uzboi or did fluvial activity terminate prior to the filling and draining of Uzboi basin? • New HiRISE images Wilson et al., 2017, in prep. 7 Topography of Uzboi Vallis Floor and the Nirgal Deposits Topography of Uzboi floor below -350 m contour: • Lowest elevations between Bond and Holden within ~75 km north and south of Nirgal Next slide is -1160 m and -1300 m, shows respectively perspective view • In the ~120 km stretch in between, elevation increases to -720 m (highest at mouth of Nirgal) • Abrupt break in slope is evidence for Nirgal deposits 8 Nirgal Deposits • Profile along Uzboi floor shows material at confluence; dashed line is projected elevation of Uzboi floor beneath deposits • Small (~40 km3) fan- shaped symmetric deposit at the mouth of Nirgal • Stratigraphically on top of a larger (~200 km3) lower mound that is offset downstream • Volume of Nirgal deposit < volume of material removed from Nirgal • Most of material from Nirgal was transported some unknown distance downstream Modified from Wilson et al., in prep. Upper fan- shaped Nirgal Deposit • Degraded, possible channels • Light-toned material, incorporates meter-scale blocks • Possible layering • Diversity of colors and textures in HiRISE images may imply different rock types 10 • Southern margin exposes sequences of light-toned, horizontal, repetitive layers • Continuous (10s to 100s of meters) • Fine-grained (lack boulders) 11 • Dip ~5 degrees to SE lower in the deposit CRISM: Upper fan-shaped Nirgal Deposit • Mafics: large olivine deposit and smaller outcrops of low-Ca pyroxene • Hints of Al-phyllosilicates in Nirgal wall and Nirgal deposit (tentative due to quality of spectra) • Material eroded from the ~10 m-thick, laterally continuous phyllosilicate layer was likely transported some distance downstream before the lake in Uzboi formed 12 Western Flank of Uzboi Vallis • Eroded surface consisting