2012

U.S. Department of the Interior

U.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Geologic Map of the MTM 85200 Quadrangle, Olympia Rupes 

Region of Mars

By James A. Skinner, Jr., and Kenneth E. Herkenhoff

Pamphlet to accompany

Scientific Investigations Map 3197

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O l y m p i a C a v i

O L Y M P I A P L A N U M

O l y m p i a U n d a e

P L A N U M

B O R E U M

B o r e a l e s S c o p u l i 

G e m i n i

S c

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u l i

G e m i n a L i n g u l a

Chasma Boreale

S C A N D I A C O L L E S

V A S T I T A S B O R E A L I

S

Scandia Tholi

Abalos Colles

Korolev

V A S T I T A S B O R E A

L

I

S

Rupes Tenuis

0° E

270° E 90° E

64° N

MTM 85080

MTM

85200

180° E
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Contents

Introduction ....................................................................................................................................................1

Physoigraphic Setting ..................................................................................................................................2

Map Base and Data ......................................................................................................................................2

Methodology ..................................................................................................................................................3

Unit Groups, Names, and Symbols ...................................................................................................3

Contact Types .......................................................................................................................................3

Feature Types .......................................................................................................................................4

Digital Drafting Parameters ...............................................................................................................4

Geomorphology .............................................................................................................................................4

Ridges ....................................................................................................................................................4

Berms ....................................................................................................................................................4

Swales ...................................................................................................................................................4

Scarps ....................................................................................................................................................5

Unconformities .....................................................................................................................................5

Sediment Transport Direction ............................................................................................................5

Craterforms ...........................................................................................................................................5

Geologic Summary ........................................................................................................................................5

Middle Amazonian Epoch ...................................................................................................................5

Middle to Late Amazonian Epochs ...................................................................................................6

Late Amazonian Epoch .......................................................................................................................7

References Cited ...........................................................................................................................................8

Table

1. Relative age determinations for geologic units in the Olympia Rupes region of Mars. .............10

Figures

1. Color shaded-relief image of MTM 85200 constructed from a digital elevation model of Mars 

Orbiter Laser Altimeter data (512 pixels/degree; ~115 m/pixel) (map sheet)

2. Excerpt of CTX image number P01_001341_2654 (centered at lat 85.21° N., long 147.19° E.; 

location shown on map) that shows local geomorphology that occurs within the Planum 

Boreum 1 unit (Ab1) .....................................................................................................................10

3. Excerpt of High Resolution Imaging Science Experiment image number PSP 001548-2755 

(centered at lat 84.39° N., long 142.94° E.; location shown on map) that shows a scarpexposed section of layered deposits south of the Olympia Rupes .....................................11

4. Excerpt of Mars Orbiter Camera narrow-angle image number e03-02719 (centered at lat 

85.14° N., long 168.76° E.; location shown on map) that shows unit relations near the 

eastern termination of a polar trough, south of the Olympia Rupes ...................................11

5. Excerpt of Context Imager (CTX) image number P02_001670_2643 (centered at lat 84.94° N., 

long 176.56° E.; location shown on map) that shows the margin of a scarp-exposed 

section of the Olympia Cavi 1 and Planum Boreum 1 units (Aoc1 and Ab1) along Olympia 

Rupes .............................................................................................................................................12

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1

Introduction

The Martian polar plateaus are likely to contain a record 

of global climate history for <107 to as much as ~3 x 109 years 

(Tanaka and others, 2008; Byrne, 2009; Banks and others, 

2010). This record is partly observable as rhythmically layered 

deposits exposed in the curvilinear troughs of the north polar 

plateau, Planum Boreum (Cutts and others, 1976, 1979; Cutts 

and Lewis, 1982; Howard and others, 1982). The north polar 

layered deposits are widely interpreted to be among the most 

youthful bedrock deposits on the Martian surface (Byrne, 2009). 

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 (for example, Byrne, 2009).

The north polar and circum-polar deposits were first 

mapped using Mariner 9 images at 1:25,000,000 scale by 

Scott and Carr (1978), who identified variously textured plains 

surrounding the polar deposits. A subsequent regional-scale 

(1:5,000,000 scale) geologic and geomorphologic map was 

produced using Viking Orbiter images by Dial (1984), who outlined boundaries of various types of polar dune deposits. This 

effort was followed by a Viking-based map (1:15,000,000 scale) 

that employed significant stratigraphic revisions (Tanaka and 

Scott, 1987). These workers delineated two Late Amazonian 

polar plateau units (unit Apl, polar layered deposits, and unit 

Api, polar ice deposits), which were interpreted to be diachronous, implying spatially variable deposition through time. Tanaka 

and Scott (1987) also mapped three discrete, Late Amazonian, 

circum-polar units (in addition to textural members of the 

circum-polar Vastitas Borealis Formation), including crescentic 

dune material (unit Adc), linear dune material (unit Adl), and 

mantle material (unit Am). Local, quadrangle-scale (1:500,000) 

mapping of Planum Boreum was completed by Dial and Dohm 

(1994) and Herkenhoff (2003) using Viking Orbiter images. 

These maps helped to improve the understanding of the internal 

characteristics of local outcrops of the polar layered deposits, 

including location and continuity of angular unconformities, 

surficial textures, and the occurrence of impact craters. 

Post-Viking Orbiter datasets significantly improve geologic map-based characterizations of the polar deposits and 

require refined mapping methodologies (for example, Tanaka 

and others, 2008). The most recent geologic map of the Martian north polar deposits is hemispheric in scale (1:15,000,000) 

and used data sets from the Mars Global Surveyor (MGS) and 

Mars Odyssey (MO) orbiters (Tanaka and others, 2005). This 

map used the Mars Orbiter Laser Altimeter (MOLA) gridded topographic data sets at ~465 m/pixel as its primary base 

map. Contacts and unit descriptions therein were refined using 

local coverage of MGS Mars Orbiter Camera (MOC) and MO 

Thermal Emission Imaging System (THEMIS) images. Tanaka 

and others (2005) discriminated two discrete units of the north 

polar layered deposits, each identified by thickness, number 

of observable laminae, and vertical location as observed in 

equator-facing slopes of the polar troughs. Tanaka and others 

(2005) instituted mapping methods that differed from previous 

approaches as a means to effectively balance the availability 

and widespread coverage of high-resolution panchromatic 

images, topographic detail, and spectral information. More 

recently, Tanaka and Fortezzo (2012) completed a 1:2,000,000-

scale regional geologic map of Planum Boreum and its immediately surrounding environments. In addition to four units mostly 

confined to the circumpolar plains, these workers identified a 

sequence of five units that collectively defines the volumetric 

and areal bulk of the north polar plateau. Their sequence is 

bracketed by the Late Hesperian rupēs unit (unit HBbr), based 

on its dominant occurrence along the scarp-forming margins of 

Planum Boreum, and the Late Amazonian Planum Boreum 3 

unit. As mapped by Tanaka and Fortezzo (2012), the geologic 

materials exposed in the north polar plateau reflect a long-lived, 

cyclic, and regional-scale history of accumulation, erosion, 

and redistribution of sand, dust, and ice deposits. Though the 

broad-scale absence of impact craters hinders the stratigraphic 

correlation of most polar materials to extant circumpolar and 

mid-latitude deposits, a certain level of inference can be made 

based on material depth, appearance, and lateral relations, as 

well as the estimated record of polar insolation through time 

(Laskar and others, 2002, 2004).

Summertime albedo features on the north polar plateau, 

such as dark bands, correspond well with the location of curvilinear troughs (Cutts, 1973). This effect is most pronounced on 

the steeper, equator-facing trough walls, where the margin of 

the residual (perennial) ice cap shifts over time. Changes in the 

extent of the residual ice cap have been noted across the north 

polar plateau (Malin and Edgett, 2001; Hale and others, 2005) 

but are limited in area and appear to be reversible over a few 

Martian years (Byrne and others, 2008). During wintertime, 

the bulk of the polar plateau is covered in a ~1-m-thick layer 

of seasonal CO2 frost (Titus and others, 2008), which generally 

obscures fine-scale surface morphology and albedo contrasts.

Data provided by topographic and satellite-based surface 

imaging instrument systems, particularly the Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science 

Experiment (HiRISE) and Context (CTX) cameras, are increasingly showing that the materials of the Mars north polar plateau 

reflect a complex history of material accumulation and degradation (Tanaka and others, 2008; Tanaka and Fortezzo, 2012). 

Analysis of post-Viking Orbiter high-resolution image data 

has confirmed and, in many instances, improved upon standing 

hypotheses concerning Martian polar geology. Such refinements 

focus on (1) temporal sequencing of layered deposit accumulation and degradation (Phillips and others, 2008; Tanaka and 

others, 2008; Banks and others, 2010), (2) geomorphic and 

stratigraphic partitioning of basal sequences from overlying 

“layered deposits” (Byrne and Murray, 2002), (3) origination 

and evolution of polar troughs and swales (Howard, 2000; Ng 

and Zuber, 2006; Smith and Holt, 2010), (4) linking of polar 

strata with obliquity cycles over the past few million years 

(Laskar and others, 2002, 2004; Milkovich and Head, 2005; 

Fishbaugh and others, 2010), (5) correlation of circumpolar 

materials to layer packages and unconformities (Tanaka and 

others, 2003, 2005, 2008), and (6) detailing of the internal composition of polar materials (Picardi and others, 2005; Murchie 

and others, 2007; Phillips and others, 2008).
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The extensive spatial coverage, high resolution, and diversity of modern (post-Viking Orbiter) data sets of Mars have 

ushered in a new era of geological map creation, presentation, 

and utility. The map described herein is of particular scientific 

relevance because it is the largest-scale geologic map produced 

for the polar deposits of Mars using post-Viking Orbiter data 

sets. As a result, it provides a point of comparison for smallerscale geologic maps, particularly the recent 1:2,000,000-scale 

regional map of Planum Boreum by Tanaka and Fortezzo 

(2012). The geologic, morphologic, and stratigraphic details 

conveyed in this 1:500,000 map of Mars Transverse Mercator 

(MTM) 85200 quadrangle offer an improved means to visualize, represent, and interpret the local geologic history of Martian 

polar deposits, as observed in the Olympia Rupēs region of 

Planum Boreum.

Physiographic Setting

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 (Smith and others, 2001). 

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 (Tanaka and others, 2005, 

2008; Tanaka and Fortezzo, 2012).

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 (fig. 1). The quadrangle includes the high-standing Planum Boreum, curvilinear 

troughs of Boreales Scopuli, deep, sinuous scarps of Olympia 

Rupēs, isolated and coalesced depressions of Olympia Cavi, 

margins of the circular polar erg Olympia Undae, and lowstanding 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 (Zuber and others, 1998; Tanaka 

and others, 2008). Areas that are perennially covered with 

bright frost are generally smooth and planar at 100-m scales 

(Cutts and others, 1979). However, MGS MOC and MRO 

HiRISE images show that much of the icy polar plateau is 

rough at decameter scale (Thomas and others, 2000; Herkenhoff and others, 2002).

We subdivide the north polar plateau in the MTM 85200 

quadrangle into the Boreum and Olympia regions to assist 

with identifying and describing units and succinctly conveying the interpreted geologic history (table 1). These regions are 

separated by an east-west-trending system of arcuate scarps 

(Olympia Rupēs) that elevate Planum Boreum by ~800 m above 

Olympia Planum (fig. 1). The Boreum region contains the highstanding, inter-trough polar flats that have high-albedo surfaces 

during the polar summer (solar longitude (Ls

) from 90° to 180°). 

Polar troughs extend from approximately –2,700 m elevation near the center of the polar cap to approximately –4,300 

m elevation along the north margins of Olympia Planum. The 

Olympia region contains the low-standing cavi margins and 

floors that have intermediate to low albedo, as well as the northern parts of Olympia Undae.

Map Base And Data

Though photogeology and geomorphology continue to 

define the cornerstone of planetary geologic mapping (Greeley and Batson, 1990), new techniques are required to most 

effectively convey stratigraphic and structural relations using 

voluminous data sets of diverse type and spatial resolution. We 

describe below (1) the construction and use of the geologic map 

base and (2) the relevance and use of supplemental mapping 

datasets.

The primary base for this geologic map was originally a 

Viking Orbiter mosaic of individual images acquired during the 

1976 Martian northern summer (LS from 133° to 135°). The 

original intent of the Viking Orbiter base photomosaic was to 

provide a time-controlled view of the extent of polar ice during 

its presumed minimal extent, similar to the MTM 85080 quadrangle mapped by Herkenhoff (2003) (see quadrangle location 

map on map sheet for location). These types of data sets were 

previously used to identify both volatile and nonvolatile units 

in both the northern and southern polar plateaus (for example, 

Herkenhoff and Murray, 1990; Herkenhoff, 2003). However, as 

noted by Herkenhoff (2003), the resolution of Viking Orbiter 

color mosaics is not generally sufficient to clearly and consistently identify geologic units based on morphologic detail and 

crosscutting relations (table 1). As a result, for the MTM 85200 

geologic map, we used the time-controlled Viking Orbiter 

mosaic in tandem with the MOLA digital elevation model 

(DEM) at 512 pixels/degree (~115 m/pixel). This “blended” 

base map provides not only summertime albedo but also topographic characteristics that are important to the identification of 

geologic units at 1:500,000 scale. The units that are mapped and 

described herein are tied to this blended base map due to its full 

areal coverage of the selected quadrangle. High-resolution data 

sets (described in subsequent sections) were used as supplements for the evaluation of contact location, identification and 

description of geologic units, and assessment of stratigraphic 

relations (table 1).

The Viking Orbiter image mosaic that was originally provided by the U.S. Geological Survey (USGS) was created using 

outdated geodetic information, resulting in an offset between 

modern data sets and the map base (Seidelmann and others, 

2002). To correct this cartographic shift, we applied a secondorder polynomial transformation to fit the USGS-produced 

Viking Orbiter mosaic to the MOLA DEM using georeferencing 

tools available in geographic information system (GIS) software 

packages. The transformation utilized 36 hand-picked points 

between easily identifiable landforms within the unregistered 

Viking Orbiter mosaic and the controlled MOLA DEM. This 

spatial transformation resulted in a root mean square (rms) error 

in horizontal placement of 354 m, which translates to 0.7 mm at 

map scale. We overlaid the rectified Viking Orbiter mosaic on 

the MOLA-derived shaded-relief image to produce the blended 
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base map rendered in the geologic map. This approach allowed 

us to tie the Viking Orbiter seasonal information to the more 

accurately controlled MOLA DEM and other modern datasets. The inclusion of MOLA topography in the base allows for 

diminished use of topographic symbology on the map (unless 

used where imperative to convey the geologic history) and 

a more concise representation of the geologic environment. 

However, topographic information is significantly reduced 

north of lat 87.1° N. due to the polar orbit of the MGS satellite 

and resultant spacing between MOLA ground tracks. Therein, 

geologic contacts were identified exclusively by examination of 

the Viking Orbiter mosaic. The resultant base-map scale is 51 

m/pixel, which translates into approximately 10 pixels/mm at 

map scale.

Though the map base provided the contextual information 

necessary to consistently map contacts and temporally correlate 

geologic units (table 1), we verified and honed these efforts by 

integrating other data sets, including MOLA-derived products 

(cell-to-cell slope, aspect, and color-shaded-relief maps) and 

the full range of THEMIS, MOC, HiRISE, and CTX images via 

web-linked image footprints in the project GIS. Though these 

data sets provided important information regarding unit texture 

and stratigraphic relations, they were considered supplemental because they did not provide complete areal coverage at 

the map scale. Supplemental data sets were used to complete 

the accompanying Description of Map Units where clear and 

consistent textural and (or) stratigraphic characteristics were 

observed.

Methodology

This geologic map was compiled using methods that both 

follow and deviate from the approaches used by Tanaka and 

others (2005) in their hemisphere-scale geologic map of the 

northern plains of Mars. We describe below the (1) rationale 

for unit groups, names, and symbols, (2) types and relevance of 

geologic contacts, (3) types and relevance of feature symbols, 

and (4) digital drafting parameters used to compile information.

Unit Groups, Names, and Symbols

We name the discrete geologic materials that occur within 

MTM 85200 using nonmorphologic and nongenetic terms to 

preserve objectivity by avoiding potentially erroneous genetic 

connotations. Tanaka and others (2005) subdivided the Martian northern plains into specific physiographic provinces and 

uniquely denoted the geologic units that occurred within these 

provinces as a means to partition the regional geologic histories 

and simplify description. The MTM 85200 quadrangle falls 

entirely within the Borealis province of the northern plains, as 

defined by Tanaka and others (2005). For clarity, we group the 

units of MTM 85200 into two regions based on IAU-approved 

nomenclature. The Boreum region units of MTM 85200 are 

those that comprise the polar flats and troughs of Planum 

Boreum, which are generally identified north of the Olympia 

Rupēs (fig. 1). The Olympia region units of MTM 85200 are 

those that occur within and directly adjacent to the Olympia 

region of Mars (centered at lat 80° N., long 160° E.) are generally identified south of the Olympia Rupēs. These groupings are 

shown on the Correlation of Map Units.

We name the units of MTM 85200 based on their geographic occurrence and relative stratigraphic position (table 

1). We identify seven geologic units within the quadrangle: 

three units of the Boreum region and four units of the Olympia 

region. The Boreum units consist, from oldest to youngest, of 

the Planum Boreum 1, Planum Boreum 2, and Planum Boreum 

3 units. The Olympia units consist, from oldest to youngest, of 

the Olympia Cavi 1, Olympia Cavi 2, and Olympia Cavi 3 units, 

as well as the Olympia Undae unit.

Units are assigned a unit label that uniquely identifies (1) 

the chronologic period (for example, A=Amazonian period), (2) 

the geographic region (for example, o=the Olympia region), (3) 

the physiographic feature name where more than one geologic 

unit occurs in a region (for example, c=Olympia Cavi), and 

(4) the interpreted stratigraphic sequence (for example, 1= the 

first/oldest of two or more related geologic units). For example, 

the unit label, Aoc1, refers to the Amazonian-age, stratigraphically lowest unit that occurs within and (or) adjacent to Olympia Cavi. We specifically note that the unit labels used herein 

differ from those applied in regional- to hemisphere-scale 

maps, because we do not indicate physiographic province (for 

example, B=Borealis province of Tanaka and others (2005) and 

Tanaka and Fortezzo (2012)). We justify this deviation because 

quadrangle-based geologic maps generally include discrete 

materials that occur within a single physiographic province. We 

denote the physiographic province within the headings in the 

Correlation of Map Units and Description of Map Units rather 

than include it in the geologic unit label. As a result, quadrangle 

schemes can utilize more simplified labels that are specific to 

large-scale geologic maps.

We acknowledge and underscore that different approaches 

to labeling and naming map units can introduce confusion 

where competing maps apply discrepant symbols for similar 

geologic units (or similar symbols for discrepant units). We 

specifically point out that some unit labels applied herein partly 

contrast and conflict with those units employed by Tanaka 

and Fortezzo (2012) in their 1:2,000,000-scale geologic map 

of Planum Boreum. However, we feel that the absence of 

physiographic province in our unit labels (as described above) 

adequately partitions potentially confusing unit labels. For 

clarity, we note association or overlap of our units with those 

of other published maps in the Geologic Summary section of 

this text.

Contact Types

We define and delineate geologic units using contacts that 

denote their interpreted level of certainty and (or) their geologic occurrence. A certain contact denotes the most precise 

contact between well-characterized geologic units that contrast 

in appearance and (or) interpreted relative age. An approximate 

geologic contact is regarded as less precise in its spatial placement due to data quality, subtlety of contact, and (or) obscura-
4

tion by secondary processes, where the existence of a geologic 

unit is generally not in question.

Feature Types

The map includes a variety of morphologic feature symbols, following precedents established in previous terrestrial 

and planetary geologic maps. Although consistent mapping of 

features is desired, it is generally not possible or practical due 

to complications that can arise from (1) locally rugged terrain; 

(2) variation in feature character, length, and (or) relief; (3) 

feature orientation relative to illumination; and (4) areas of low 

data resolution and (or) data coverage. The features that are 

mapped in the MTM 85200 quadrangle grade in appearance 

and overlap in places and should be considered cartographic 

rather than exact representations, in regard to spatial placement 

and density. We mapped ridges, berms, unconformities, swales, 

scarps, knobs, depressions, sediment transport directions, and 

craterforms.

Digital Drafting Parameters

We used the Environmental Systems Research Institute, 

Inc. (v. 9.3, 1982–2011, Redlands, CA) ArcGIS software package to co-register and analyze available datasets. We digitized 

points, lines, and polygons using the digital streaming capability 

in ArcGIS. We digitized vector linework at a consistent scale of 

1:125,000 (25% of the publication map scale), which allowed 

for substantially detailed lines for use in a hard-copy map 

publication, as well as a digital product, while remaining true 

to the publication map scale of 1:500,000. Vertex spacing was 

controlled in ArcGIS and was set to place spatial points within 

linework every 125 m (1 vertex per 1 mm at 1:125,000 digitizing map scale).

Linework was streamed using a WACOM Intuos 3 digital 

mapping tablet into an ArcGIS digital geodatabase in polar 

stereographic projection. The geodatabase housed the attribute 

information for each digitized feature. Attributes for linework 

were assigned iteratively using attribute domain settings stored 

within the geodatabase. Geologic map symbols were derived 

from Federal Geographic Data Committee Digital Cartographic 

Standards for Geologic Map Symbolization (FGDC, 2006) and 

were adapted where necessary to convey the geologic information unique to the quadrangle. Subsequent editorial iterations of 

the digitized linework allowed for refinement of contact placement and unit descriptions based on cross-comparison between 

the map base and supplemental data sets. Contact linework was 

cleaned (dangles removed) and used to build unit polygons. To 

balance base map resolution and clear cartographic representation, all unit outcrops are >1 km2.

Geomorphology

The spatial and morphologic characteristics of landforms 

that occur within the geologic units of the MTM 85200 quadrangle provide information helpful to the discernment of the 

regional geologic history.

Ridges

Ridges are the peaks of relatively sharp crested lineaments 

that occur on the inter-trough polar plateau, generally south of 

Olympia Rupēs. Ridges are most commonly oriented oblique 

to the axes of nearby polar troughs. These features generally 

parallel low-albedo surface streaks (“veneers” of Rodriguez 

and others, 2007), as well as the sediment transport direction as 

determined from duneform shape. We mapped 58 ridges within 

the quadrangle ranging in length from 1.4 to 5.6 km (2.6 km 

mean), though high-resolution images show a greater density 

than we identify using the basemap. Ridges occur entirely 

within the boundary of the Planum Boreum 3 unit.

Berms

Berms are the crestlines of curvilinear, convex-upward 

ridges that occur on equator-facing trough walls north of 

Olympia Rupēs (fig. 2). Berms are several hundreds of meters 

wide and occur adjacent to (and can linearly grade into and 

from) scarps and unconformities. The features sometimes 

form the margins of lens-shaped benches within polar troughs 

(for example, lat 86.08° N., long 164.47° E.). We mapped 21 

berms within the quadrangle ranging in length from 3.7 to 30.4 

km (11.3 km mean). Berms occur entirely within the Planum 

Boreum 1 unit and may represent layers of contrasting friability 

within the polar stratigraphic sequence (see Geologic Summary 

section for elaboration).

Swales

Swales are the axes of shallow, asymmetric, curvilinear depressions that occur on the inter-trough flats of Planum 

Boreum both north and south of Olympia Rupēs. These features 

were initially recognized and mapped by Cutts and others (1979) 

and Howard and others (1982) and assist in defining the shallowly undulating, wavelike topography between troughs of the 

north polar plateau. We use the term “swale” to denote the axial 

trend of depressions that occur within otherwise horizontal intertrough flats as opposed to the term “undulations” of Howard and 

others (1982), which emphasizes the collective wavelike characteristics of inter-trough regions based on lower resolution Viking 

Orbiter-based topography. As mapped herein, swales generally 

range from ~1 to >5 km wide, are as much as 100 m deep, and 

are bounded on both sides by low-angle slopes. Bounding slopes 

are generally less than 5° and we subdivide these based on 

bounding slopes greater than and less than 2°. Slopes as high as 

8° occur locally where swales curve into trough terminations. We 

mapped 89 swales within the quadrangle ranging in length from 

3.1 to 60.2 km (11.3 mean). Swales with bounding slopes <1° 

were not mapped. Swales occur predominantly within Planum 

Boreum 2 unit, though genetic association with the Planum 

Boreum 1 and 3 units cannot be ruled out.
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Scarps

Scarps are the sharp or subdued breaks in slope that form 

the margins of topographic ledges within equator-facing walls 

of polar troughs (for example, lat 86.74° N., long 173.78° E.), 

depressions within Olympia Cavi (for example, lat 85.26° N., 

long 160.05° E.), and topographically shallow inflections on the 

polar plateau south of Olympia Rupēs (for example, lat 84.00° 

N., long 174.30° E). Scarps can be partly or fully enclosed to 

form knobs or depressions and locally appear to grade laterally 

into berms. Scarps form within the Olympia Cavi 1, Planum 

Boreum 2, and Planum Boreum 3 units and appear to be buried 

by the Planum Boreum 3 and Olympia Cavi 3 units.

Unconformities

Unconformities are curvilinear features that demarcate 

the truncation of stratigraphic layers exposed in the equatorfacing walls of polar troughs. Some sections of truncated strata 

are linked by layers with discrepant morphologic or albedo 

characteristics (for example, lat 85.18° N., long 146.85° E.). 

Unconformities are interpreted to represent periods of erosion. 

We mapped 12 unconformities within the quadrangle ranging 

in length from 1.3 to 23.1 km (5.8 km mean). Unconformities 

are mapped only within the Planum Boreum 1 unit, although the 

variable topographic character of the upper stratigraphic boundaries of the Olympia Cavi 1, Planum Boreum 1, Olympia Cavi 

2, and Planum Boreum 2 units are interpreted as unconformities 

(see Descriptions of Map Units for elaboration).

Sediment Transport Direction

Sediment transport direction was inferred from duneform 

shape as evidenced in both regional base maps and high-resolution images. Barchan and crescent duneforms are the dominant 

landform within the Olympia Undae unit, which occurs on the 

floors of Olympia Cavi as well as along the northern margin 

of Olympia Planum. Sediment transport direction is meant to 

convey the overall sense of transport and wind direction, though 

it is generally not known whether saltation is a current geologic 

process affecting the materials of the north polar plateau. Sediment transport directions show general transport in the southwesterly direction, though local topography within Olympia 

Cavi appears to have caused localized deviations in these directions (Tanaka and others, 2008).

Craterforms

We mapped two craterforms within the quadrangle (informally named “crater A” and “crater B”). Each is defined by 

circular depressions with very subdued rims and no associated 

surrounding deposits. Crater A is an approximately 150-m-diameter bowl-shaped feature that resides within the Olympia Cavi 1 

unit at lat 84.37° N., long 142.80° E (fig. 3). Based on measurement in MOLA gridded data, the feature forms the top of a 160-

m-tall topographic promontory within a cavus. The craterform 

feature may be an eroded vestige of an impact crater within the 

Olympia Cavi 1 unit. As such, the bowl-shaped features may 

only represent a part of the interior wall and floor of the original 

impact structure of a larger diameter than measured. Crater B 

is an approximately 200-m-diameter bowl-shaped feature that 

resides within the Planum Boreum 1 unit at lat 85.15° N., long 

168.88° E. (fig. 4). The craterform is partly filled or abutted by 

Planum Boreum 2 unit near a trough termination. In addition to 

the two craterforms that we mapped herein, Banks and others 

(2010) identified four other similarly shaped features within the 

quadrangle boundary, based on analysis of CTX and HiRISE 

images; these range from 39 to 52 m in diameter and occur primarily in the Planum Boreum 2 and 3 units.

Geologic Summary

The stratigraphic architecture of the north polar plateau 

of Mars is refined through quadrangle-scale geologic mapping, wherein local unit characteristics and geologic relations 

can be conveyed cartographically (table 1). The evolution of 

the materials that make up the north polar plateau (including 

those within the MTM 85200 quadrangle) is not temporally 

well constrained due to a paucity of impact craters on the north 

polar plateau (Herkenhoff and Plaut, 2000; Herkenhoff, 2003; 

Tanaka and others, 2008; Banks and others, 2010; Tanaka and 

Fortezzo, 2012), the densities of which are helpful in providing some stratigraphic control. Recent regional-scale mapping 

(Tanaka and others, 2005; Tanaka and Fortezzo, 2012) and 

topical science investigations (Tanaka, 2005; Tanaka and others, 

2008; Banks and others, 2010) help provide some stratigraphic 

control for the materials of Planum Boreum, including geologic 

and (or) stratigraphic equivalents to those mapped herein. We 

employ these controls where possible. However, we note that 

some temporal context gleaned from other investigations is 

based on observations made outside of the MTM 85200 map 

area, including crosscutting relations and small population 

crater densities (with inherent large uncertainties). The geologic 

summary discussed herein is portrayed graphically in the Correlation of Map Units. Details of the stratigraphic relations are 

provided in the Description of Map Units.

Middle Amazonian Epoch

The oldest unit that we identify is the Olympia Cavi 1 unit 

(Aoc1), which is distinguished from adjacent units by its variable (though generally low) albedo and rugged, tabular appearance (figs. 3, 5). Unit Aoc1 forms most of the lower elevation 

walls and floors of Olympia Cavi. Crossbeds are locally 

apparent on clean (unmantled) surfaces. The unit’s lowermost 

sections (outcropping in the deepest cavi) are commonly made 

up of evenly layered light and dark material (fig. 5), which 

often reveals intricate, light-dark swirled patterns when eroded. 

Because it is the lowest exposed unit within the quadrangle, 

the stratigraphic nature of the Olympia Cavi 1 unit’s lowermost 

contact is temporally unconstrained. Based on the identification 

of an equivalent unit by Tanaka and others (2008), we infer a 
6

commencement of deposition near the beginning of the Middle 

Amazonian and a cessation at an unspecified time during the 

Middle Amazonian. However, a paucity of impact craters on 

this unit both within and outside of the MTM 85200 quadrangle 

means this emplacement range should be viewed as approximate (Tanaka and others, 2008).

Unit Aoc1 is generally equivalent to the more regionally 

occurring Planum Boreum cavi unit of Tanaka and Fortezzo 

(2012). However, those workers do not map the equivalent unit 

as pervasively within the Olympia Cavi as we do herein, opting 

instead to map a younger, tens-of-meters-thick lag deposit 

that obscures the discriminating characteristics of subjacent 

material. The lag deposit is identified herein as the Olympia 

Cavi 2 unit (Aoc2), described below. Tanaka and Fortezzo 

(2012) also suggest that the younger lag deposit buries a Late 

Hesperian unit that broadly underpins the north polar plateau, 

termed the “Planum Boreum rupēs unit.” We do not identify a 

unit that is equivalent to the rupēs unit of Tanaka and Fortezzo 

(2012) within the boundaries of the MTM 85200 quadrangle; 

we instead interpret the Olympia Cavi 1 unit as the stratigraphically lowest exposed unit (see cross sections, map sheet). 

Regional observations, however, suggest the possibility that 

the Planum Boreum rupēs unit of Tanaka and Fortezzo (2012) 

exists beneath the materials mapped herein, but that it is either 

not exposed within the Olympia Cavi or it is thinly buried by 

younger units. For context and comparison, we have shown the 

stratigraphic position of unit AHBr from Tanaka and Fortezzo 

(2012) in the Correlation of Map Units. The Olympia Cavi 

1 unit is also partly equivalent to the platy unit of Byrne and 

Murray (2002), the mantle unit of Herkenhoff (2003), and the 

basal unit of Fishbaugh and Head (2005).

We interpret the Olympia Cavi 1 unit to have been deposited as layers of dark lithic sand and dust, layered with (and perhaps bound by) ice during varied atmospheric conditions driven 

by orbital/axial oscillations. Occasional evidence of crossbedding indicates that the unit is partly aeolian in origin, though 

sparsely occurring light and dark near-horizontal layers perhaps point toward intermittent deposition through atmospheric 

fallout. The unit may have partly accumulated in similar fashion 

to the layered deposits of the Planum Boreum 1 and 2 units 

(described below), with a supply of dark sand being exhausted 

before the Planum Boreum 1 unit was emplaced (Herkenhoff 

and others, 2007). Based on the rugged, slabby appearance 

and thin scale of internal stratification, we suggest that unit 

Aoc1 underwent intensive, syndepositional or postdepositional 

devolatilization, compaction, and eventual erosion and surface 

deflation; primary depositional characteristics do not always 

appear to be well preserved.

Variable elevation of the uppermost surface of the Olympia Cavi 1 unit suggests that the unit’s vertical accumulation 

was followed by erosion, deflation, and localized downcutting 

(see cross sections), perhaps driven by off-pole katabatic winds 

(Howard, 2000; Kolb and Tanaka, 2001; Byrne and Murray, 

2002; Tanaka and others, 2008). This activity may have formed 

an uneven, undulating paleosurface onto which younger deposits were emplaced. Localized erosion may also have resulted 

in the formation of prototroughs—topography that persisted 

during the accumulation of subsequent geologic sequences.

Following an indeterminate period of time, the Planum 

Boreum 1 unit (Ab1) began accumulating on the unit Aoc1

paleosurface. Unit Ab1 is made up almost entirely of a largely 

undifferentiable sequence of rhythmic layers (figs. 3, 5), with 

unconformities (documented by truncated layers) occurring 

intermittently within the stratigraphic sequence (fig. 4). Because 

such unconformities are not concentrated near the bottom of the 

exposed unit Ab1 sequence, we suggest that the lower contact 

of this unit is stratigraphically certain, implying commencement of deposition within a narrow window of geologic time. 

This contrasts with the regional observations and interpretations 

of Tanaka and Fortezzo (2012), who perceive the transition 

between their regionally equivalent units to be time-transgressive. These contrasting interpretations indicate that timetransgression between units Aoc1 and Ab1 suggested by Tanaka 

and Fortezzo (2012) was spatially confined to particular regions 

of Planum Boreum and did not occur inside of the MTM 85200 

quadrangle boundary. The Planum Boreum 1 unit is generally 

equivalent to polar layered deposits of Tanaka and Scott (1987), 

the layered deposits of Herkenhoff (2003), the lower layered 

deposits of Tanaka (2005), and the Planum Boreum 1 unit of 

Tanaka and others (2008) and Tanaka and Fortezzo (2012).

The rhythmic and laterally continuous layers within the 

Planum Boreum 1 unit implicate accumulation of ice and dust 

through atmospheric fallout, perhaps as the result of orbit-controlled climate oscillations (Laskar and others, 2002, 2004) and 

(or) far-field volcanism and surface impact. Consistent traceability and thick outcrops suggest that unit Ab1 constitutes the 

volumetric bulk of the north polar plateau (see cross sections). 

However, unit accumulation appears to have been spatially 

inconsistent, as evidenced by unconformities within the unit. 

Localized erosion may have resulted, in part, from the focusing of katabatic winds in surface swales, which preferentially 

accumulated dust when deposition resumed. The occurrence of 

curvilinear ridges (mapped as berms; fig. 2) and scarps within 

unit Ab1 and their lateral traceability to unconformities suggests that erosion may have been partly influenced by strength 

differences between adjacent layers, perhaps due to variation 

in grain size, dust content, or degree of material induration/ice 

annealing. Such variations may have formed when the ice-rich 

material was exposed to insolation or during a resumption of 

deposition following localized depositional hiatus or erosion.

Middle to Late Amazonian Epochs

The widespread deposition of the Planum Boreum 1 unit 

within the MTM 85200 quadrangle was followed by widespread erosion, surface deflation, and localized downcutting, 

which resulted in the formation of an upper unconformity upon 

which younger units were deposited (see cross sections). Tanaka 

(2005) suggested that the eroded surface of this unit is ~5 m.y. 

old, based on crater retention ages, perhaps corresponding to 

a major decrease in Mars’ obliquity and a resultant decrease 

in insolation on the north polar plateau (Laskar and others, 

2002). However, the paucity of craters on this unit, as well as 

the interpretation that the unit underwent large-scale erosion 

(presumably destroying any accumulated craters), indicates that 
7

crater-based temporal constraints provide only an indication of 

when erosion significantly waned (Tanaka and Fortezzo, 2012). 

Thus, the time period over which unit Ab1 not only accumulated but also eroded is poorly constrained, as noted by Tanaka 

and others (2008).

The Olympia Cavi 2 (Aoc2) unit was deposited on the 

eroded vestiges of units Aoc1 and Ab1, where it formed a draping unit of variable thickness. We note, however, that the lower 

contact of unit Aoc2 is not well constrained, perhaps indicating 

that accumulation of that unit was partly coeval with the erosion 

of unit Ab1. Unit Aoc2 is characterized by its low to intermediate albedo and its occurrence at the base of trough scarps and on 

cavi slopes and floors, mostly south of Olympia Rupēs (figs. 3, 

5). We interpret the Olympia Cavi 2 unit as discontinuous layers 

of sediment and ice, perhaps eroded from the underlying Olympia Cavi 1 and the Planum Boreum 1 units. Erosion of unit Ab1

may have resulted from sublimation of layered and interstitial 

ices, which allowed for the mobilization, transport, and redeposition of lithic materials as unit Aoc2. Lithic inputs from outside 

of the map region, perhaps sourced from deep cavi immediately 

to the east of the map area (Tanaka and others, 2008), may also 

have added significant components to unit Aoc2.

Unit Aoc2 drapes older units, suggesting that it preferentially accumulated in topographic lows of the polar flats and 

cavi. However, the paleosurface of this unit, as it occurs beneath 

mapped materials, is difficult to determine. Traceability of unit 

Aoc2 outcrops shows its characteristic occurrence between 

units Ab1 and Ab2 (fig. 5) and thickening in topographic catchments, perhaps implying an aeolian origin (also suggested by 

Tanaka and Fortezzo, 2012). It is important to recognize that 

the Olympia Cavi 2 unit mapped herein is roughly equivalent to 

the “intermediate unit” of Rodriguez and others (2007) and the 

“Planum Boreum 2 unit” of Tanaka and Fortezzo (2012). Each 

of these units is interpreted to be a Middle to Late Amazonian 

dust and sand lag deposit derived from the erosion of subjacent units. In contrast to regional mapping results of Tanaka 

and Fortezzo (2012), however, we explicitly discriminate and 

partition those units of the MTM 85200 quadrangle that we 

interpret to have accumulated primarily through the widespread, 

rhythmic deposition of dust and ice (Boreum region units) and 

those that we interpret were deposited as eroded and redeposited 

lithic components from subjacent units (Olympia region units). 

In this geologic scenario, the erosion of Boreum region units is 

succeeded by the deposition of Olympia region units (and vice 

versa).

The Olympia Undae unit (Aou) comprises low-albedo 

duneforms and overlies and locally grades with unit Aoc2 (fig. 

5). We interpret unit Aou as accumulations of eroded and redistributed sand. Preserved duneforms within the unit indicate that 

bulk transport directions were off the pole to the south and west. 

Due to proximity of outcrops, a source of unit Aou sand may be 

the Olympia Cavi 1 unit, which is interpreted to be less resistant 

to erosion than the overlying Planum Boreum 1 unit (Byrne and 

Murray, 2002; Tanaka and others, 2008). Alternatively (or perhaps in addition to), apparent gradation between unit Aou and 

unit Aoc2 suggests that the latter may be a significant source 

for the dune sand. Though superposition relations with adjacent 

geologic units indicates that the Olympia Undae unit is one of 

the most recently active units within the quadrangle, its lowermost stratigraphic contact is uncertain. The Olympia Undae unit 

is partly equivalent to the crescentic dune material, linear dune 

material, and mantle material of Tanaka and Scott (1987) and 

the dune and mantle material of Tanaka (2005).

Late Amazonian Epoch

The Planum Boreum 2 unit (Ab2) was emplaced above 

eroded vestiges of unit Aoc2 as the result of seasonal to decadal 

climatic variations (Laskar and others, 2002, 2004), though the 

temporal nature of individual layers cannot be directly ascertained. The lithic fraction of unit Ab2, which is likely to be volumetrically minimal (Byrne, 2009), was perhaps derived from 

the erosion of subjacent units and (or) through atmospheric fallout. The lower contact grades into (and locally extends beneath) 

the Olympia Undae unit (see cross sections), particularly on 

pole-facing polar trough walls. Unit Ab2 mapped herein is correlative to the Planum Boreum 3 unit of Tanaka and Fortezzo 

(2012), who interpret the unit as Late Amazonian water ice 

with minor dust components and local thermokarst degradation. 

In addition, the Planum Boreum 3 unit is partly equivalent to 

the banded terrain of Howard and others (1982), partial frost 

cover of Herkenhoff (2003), and the upper layered deposits of 

Tanaka (2005). Unit Ab2 may represent the most recent episode 

of polar layered-deposit formation, because it unconformably 

overlies the bulk of Planum Boreum 1 unit outcrops and appears 

to fill small craters (Banks and others, 2010).

The Olympia Cavi 3 unit (Aoc3) is thin (perhaps several 

meters thick) and has a low albedo and a smooth and diffusely 

lineated surface. Unit Aoc3, which is not differentiated as a separate unit in previous geologic maps, occurs south of the Olympia Rupēs on pole-facing, low-angle slopes and shallow swales 

on polar flats (cross section B–B′). Unit Aoc3 forms irregularly 

shaped outcrops within unit Ab2 and appears to locally grade 

with units Ab3 and Aou. Characteristic mixed albedo swirled 

patterns, which are mostly apparent in high-resolution images 

(for example, CTX P02_001670_2643), allude to weak, nearhorizontal internal stratification. We interpret the Olympia Cavi 

3 unit to be composed of lithic particles, perhaps derived from 

the transport and redeposition of proximal outcrops of unit Aou, 

as well as topographically higher exposures of older units. The 

Olympia Cavi 3 unit represents localized sand sheets of equivalent composition to unit Aou but with a larger fraction of ice 

(based on albedo), perhaps occurring as poorly formed interbeds 

and (or) interstitial ice. 

The Planum Boreum 3 unit (Ab3) represents the extent of 

the north polar residual water ice cap (figs. 3, 4), as observed 

by Viking Orbiter 2 in 1976. It is interpreted to be a thin (<1 m 

thick; Byrne and others, 2008) layer of water ice that unconformably overlies all other units within the map region; the 

lower contact represents the inferred onset of deposition. South 

of the Olympia Rupēs, clusters of narrow ridges occur within 

the boundaries of unit Ab3. These features may have formed 

within either unit Ab3 or the subjacent unit Ab2 through the 

action of cross-polar aeolian scour (ridges are oriented in the 

same direction as the inferred transport direction of regional 
8

dune fields). The Planum Boreum 3 unit is partly equivalent to 

the polar ice deposits of Tanaka and Scott (1986), the polar ice 

cap of Herkenhoff (2003), the Planum Boreum 2 unit of Tanaka 

and others (2003), the Planum Boreum 4 unit of Tanaka and 

others (2008), and the residual ice cap of Tanaka and Fortezzo 

(2012).

Hyperspectral observations indicate that the water ice in 

the residual cap is coarse grained, implying that the ice is old 

enough to have annealed (Byrne, 2009). Areas of lower albedo 

within the unit typically coincide with shallow topographic 

lows. In such instances, the lower albedo of subjacent unit Ab2

may be visible through the higher albedo, coarse-grained ice of 

unit Ab3. Alternatively, dust may be more concentrated in the 

low areas (Rodriguez and others, 2007). Spectral observations 

and high-resolution images of the unit’s texture suggest that ice 

deposition has not occurred recently on the cap (Byrne, 2009). 

All units within the MTM 85200 quadrangle are covered annually by the north polar seasonal cap composed of carbon dioxide 

(Titus and others, 2008), which is not mapped herein.

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10

Table 1. Relative age determinations for geologic units in the Olympia Rupes region of Mars.

Geographic Unit Unit Relative Area2 Superposition

 region name label age1 (103 km2) relations3

Boreum Boreum 3 unit Ab3 LA 38.78 <Aou, Aoc3, Ab2, Aoc2, Ab1, ~Aou

 Olympia Olympia Undae unit Aou LA 6.57 >Ab3, ~Ab3, Aoc3, Ab2, Aoc2, <Ab1, Aoc1

 Olympia Olympia Cavi 3 unit Aoc3 LA 1.10 >Ab3, ~Aou, Ab2, <Aou, Ab2, Aoc2, Ab1, Aoc1

 Boreum Boreum 2 unit Ab2 LA 11.66 >Ab3, Aou, Aoc3, ~Aou, Aoc3, <Aoc2, Ab1, Aoc1

 Olympia Olympia Cavi 2 unit Aoc2 MA–LA (?) 1.11 >Ab3, Aou, Aoc3, Ab2, ~Aou, <Ab1, Aoc1

 Boreum Boreum 1 unit Ab1 MA–LA (?) 3.67 >Ab3, Aou, Aoc3, Ab2, Aoc2, <Aoc1

 Olympia Olympia Cavi 1 unit Aoc1 MA 1.11 >Ab3, Aou, Aoc3, Ab2, Aoc2, Ab1

1Relative age determined by superposition relations only: LA, Late Amazonian; MA, Middle Amazonian (after Tanaka, 1986).

2Area includes exposed area of unit only.

3Symbols: <, younger than; ~, overlaps in time with; >, older than.

Ab1

Ab1

depression

knob

0 1 KILOMETER

N

Figure 2. Excerpt of CTX image number P01_001341_2654 (centered at lat 

85.21° N., long 147.19° E.; location shown on map) that shows local geomorphology that occurs within the Planum Boreum 1 unit (Ab1). These include berms 

(denoted by black arrows), as well as a knob and depression.
11

Ab3

Ab1

Aoc2

Aoc1

Aoc2

0 1 KILOMETER

craterform

N

Figure 3. Excerpt of High Resolution Imaging Science 

Experiment image number PSP 001548-2755 (centered at lat 

84.39° N., long 142.94° E.; location shown on map) that shows 

a scarp-exposed section of layered deposits south of the 

Olympia Rupēs. Vertical section is ~400 m thick within the 

image excerpt. Sinuous high- and low-albedo layers define the 

margin of the Planum Boreum 3 unit (Ab3), which buries 

Olympia Cavi 2 unit (Aoc2) above the scarp. Unit Aoc1 forms 

a medium-albedo, smooth surface above the rhythmic layering 

of the Planum Boreum 1 unit (Ab1). A craterform feature, 

crater A (~150 m diameter), interpreted as the eroded vestige of 

an impact crater, resides in the Olympia Cavi 1 unit (Aoc1).

Ab2

Ab2

Ab1

Ab1

Ab3

Aoc2?

(unmapped)

0 1 KILOMETER

craterform

Figure 4. Excerpt of Mars Orbiter Camera narrow-angle image 

number e03-02719 (centered at lat 85.14° N., long 168.76° E.; 

location shown on map) that shows unit relations near the 

eastern termination of a polar trough, south of the Olympia 

Rupēs. Excerpt is centered near a craterform feature, crater B, 

in the Planum Boreum 1 unit (Ab1). Two subparallel angular 

truncations (unconformities) are visible in the unit Ab1 (white 

arrows). Uppermost sequences of unit Ab1 are locally overlain 

by Planum Boreum 2 and 3 units (Ab2, Ab3), demonstrating 

the unconformable nature of the polar layered sequences. 

Though it is not visible at map scale, the massive, darker 

material that underlies high-albedo patches of the Planum 

Boreum 3 unit located at top center may be a small outcrop of 

the Olympia Cavi 2 unit (Aoc2?).

N
12

Ab2

Ab1

Aoc2

Aoc1

Aoc2

Ab2

Ab2

Ab2

Aou

Aoc2

Ab2

Ab2

Ab2

Ab1

0 1 KILOMETER

N

Figure 5. Excerpt of Context Imager (CTX) image number P02_001670_2643 

(centered at lat 84.94° N., long 176.56° E.; location shown on map) that shows 

the margin of a scarp-exposed section of the Olympia Cavi 1 and Planum 

Boreum 1 units (Aoc1 and Ab1) along Olympia Rupēs. The Olympia Cavi 2 

unit (Aoc2) resides both above and below the scarp. The scarp wall is made up 

of unit Aoc1, which is interpreted to extend beneath younger units to underpin 

the exposed cavi floor materials. The Planum Boreum 2 unit (Ab2), though 

locally unmapped, can be seen as high-albedo patches (white arrows) beneath 

the Olympia Undae unit (Aou), both of which overlie unit Aoc2.