Geology topics

James A. Skinner

Publications and source records attributed to James A. Skinner.

18 recordsLinked to original sources

USGS2022

Planetary geologic mapping protocol—2022

The Planetary Geologic Mapping Protocol covers the idealized process of compiling a NASA-funded map product of a non-terrestrial solid surface planetary body for U.S. Geological Survey (USGS) publication and summarizes technical specifications of the Mapping Process for authors and reviewers. Directed by community and programmatic recommendations, the USGS Planetary Geologic Map Coordination Group assembled the content herein to aid the timely production of USGS map products. This document can be also used as a reference document by those researchers who are completing geologic maps that will be published outside the USGS.

Techniques and Methods↗

USGS2021

A geologic field guide to S P Mountain and its lava flow, San Francisco Volcanic Field, Arizona

Introduction We created this guide to introduce the user to the San Francisco Volcanic Field as a terrestrial analog site for planetary volcanic processes. For decades, the San Francisco Volcanic Field has been used to teach scientists to recognize the products of common types of volcanic eruptions and associated volcanic features. The volcanic processes and products observed in this volcanic field are like those observed on lunar and Martian surfaces. As a result, this region has been a favored location for training National Aeronautics and Space Administration astronauts and engineers since the Apollo missions. Though the San Francisco Volcanic Field has more than 600 volcanic vents and flows, this guide will focus on S P Mountain (known locally as S P Crater, located ~30 miles north of Flagstaff, Arizona), one of the best preserved and most accessible of the volcanic cones and lava flows. S P Mountain presents both major types of basaltic eruptions—explosive and effusive—as well as some commonly associated tectonic landforms. We assume that the user has a basic understanding of geologic concepts and terminology. For more specialized terminology, we include tables showing the classification scheme for lava compositions, styles of eruptions, and tephra sizes (tables 1, 2, and 3). If a further introduction or refresher in volcanological terminology is desired, we suggest reviewing such terms on the U.S. Geological Survey Volcano Science Center’s online glossary ( https://volcanoes.usgs.gov/vsc/glossary/ ). One term requires clarification at the start of this guide—the term cinder. The terms cinder and cinder cone are widely used to describe the material and edifice produced by lava fountains. However, the term comes from the mining and construction industries and has no clear or formal definition. The international committees in geology and volcanology have chosen the term tephra to be the general term to describe pyroclasts (material ejected through a volcanic explosion or from a volcanic vent). Therefore, in this guide, we use the term tephra rather than cinder. This guide is outlined as follows: A brief tour of volcanism across the solar system A brief geologic history of the Colorado Plateau and San Francisco Volcanic Field Background on distributed volcanism and S P Mountain Driving directions and field stops Questions for discussion Each field stop includes a brief description, learning goals, tasks, and a summary of key points. At the end of the field guide are discussion points and questions that will ask the user to consider what they have observed and learned and how such knowledge can be used to better our understanding of geologic processes on other planetary bodies. Upon the completion of this field guide, we expect the user to: Have a basic understanding of the volcanic processes relevant to S P Mountain and its lava flow. Be able to identify different volcanic textures that are associated with tephra cones. Be aware of the different observations one can make at different scales (for example, observing lava flow morphology from aerial or satellite imagery versus tephra characteristics in the field).

Arizona↗

USGS2021

Exposure of an early to middle Noachian valley network in three dimensions on Mars

We document a set of channels in a section of the Martian cratered highlands located between crustal massifs northeast of Hellas Planitia that are visible in cross section and planview >200 m below the surface. The morphometry and spatial distribution of the outcrops provide concrete geological evidence of a dynamic aqueous system in a structural sub-basin during the Early to Middle Noachian, bolstering evidence of conditions compatible with sustained liquid water on the surface very early in Mars' history.

Icarus↗

USGS2019

Planetary geologic mapping—Program status and future needs

The United States Geological Survey’s (USGS) Planetary Geologic Map Coordination Group (Flagstaff, Ariz.) surveyed planetary geoscience map makers and users to determine the importance, relevance, and usability of such products to their planetary science research and to current and future needs of the planetary science community. This survey was prepared because the planetary science community lacks a modern assessment of the value invested in geoscience map products and processes (including the diverse scientific and technical personnel who add to and maintain this infrastructure) and a strategy that ensures these efforts appropriately prioritize mapping efforts across all solid surface bodies in the Solar System. A 30-question survey was conducted through an online questionnaire and was designed to (1) take <10 minutes, (2) instill a sense that responses would be acted upon, and (3) encourage community participation through a user-friendly interface. The survey made a distinction between “standardized” geoscience maps (those published by the USGS that require adherence to specific cartographic standards, conventions, and principles) and “non-standardized” geoscience maps (those published by other venues such as peer-reviewed journals that are not required to, but might, adhere to some cartographic standards, conventions, and principles). The survey was opened on Sunday, March 18, 2017 (to coincide with the annual Lunar and Planetary Science Conference in The Woodlands, Tex.) and was closed on Thursday, May 25, 2017. There was a total of 265 unique responses that were formulated into 17 unique findings that were matched with one or more recommendations to be addressed by the planetary science community.

Open-File Report↗

USGS2018

Geologic map of the Nepenthes Planum Region, Mars

This map product contains a map sheet at 1:1,506,000 scale that shows the geology of the Nepenthes Planum region of Mars, which is located between the cratered highlands that dominate the southern hemisphere and the less-cratered sedimentary plains that dominate the northern hemisphere. The map region contains cone- and mound-shaped landforms as well as lobate materials that are morphologically similar to terrestrial igneous or mud vents and flows. This map is part of an informal series of small-scale (large-area) maps aimed at refining current understanding of the geologic units and structures that make up the highland-to-lowland transition zone. The map base consists of a controlled Thermal Emission Imaging System (THEMIS) daytime infrared image mosaic (100 meters per pixel resolution) supplemented by a Mars Orbiter Laser Altimeter (MOLA) digital elevation model (463 meters per pixel resolution). The map includes a Description of Map Units and a Correlation of Map Units that describes and correlates units identified across the entire map region. The geologic map was assembled using ArcGIS software by Environmental Systems Research Institute ( http://www.esri.com ). The ArcGIS project, geodatabase, base map, and all map components are included online as supplemental data.

Scientific Investigations Map↗

USGS2017

Large crater clustering tool

In this paper we present the Large Crater Clustering (LCC) tool set, an ArcGIS plugin that supports the quantitative approximation of a primary impact location from user-identified locations of possible secondary impact craters or the long-axes of clustered secondary craters. The identification of primary impact craters directly supports planetary geologic mapping and topical science studies where the chronostratigraphic age of some geologic units may be known, but more distant features have questionable geologic ages. Previous works (e.g., McEwen et al., 2005; Dundas and McEwen, 2007) have shown that the source of secondary impact craters can be estimated from secondary impact craters. This work adapts those methods into a statistically robust tool set. We describe the four individual tools within the LCC tool set to support: (1) processing individually digitized point observations (craters), (2) estimating the directional distribution of a clustered set of craters, back projecting the potential flight paths (crater clusters or linearly approximated catenae or lineaments), (3) intersecting projected paths, and (4) intersecting back-projected trajectories to approximate the local of potential source primary craters. We present two case studies using secondary impact features mapped in two regions of Mars. We demonstrate that the tool is able to quantitatively identify primary impacts and supports the improved qualitative interpretation of potential secondary crater flight trajectories.

Computers & Geosciences↗

USGS2014

Widespread loess-like deposit in the Martian northern lowlands identifies Middle Amazonian climate change

Consistently mappable units critical to distinguishing the style and interplay of geologic processes through time are sparse in the Martian lowlands. This study identifies a previously unmapped Middle Amazonian (ca. 1 Ga) unit (Middle Amazonian lowland unit, mAl) that postdates the Late Hesperian and Early Amazonian lowland plains by >2 b.y. The unit is regionally defined by subtle marginal scarps and slopes, has a mean thickness of 32 m, and extends >3.1 × 10 6 km 2 between lat 35°N and 80°N. Pedestal-type craterforms and nested, arcuate ridges (thumbprint terrain) tend to occur adjacent to unit mAl outcrops, suggesting that current outcrops are vestiges of a more extensive deposit that previously covered &sim;16 × 10 6 km 2 . Exposed layers, surface pits, and the draping of subjacent landforms allude to a sedimentary origin, perhaps as a loess-like deposit emplaced rhythmically through atmospheric fallout. We propose that unit mAl accumulated coevally with, and at the expense of, the erosion of the north polar basal units, identifying a major episode of Middle Amazonian climate-driven sedimentation in the lowlands. This work links ancient sedimentary processes to climate change that occurred well before those implied by current orbital and spin axis models.

Geology↗

USGS2014

Geologic map of Mars

This global geologic map of Mars, which records the distribution of geologic units and landforms on the planet's surface through time, is based on unprecedented variety, quality, and quantity of remotely sensed data acquired since the Viking Orbiters. These data have provided morphologic, topographic, spectral, thermophysical, radar sounding, and other observations for integration, analysis, and interpretation in support of geologic mapping. In particular, the precise topographic mapping now available has enabled consistent morphologic portrayal of the surface for global mapping (whereas previously used visual-range image bases were less effective, because they combined morphologic and albedo information and, locally, atmospheric haze). Also, thermal infrared image bases used for this map tended to be less affected by atmospheric haze and thus are reliable for analysis of surface morphology and texture at even higher resolution than the topographic products.

Scientific Investigations Map↗

USGS2013

The role of photogeologic mapping in traverse planning: Lessons from DRATS 2010 activities

We produced a 1:24,000 scale photogeologic map of the Desert Research and Technology Studies (DRATS) 2010 simulated lunar mission traverse area and surrounding environments located within the northeastern part of the San Francisco Volcanic Field (SFVF), north-central Arizona. To mimic an exploratory mission, we approached the region “blindly” by rejecting prior knowledge or preconceived notions of the regional geologic setting and focused instead only on image and topographic base maps that were intended to be equivalent to pre-cursor mission “orbital returns”. We used photogeologic mapping techniques equivalent to those employed during the construction of modern planetary geologic maps. Based on image and topographic base maps, we identified 4 surficial units (talus, channel, dissected, and plains units), 5 volcanic units (older cone, younger cone, older flow, younger flow, and block field units), and 5 basement units (grey-toned mottled, red-toned platy, red-toned layered, light-toned slabby, and light-toned layered units). Comparison of our remote-based map units with published field-based map units indicates that the two techniques yield pervasively similar results of contrasting detail, with higher accuracies linked to remote-based units that have high topographic relief and tonal contrast relative to adjacent units. We list key scientific questions that remained after photogeologic mapping and prior to DRATS activities and identify 13 specific observations that the crew and science team would need to make in order to address those questions and refine the interpreted geologic context. We translated potential observations into 62 recommended sites for visitation and observation during the mission traverse. The production and use of a mission-specific photogeologic map for DRATS 2010 activities resulted in strategic and tactical recommendations regarding observational context and hypothesis tracking over the course of an exploratory mission.

Acta Astronautica↗

USGS2013

Crater-based dating of geological units on Mars: methods and application for the new global geological map

The new, post-Viking generation of Mars orbital imaging and topographical data provide significant higher-resolution details of surface morphologies, which induced a new effort to photo-geologically map the surface of Mars at 1:20,000,000 scale. Although from unit superposition relations a relative stratigraphical framework can be compiled, it was the ambition of this mapping project to provide absolute unit age constraints through crater statistics. In this study, the crater counting method is described in detail, starting with the selection of image data, type locations (both from the mapper’s and crater counter’s perspectives) and the identification of impact craters. We describe the criteria used to validate and analyse measured crater populations, and to derive and interpret crater model ages. We provide examples of how geological information about the unit’s resurfacing history can be retrieved from crater size–frequency distributions. Three cases illustrate short-, intermediate, and long-term resurfacing histories. In addition, we introduce an interpretation-independent visualisation of the crater resurfacing history that uses the reduction of the crater population in a given size range relative to the expected population given the observed crater density at larger sizes. From a set of potential type locations, 48 areas from 22 globally mapped units were deemed suitable for crater counting. Because resurfacing ages were derived from crater statistics, these secondary ages were used to define the unit age rather than the base age. Using the methods described herein, we modelled ages that are consistent with the interpreted stratigraphy. Our derived model ages allow age assignments to be included in unit names. We discuss the limitations of using the crater dating technique for global-scale geological mapping. Finally, we present recommendations for the documentation and presentation of crater statistics in publications.

Icarus↗

USGS2012

Geologic map of the MTM 85200 quadrangle, Olympia Rupes region of Mars

The north polar region of Mars is dominated by Planum Boreum, a roughly circular, domical plateau that rises >2,500 m above the surrounding lowland. Planum Boreum is >1,500 km in diameter, contains deep, curvilinear troughs and chasmata, isolated cavi, and marginal scarps and slopes. The north polar plateau is surrounded by low-lying and nearly horizontal plains of various surface texture, geologic origin, and stratigraphic significance. The MTM 85200 quadrangle spans 5° of latitude (lat 82.5° to 87.5° N.) and 40° of longitude (long 140° to 180° E.) within the eastern hemisphere of Mars. The quadrangle includes the high-standing Planum Boreum, curvilinear troughs of Boreales Scopuli, deep, sinuous scarps of Olympia Rupes, isolated and coalesced depressions of Olympia Cavi, margins of the circular polar erg Olympia Undae, and low-standing Olympia Planum. The surface of Planum Boreum within the MTM 85200 quadrangle is characterized by smoothly sculptured landforms with shallow slopes and variable relief at kilometer scales. Areas that are perennially covered with bright frost are generally smooth and planar at 100-m scales. However, MGS MOC and MRO HiRISE images show that much of the icy polar plateau is rough at decameter scale. The Martian polar plateaus are likely to contain a record of global climate history for >107 to as much as ~3 x 109 years. This record is partly observable as rhythmically layered deposits exposed in the curvilinear troughs of the north polar plateau, Planum Boreum. The north polar layered deposits are widely interpreted to be among the most youthful bedrock deposits on the Martian surface. These materials and their stratigraphic and structural relations provide a glimpse into some of the more recent geologic processes that have occurred on Mars. The ability of the massive polar deposits to periodically trap and release both volatiles and lithic particles may represent a globally important, recurring geologic process for Mars.

Scientific Investigations Map↗

USGS2011

History of plains resurfacing in the Scandia region of Mars

We present a preliminary photogeologic map of the Scandia region of Mars with the objective of reconstructing its resurfacing history. The Scandia region includes the lower section of the regional lowland slope of Vastitas Borealis extending about 500–1800 km away from Alba Mons into the Scandia sub-basin below −4800 m elevation. Twenty mapped geologic units express the diverse stratigraphy of the region. We particularly focus on the materials making up the Vastitas Borealis plains and its Scandia sub-region, where erosional processes have obscured stratigraphic relations and made the reconstruction of the resurfacing history particularly challenging. Geologic mapping implicates the deposition, erosion, and deformation/degradation of geologic units predominantly during Late Hesperian and Early Amazonian time (~3.6–3.3 Ga). During this time, Alba Mons was active, outflow channels were debouching sediments into the northern plains, and basal ice layers of the north polar plateau were accumulating. We identify zones of regional tectonic contraction and extension as well as gradation and mantling. Depressions and scarps within these zones indicate collapse and gradation of Scandia outcrops and surfaces at scales of meters to hundreds of meters. We find that Scandia Tholi display concentric ridges, rugged peaks, irregular depressions, and moats that suggest uplift and tilting of layered plains material by diapirs and extrusion, erosion, and deflation of viscous, sedimentary slurries as previously suggested. These appear to be long-lived features that both pre-date and post-date impact craters. Mesa-forming features may have similar origins and occur along the southern margin of the Scandia region, including near the Phoenix Mars Lander site. Distinctive lobate materials associated with local impact craters suggest impact-induced mobilization of surface materials. We suggest that the formation of the Scandia region features potentially resulted from crustal heating related to Alba Mons volcanism, which acted upon a sequence of lavas, outflow channel sediments, and polar ice deposits centered within the Scandia region. These volatile-enriched sediments may have been in a state of partial volatile melt, resulting in the mobilization of deeply buried ancient materials and their ascent and emergence as sediment and mud breccia diapirs to form tholi features. Similar subsurface instabilities proximal to Alba Mons may have led to surface disruption, as suggested by local and regional scarps, mesas, moats, and knob fields.

Planetary and Space Science↗

USGS2011

Geologic map of the Metis Mons quadrangle (V–6), Venus

The Metis Mons quadrangle (V–6) in the northern hemisphere of Venus (lat 50° to 75° N., long 240° to 300° E.) includes a variety of coronae, large volcanoes, ridge and fracture (structure) belts, tesserae, impact craters, and other volcanic and structural features distributed within a plains setting, affording study of their detailed age relations and evolutionary development. Coronae in particular have magmatic, tectonic, and topographic signatures that indicate complex evolutionary histories. Previously, the geology of the map region has been described either in general or narrowly focused investigations. Based on Venera radar mapping, a 1:15,000,000-scale geologic map of part of the northern hemisphere of Venus included the V–6 map region and identified larger features such as tesserae, smooth and hummocky plains materials, ridge belts, coronae, volcanoes, and impact craters but proposed little relative-age information. Global-scale mapping from Magellan data identified similar features and also determined their mean global ages with crater counts. However, the density of craters on Venus is too low for meaningful relative-age determinations at local to regional scales. Several of the coronae in the map area have been described using Venera data (Stofan and Head, 1990), while Crumpler and others (1992) compiled detailed identification and description of volcanic and tectonic features from Magellan data. The main purpose of this map is to reconstruct the geologic history of the Metis Mons quadrangle at a level of detail commensurate with a scale of 1:5,000,000 using Magellan data. We interpret four partly overlapping stages of geologic activity, which collectively resulted in the formation of tesserae, coronae (oriented along structure belts), plains materials of varying ages, and four large volcanic constructs. Scattered impact craters, small shields and pancake-shaped domes, and isolated flows superpose the tectonically deformed materials and appear to be the most youthful materials in the map region.

Scientific Investigations Map↗

USGS2009

Martian mud volcanism: Terrestrial analogs and implications for formational scenarios

The geology of Mars and the stratigraphic characteristics of its uppermost crust (mega-regolith) suggest that some of the pervasively-occurring pitted cones, mounds, and flows may have formed through processes akin to terrestrial mud volcanism. A comparison of terrestrial mud volcanism suggests that equivalent Martian processes likely required discrete sedimentary depocenters, volatile-enriched strata, buried rheological instabilities, and a mechanism of destabilization to initiate subsurface flow. We outline five formational scenarios whereby Martian mud volcanism might have occurred: (A) rapid deposition of sediments, (B) volcano-induced destabilization, (C) tectonic shortening, (D) long-term, load-induced subsidence, and (E) seismic shaking. We describe locations within and around the Martian northern plains that broadly fit the geological context of these scenarios and which contain mud volcano-like landforms. We compare terrestrial and Martian satellite images and examine the geological settings of mud volcano provinces on Earth in order to describe potential target areas for piercement structures on Mars. Our comparisons help to evaluate not only the role of water as a functional component of geological processes on Mars but also how Martian mud volcanoes could provide samples of otherwise inaccessible strata, some of which could contain astrobiological evidence.

Marine and Petroleum Geology↗

USGS2009

Extraterrestrial GIS

No abstract available.

Book chapter↗

USGS2008

North polar region of Mars: Advances in stratigraphy, structure, and erosional modification

We have remapped the geology of the north polar plateau on Mars, Planum Boreum, and the surrounding plains of Vastitas Borealis using altimetry and image data along with thematic maps resulting from observations made by the Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter spacecraft. New and revised geographic and geologic terminologies assist with effectively discussing the various features of this region. We identify 7 geologic units making up Planum Boreum and at least 3 for the circumpolar plains, which collectively span the entire Amazonian Period. The Planum Boreum units resolve at least 6 distinct depositional and 5 erosional episodes. The first major stage of activity includes the Early Amazonian (∼3 to 1 Ga) deposition (and subsequent erosion) of the thick (locally exceeding 1000 m) and evenly-layered Rupes Tenuis unit (A b rt), which ultimately formed approximately half of the base of Planum Boreum. As previously suggested, this unit may be sourced by materials derived from the nearby Scandia region, and we interpret that it may correlate with the deposits that regionally underlie pedestal craters in the surrounding lowland plains. The second major episode of activity during the Middle to Late Amazonian ( ∼ < "> ∼< 1 Ga) began with a section of dark, sand-rich and light-toned ice-rich irregularly-bedded sequences (Planum Boreum cavi unit, A b b c ) along with deposition of evenly-bedded light-toned ice- and moderate-toned dust-rich layers (Planum Boreum 1 unit, A b b 1 ). These units have transgressive and gradational stratigraphic relationships. Materials in Olympia Planum underlying the dunes of Olympia Undae are interpreted to consist mostly of the Planum Boreum cavi unit (A b b c ). Planum Boreum materials were then deeply eroded to form spiral troughs, Chasma Boreale, and marginal scarps that define the major aspects of the polar plateau's current regional topography. Locally- to regionally-extensive (though vertically minor) episodes of deposition of evenly-bedded, light- and dark-toned layered materials and subsequent erosion of these materials persisted throughout the Late Amazonian. Sand saltation, including dune migration, is likely to account for much of the erosion of Planum Boreum, particularly at its margin, alluding to the lengthy sedimentological history of the circum-polar dune fields. Such erosion has been controlled largely by topographic effects on wind patterns and the variable resistance to erosion of materials (fresh and altered) and physiographic features. Some present-day dune fields may be hundreds of kilometers removed from possible sources along the margins of Planum Boreum, and dark materials, comprised of sand sheets, extend even farther downwind. These deposits also attest to the lengthy period of erosion following emplacement of the Planum Boreum 1 unit. We find no evidence for extensive glacial flow, topographic relaxation, or basal melting of Planum Boreum materials. However, minor development of normal faults and wrinkle ridges may suggest differential compaction of materials across buried scarps. Timing relations are poorly-defined mostly because resurfacing and other uncertainties prohibit precise determinations of surface impact crater densities. The majority of the stratigraphic record may predate the recent (<20 Ma) part of the orbitally-driven climate record that can be reliably calculated. Given the strong stratigraphic but loose temporal constraints of the north polar geologic record, a comparison of north and south polar stratigraphy permits a speculative scenario in which major Amazonian depositional and erosional episodes driven by global climate activity is plausible.

Icarus↗

USGS2005

Geologic map of the northern plains of Mars

The northern plains of Mars cover nearly a third of the planet and constitute the planet's broadest region of lowlands. Apparently formed early in Mars' history, the northern lowlands served as a repository both for sediments shed from the adjacent ancient highlands and for volcanic flows and deposits from sources within and near the lowlands. Geomorphic evidence for extensive tectonic deformation and reworking of surface materials through release of volatiles occurs throughout the northern plains. In the polar region, Planum Boreum contains evidence for the accumulation of ice and dust, and surrounding dune fields suggest widespread aeolian transport and erosion. The most recent regional- and global-scale maps describing the geology of the northern plains are largely based on Viking Orbiter image data (Dial, 1984; Witbeck and Underwood, 1984; Scott and Tanaka, 1986; Greeley and Guest, 1987; Tanaka and Scott, 1987; Tanaka and others, 1992a; Rotto and Tanaka, 1995; Crumpler and others, 2001; McGill, 2002). These maps reveal highland, plains, volcanic, and polar units based on morphologic character, albedo, and relative ages using local stratigraphic relations and crater counts. This geologic map of the northern plains is the first published map that covers a significant part of Mars using topography and image data from both the Mars Global Surveyor and Mars Odyssey missions. The new data provide a fresh perspective on the geology of the region that reveals many previously unrecognizable units, features, and temporal relations. In addition, we adapted and instituted terrestrial mapping methods and stratigraphic conventions that we think result in a clearer and more objective map. We focus on mapping with the intent of reconstructing the history of geologic activity within the northern plains, including deposition, volcanism, erosion, tectonism, impact cratering, and other processes with the aid of comprehensive crater-density determinations. Mapped areas include all plains regions within the northern hemisphere of Mars, as well as an approximately 300-km-wide strip of cratered highland and volcanic regions, which border the plains. Note that not all of the contiguous northern plains are mapped, because some minor parts of Elysium and Amazonis Planitiae lie south of the equator.

Scientific Investigations Map↗

USGS2003

Resurfacing history of the northern plains of Mars based on geologic mapping of Mars Global Surveyor data

Geologic mapping of the northern plains of Mars, based on Mars Orbiter Laser Altimeter topography and Viking and Mars Orbiter Camera images, reveals new insights into geologic processes and events in this region during the Hesperian and Amazonian Periods. We propose four successive stages of lowland resurfacing likely related to the activity of near-surface volatiles commencing at the highland-lowland boundary (HLB) and progressing to lower topographic levels as follows (highest elevations indicated): Stage 1, upper boundary plains, Early Hesperian, <-2.0 to -2.9 km; Stage 2, lower boundary plains and outflow channel dissection, Late Hesperian, <-2.7 to -4.0 km; Stage 3, Vastitas Borealis Formation (VBF) surface, Late Hesperian to Early Amazonian, <-3.1 to -4.1 km; and Stage 4, local chaos zones, Early Amazonian, <-3.8 to -5.0 km. At Acidalia Mensa, Stage 2 and 3 levels may be lower (<-4.4 and -4.8 km, respectively). Contractional ridges form the dominant structure in the plains and developed from near the end of the Early Hesperian to the Early Amazonian. Geomorphic evidence for a northern-plains-filling ocean during Stage 2 is absent because one did not form or its evidence was destroyed by Stage 3 resurfacing. Remnants of possible Amazonian dust mantles occur on top of the VBF. The north polar layered deposits appear to be made up of an up to kilometer-thick lower sequence of sandy layers Early to Middle Amazonian in age overlain by Late Amazonian ice-rich dust layers; both units appear to have outliers, suggesting that they once were more extensive.

Journal of Geophysical Research E: Planets↗