**GEOLOGIC MAP**

**OF THE**

**TOLSTOJ (H-8)**

**QUADRANGLE**

**OF MERCURY**

By

Gerald G. Schaber and John F. McCauley

1980

Prepared for the

National Aeronautics and Space Administration

by

U.S. Department of the Interior, U.S. Geological Survey

(Published in hardcopy as USGS Miscellaneous Investigations Series Map
I--1199, as part of the Atlas of Mercury, 1:5,000,000 Geologic Series.
Hardcopy is available for sale from U.S. Geological Survey, Information
Services, Box 25286, Federal Center, Denver, CO 80225)

Please direct questions or comments about the digital version to:

Richard Kozak

U.S. Geological Survey

2255 N. Gemini Drive

Flagstaff, AZ 86001

e-mail: rkozak@flagmail.wr.usgs.gov

**DESCRIPTION OF MAP UNITS**

**PLAINS MATERIALS**

pvs VERY SMOOTH PLAINS MATERIAL---Occurs as smooth planar surfaces only
in floors of c4 and c5 craters (FDS 529126). Very few superposed craters
at Mariner 10 resolutions. *Interpretation:* Probably shock-melt and
fallback associated with formation of individual craters

ps SMOOTH PLAINS MATERIAL---Forms flat to gently rolling plains in
extremely large annulus surrounding Caloris Basin, and in southeastern
part of quadrangle in smaller patches associated with basin (for
example, Tolstoj; FDS 166912) and crater (c1 to c3) floors (FDS 223).
Also occurs in southeast in vague topographic lows of uncertain origin.
Characterized by abundant small bright-halo craters (FDS 214, 221) and
sharp overlapping contacts with older terrain. Sinuous to lobate ridges
are abundant. *Interpretation:* Uncertain; some areas may consist of
fluidized shock-melt materials created by Caloris impact. Alternatively,
may be post-Caloris volcanic flows. Unequivocal volcanic vents and
lava-flow fronts not observed. Unit appears to be equivalent to and
younger than a late c3 crater age, as indicated by relatively few
superposed c3 craters \>30 km in diameter

psi INTERMEDIATE PLAINS MATERIAL---Forms flat to rolling plains in c1 to
c3 craters and in topographic depressions. Contains fewer superposed
craters than intercrater plains but may form more convex, overlapping
contacts with older terrain than do smooth plains (see craters Goya and
Rublev: FDS 166907, 246, 223). *Interpretation:* Uncertain; may consist
of shock-melt and breccia materials moved into topographic depressions
by mass-wasting processes; may also consist in part of volcanic
materials. Age relations to intercrater and smooth plains uncertain,
primarily because of insufficient resolution. May have origin similar to
smooth plains but deposited over a considerably longer period. Possibly
analogous to widespread light lunar plains

pi INTERCRATER PLAINS MATERIAL---Forms very extensive, gently rolling
ground between large c1 and c2 craters, primarily in southeast half of
quadrangle (FDS 124). Overlaps parts of Tolstoj Basin. Unit has high
density of superposed 5- to 10-km-diameter craters, many of which are
elongate and shallow or open on one side, features suggesting
secondary-crater morphology (FDS 223). Surface nearly saturated with
visually indistinct but topographically distinct circular depressions
but essentially free of well-defined craters larger than about 50 km in
diameter. *Interpretation:* A generally planar surface consisting of
highly eroded remnants of large craters and basins that are only very
shallow circular depressions (Malin, 1976). May consist of mixture of
basin-related ejecta (breccia) and volcanic deposits emplaced during
later stages of heavy bombardment (Strom, 1977). Embayment and
transection relations of intercrater plains to craters and basins (for
example, Tolstoj) suggest a complex history of contemporaneous formation

cfp CALORIS FLOOR PLAINS MATERIAL---Closely resembles very smooth plains
material (unit pvs) but exhibits more intense secondary deformation (FDS
106, 110, 195, 229). Sinuous ridges cut by tension fractures form
grossly polygonal pattern dominated by two major trends, one concentric
with respect to border of basin and the other

radial. Intensity of deformation appears to increase from basin rim
inward. Surface of unit slopes toward center of basin floor (Hapke and
others, 1975). *Interpretation:* Uncertain; probably post-Caloris
volcanic flow materials, although unit could in part be shock-melt
material associated with Caloris impact. Ridges and fractures suggest
basin floor subsidence (ridge formation) followed by possible slight
uplift forming tension fractures (Strom and others, 1975)

**BASIN MATERIALS**

> CALORIS GROUP

cm Caloris Montes Formation---Consists of numerous rectilinear massifs 1
to 2 km high and about 10 to 50 km long, mostly elongate radially from
center of basin and separated by hackly-floored, radial troughs and
gougelike structures. Surfaces of massifs are hackly. Best developed
along inner edge of basin where steep inward-facing scarps are common,
grading outward into smaller massifs and blocks. Marks crest of most
prominent ring structure around Caloris. Type area: region near 18°,
184.5° (FDS 229) (McCauley and others, 1980). *Interpretation:* Uplifted
prebasin bedrock covered by deep-seated late ejecta from Caloris. Inner
boundary is approximate outer limit of crater of excavation

cn Nervo Formation---Consists of patches of hackly to rolling plains
that lie above both Caloris floor plains (unit cfp) and smooth plains
surrounding Caloris Basin. Locally appears to be draped over subjacent
blocks and massifs of the Caloris Montes Formation (unit cm). Type area:
region in Shakespeare quadrangle near 40°, 177.5° (FDS 193) and south of
unrelated crater Nervo (McCauley and others, 1980). *Interpretation:*
Probably fallback mixed with impact melt

co Odin Formation---Patches of hummocky plains that generally lie beyond
major Caloris Montes scarp for as much as 1000 km. Consists of closely
spaced to isolated smooth hummocks, usually 1 to 2 km across, separated
by smooth plains; these plains resemble unit ps but for mapping
convenience are included in this unit. Locally, hills are concentric
with Caloris scarps. Type area: region around 25°, 170° (FDS 191, 72)
(McCauley and others, 1980). *Interpretation:* Ejecta from Caloris
embayed by smooth plains material. Individual hummocks and hills may
represent coherent blocks of material from deeper parts of basin below
mercurian megaregolith

com Odin Formation (Mantling Preexisting Terrain)---Extensive patches of
hummocky deposits that mantle preexisting crater and plains units as far
as 1200 km beyond edge of Caloris Basin. Consists of closely spaced,
rounded hummocks usually 3 to 8 km across, separated by smoother plains
that resemble units pi and psi. Preexisting topography apparent. Type
area: region around --2.5°, 170° (FDS 56, 63). *Interpretation:* Ejecta
from Caloris; individual hummocks and hills may represent coherent
blocks of material from deeper parts of basin. Most contacts of unit
farthest from Caloris with intercrater plains (unit pi) and c1 and c2
crater materials difficult to map with confidence

cvl Van Eyck Formation, Lineated Facies---Scattered outward from about
limit of weak outer Caloris scarp for distance of about 1000 km. Forms
long, wavy radial ridges and intervening grooves. Some ridges sharply
bounded by scarps. Extensively embayed by smooth plains material (unit
ps). Type area: southwest edge of Van Eyck Basin in Shakespeare

quadrangle near 40°, 163° (FDS 189) (McCauley and others, 1980).
*Interpretation:* Ejecta from Caloris and radial structures formed by
Caloris event. Secondary cratering played important role in development
of surface morphology. Most grooves are probably buried secondaries.
Ejecta from shallower layers in pre-Caloris surface than those that were
source of other formations in Caloris Group

cvs Van Eyck Formation, Secondary-Crater Facies---Consists of chains and
clusters of craters that are generally superposed on cratered terrain in
southeast but are also locally gradational with distal ends of lineated
facies (unit cvl). Degree of freshness uniform except near large younger
craters. Type area: crater cluster on rim of crater Po Chu-I at --7°,
166° (FDS 0529131) (McCauley and others, 1980). *Interpretation:*
Far-flung secondaries from Caloris that lie within and beyond limit of
thin, discontinuous part of ejecta blanket. These secondaries were
little affected by later arriving ejecta and other secondaries, as was
lineated facies of the Van Eyck

trl TOLSTOJ BASIN RIM MATERIAL---Rises slightly above intercrater
plains. Characterized by radial lineations and grooves. Rectilinear map
pattern. Pronounced dark albedo between inner and outer rings of basin.
Type area: regions around --10°, 153° (FDS 246) and --21°, 171° (FDS
166912). *Interpretation:* Radially lineated and grooved rim ejecta from
Tolstoj Basin, consisting of shock breccia and shock-melt materials
derived from depth. Lineations and grooves represent secondary-crater
scour and in part post- depositional structural deformation. Rectilinear
map pattern of unit discussed in text. Dark albedo of some materials
probably compositional (Hapke and others, 1975). Overlap of intercrater
plains (unit pi) onto northwest side of Tolstoj suggests that basin
impact was an early pi event

**CRATER MATERIALS**

> CRATER MATERIALS---Craters \>30 km in diameter

c5 Material with very fresh appearing, sharp rims, wall terraces, and
central peaks (FDS 121, 126, 529126). Well-defined, continuous fields of
relatively sharp secondary-crater clusters and chains, of which only the
larger are mapped (sc). Sharp break between wall and floor materials.
Floors generally filled with very smooth plains material (unit pvs)

c4 Material with prominent, continuous, only slightly subdued rims, wall
terraces, and central peaks (FDS 116, 166827). Most floors partly filled
with very smooth plains materials (unit pvs). Break at base of wall
still distinct. Floor and wall contact not visible in crater Mozart.
Well-preserved, extensive secondary-crater fields; low number of
superposed craters

c3 Material with low, rounded, but continuous rims and with flat floors
generally filled with intermediate plains and smooth plains materials
(units psi and ps) (FDS 222, 166912). Floor-wall boundary indistinct in
many craters. No well-defined secondary-crater fields surround craters
\<100 km across. Craters \>100 km in diameter have discontinuous, only
partly preserved secondaries. Moderate number of superposed craters

c2 Material with a pan-shaped profile and with low, greatly subdued,
continuous to discontinuous rims (FDS 166912, 166913). No wall terraces,
rare central peaks. Crater floor generally filled with

intermediate plains and smooth plains materials (units psi and ps).
Floor-wall boundary very indistinct. No visible secondaries; many
superposed craters

c1 Material of flat-floored craters with extremely low and discontinuous
rims rising only slightly above intercrater plains. Rare central peaks;
wall structure and satellite craters are absent (FDS 62, 124, 215). Most
craters \>100 km in diameter, many filled with intermediate plains and
smooth plains materials (units psi and ps) *Interpretation:* Impact
craters displaying various degrees of degradation and infilling; state
of degradation can be misleading age criterion because of proximity of
crater to younger impact craters and associated secondary-crater effects
that are more intense near a primary on Mercury than on the Moon.
Morphologic classification of mercurian craters \<100 km and \>100 km in
diameter has been described by N. J. Trask (fig. 10 *in* McCauley and
others, 1980)

cp CENTRAL PEAK MATERIAL---Occurs in c2 to c5 craters as single hills or
small semicircular clusters of hills rising above crater floors near
their centers (FDS 116, 529126). *Interpretation:* Precrater brecciated
rocks uplifted and deformed at time of impact

cs SECONDARY CRATER MATERIAL---Occurs as chains and clusters of
moderately fresh to subdued craters, many relatable to nearby primary
craters. *Interpretation:* Near- and far-flung secondary ejecta from
primary impacts. Only larger secondary-crater chains and clusters from
each crater and basin are mapped (for example, crater Mozart)

> NOTE The area shown as geologic unit cvl (Caloris Group---Van Eyck
> Formation) centered at lat. 23° N. and long. 162°, should be unit com
> (Odin Formation mantling preexisting terrain). The preexisting terrain
> at this locality is considered to be ejecta rim deposits from an
> unnamed 355-km-diameter basin centered at lat. 28° N, and long.
> 158.5°, in the Shakespeare quadrangle (H-3).
>
> Contact---Queried where doubtful
>
> Scarp, probably a fault---Dashed where subdued. Bar and ball on
> downthrown side
>
> Lineament---Probably a fault with no vertical relief or uncertain
> displacement. May include unresolved crater chains of secondary-impact
> origin
>
> Narrow to broad ridge---Interpreted within units cvl and co and plains
> materials (units pi, ps, and cfp) as mare-type wrinkle ridge due to
> shrinkage. Symbol on ridge crest
>
> Ridge scarp, linear to arcuate---Interpreted within units co and com
> and plains materials (units psi, ps, and cfp) as edge of mare-type
> wrinkle ridge. Line marks base of slope; barb points downslope
>
> Fissure or depression of structural origin, linear to
> arcuate---Restricted to unit cfp in floor of Caloris Basin. Barbs
> point downslope
>
> Scarp, linear to arcuate---Interpreted as fault scarp or depression of
> structural or erosional origin
>
> Crater rim crest
>
> Crater rim crest---Greatly subdued or buried
>
> Crestline of concentric rings surrounding Caloris and Tolstoj
> Basins---Interpreted as structural ring fractures formed during and
> immediately after these impacts
>
> Area of bright crater-ray material---Interpreted as ejecta from very
> fresh impacts
>
> Area of abnormally low albedo

**INTRODUCTION**

The Tolstoj quadrangle in the equatorial region of Mercury contains the
southern part of Caloris Planitia, which is the largest and best
preserved basin seen by Mariner 10. This basin, about 1300 km in
diameter, is surrounded by a discontinuous annulus of ejecta deposits of
the Caloris Group (units cm, cn, co, cvl, and cvs) that are embayed and
covered by broad expanses of smooth plains (unit ps). The southeast half
of the quadrangle is dominated by ancient crater deposits (units cl, c2,
and c3), by nondescript rolling to hummocky plains materials (unit pi)
between individual craters, and by isolated patches of nondescript
plains (units pi--ps). The ancient and degraded Tolstoj multiring basin,
about 350 km in diameter, is in the south-central part of the quadrangle
The large, well-preserved crater Mozart (285 km diameter) is a prominent
feature in the western part of the area; its extensive ejecta blanket
and secondary crater field are superposed on the smooth plains (unit ps)
surrounding Caloris.

Low-albedo features Solitudo Neptunii and Solitudo Helii, adopted from
telescopic mapping, appear to be associated with the smooth plains
material (unit ps) surrounding Caloris; a third low-albedo feature,
Solitudo Maiae, appears to be associated with the Tolstoj Basin. (For
location of albedo features see Davies and others, 1978, p. 15.)

Mercury's rotation period of 58.64 days is in two-thirds resonance with
its orbital period of 87.97 days Therefore, at its equator, longitudes
0° and 180° are subsolar points ("hot poles") near alternate perihelion
passage (Davies and others, 1978). The "hot pole" at 180° lies within
the Tolstoj quadrangle; at perihelion, equatorial temperatures range
from about 100° K at local midnight to 700° K at local noon. This daily
range of 600° K is greater than that on any other body in the solar
system (Davies and others, 1978).

Mariner 10 photographic coverage was available for only the eastern
two-thirds of the Tolstoj quadrangle. Image data from three Mariner 10
encounters with Mercury were used in mapping the quadrangle.

**STRATIGRAPHY**

**OLDER PLAINS MATERIALS**

The rolling to hummocky plains that lie between large craters in the
southeastern part of the quadrangle make up the oldest recognizable map
unit, the intercrater plains material (unit pi). The plains were
originally described as intercrater by Trask and Guest (1975), who noted
their level to gently rolling appearance and their general lack of
well-defined craters larger than about 50 km in diameter. Malin (1976)
showed the plains to contain highly eroded remnants of large craters and
basins that are only very shallow circular depressions. These
intercrater plains are, however, marked by a very high density of
superposed craters that are small (5-10 km diameter), elongate, shallow,
and probably secondary to the many large craters superposed on the
plains. Superposition of crater ejecta over parts of intercrater plains
in other areas indicates that some large craters formed in a preexisting
intercrater plains unit. On the other hand, the intercrater plains
material partly postdates some of the major cratering events on Mercury,
according to apparent superposition relations (Malin, 1976; Guest and
O'Donnell, 1977). In particular, the unit appears to overlap the entire
northwest side of the Tolstoj Basin, a feature indicating that the
intercrater plains in this region probably do not represent the remains
of the primordial surface of the planet. A complex history of
contemporaneous craters and plains formation is therefore suggested. A
detailed discussion of the origin of the intercrater plains on the Moon
and Mercury was given by Strom (1977).

Patches of less cratered, smoother, less rolling plains occur throughout
the quadrangle, but their recognition is highly dependent on the
resolution and lighting of individual Mariner 10 frames. Therefore,
because their distribution cannot now be mapped accurately, many of
these patches are included with the smooth plains material (unit ps).
Certain patches of these intermediate plains, where clearly rougher and
possibly older than unit ps, are mapped as the intermediate plains
material (unit psi). These patches occur mostly within the floors of cl,
c2, and c3 craters and are distinguished by a slightly greater density
of small craters and a lower incidence of small bright-halo craters than
are found on the smooth plains material (unit ps). The presence of
plains intermediate in roughness and crater density between the oldest
plains (unit pi) and the post-Caloris plains (unit ps) suggests that
plains formation was a more or less continuous process that spanned much
of the early geologic history of Mercury.

**BASIN MATERIALS**

The impact that produced the Tolstoj Basin occurred very early in the
history of the quadrangle. Two ragged, discontinuous rings approximately
356 km and 510 km in diameter encompass the structure but are poorly
developed on its north and northeast sides; a third partial ring with a
diameter of 466 km occurs on its southeast side. Diffuse patches of
material of dark albedo lie outside the innermost ring. The central part
of the basin is covered by smooth plains material (unit ps). Hapke and
others (1975) have suggested that the dark-albedo materials associated
with the Tolstoj Basin margins are distinctly bluer than the surrounding
terrain, whereas the plains filling the interior are distinctly redder.

Despite Tolstoj's great age and its embayment by the ancient inter-
crater plains, it retains an extensive and remarkably well preserved,
radially lineated ejecta blanket (unit trl) around two-thirds of its
cir- cumference. The ejecta tends to be blocky and only weakly lineated
between the inner and outer rings. Radial lineations with a slight
swirly pattern are best seen on the southwest side of Tolstoj. The
unusual rectilinear map pattern of the ejecta suggests: (1) control of
the ejecta pattern by prebasin structures, (2) preferential burial along
structural trends of an originally symmetrical ejecta blanket by the
intercrater plains material (unit pi), or (3) formation of Tolstoj by an
oblique impact from the northwest that produced an ejecta blanket with
bilateral symmetry and little or no deposition uprange. Analysis of
stereo- photography of Tolstoj ejecta northeast of the crater suggests
that this deposit has been upwarped to a higher elevation relative to
the surrounding plains.

Caloris Group

The Caloris Basin is especially significant from a stratigraphic
standpoint. Like the Imbrium and Orientale Basins on the Moon, it is
surrounded by an extensive and well-preserved ejecta blanket (Strom and
others, 1975; Trask and Guest, 1975; Guest and O'Donnell, 1977). As on
the Moon, where ejecta from the better preserved basins was used to
construct a stratigraphy, the ejecta from the Caloris Basin also can be
used as a marker horizon. This ejecta is recognizable to a distance of
about one basin diameter in the Tolstoj quadrangle and the adjacent
Shakespeare quadrangle to the north. Undoubtedly, the ejecta also
influences a large part of the as-yet-unseen terrain to the west. A
stratigraphic and structural comparison between the Orientale and
Caloris Basins has been made by McCauley (1977). McCauley and others
(1980) have proposed a formal rock stratigraphy for the Caloris Basin
that we have adopted on the present map. This stratigraphy is patterned
after that used in and around the Orientale Basin on the Moon (Scott and
others, 1977) and should aid in the future recognition of pre- and
post-Caloris events over a broad expanse of the surface of Mercury.
Crater degradation chronologies, such as the one modified from Trask
(McCauley and others, 1980), and correlations between

plains units on the basis of crater frequency may aid in tying much of
the remainder of the surface of Mercury to the Caloris event.

Unlike the Imbrium-related stratigraphy of Shoemaker and Hackman (1962),
that devised for Mercury is a rock rather than a time stratigraphy. It
recognizes the existence of an orderly, in essence isochro- nous
sequence of mappable units around Caloris that are similar in character
to those recognized around the better preserved impact basins of the
Moon such as Orientale, Imbrium, and Nectaris.

McCauley and others (1980) have proposed the name "Caloris Group" to
include the mappable units created by the impact that formed the Caloris
Basin and have formally named four formations within the group, which
were first recognized and named informally by Trask and Guest (1975).
The formations are here described from the basin rim outward.

The [Caloris Montes Formation]{.underline} (unit cm), which was
informally called the Caloris mountains terrain by Trask and Guest
(1975), consists of a jumbled array of smooth-appearing but highly
segmented mountain massifs that rise 1-2 km above the surrounding
terrain. These massifs mark the crestline of the most prominent scarp or
ring of the Caloris Basin and grade outward into smaller blocks and
lineated terrain. The Caloris Montes Formation is very similar in
morphology to and is considered the equivalent of the massif facies of
the Montes Rook Formation around the Orientale Basin (Scott and others,
1977; McCauley and others, 1980). The Caloris Montes is interpreted as
basin rim deposits consisting of ejecta from deep within Caloris that is
mixed with but generally overlies uplifted and highly fractured prebasin
bedrock (McCauley, 1977).

The [Nervo Formation]{.underline} (unit cn) consists of rolling to
locally hummocky plains that lie in intermassif depressions. The plains
generally lie within the annulus of rugged terrain marked by the Caloris
Montes Formation and locally appear to drape and overlie some of the
more low-lying massifs. The Nervo bears some resemblance to the Apennine
Bench Formation around the Imbrium Basin (Hackman, 1966); its closest
counterpart in Orientale is the knobby facies of the Montes Rook
Formation (Scott and others, 1977). The Nervo Formation was originally
designated the intermontane plains by Trask and Guest (1975) and has
been interpreted by them as fallback ejecta, an interpretation that
seems to explain its distribution pattern and relative roughness as well
as the fact that it is generally perched above the smooth plains (unit
ps) that encompass Caloris.

The [Odin Formation]{.underline} (units co and com), which was
originally called the hummocky plains by Trask and Guest (1975), was
described by them as consisting of closely spaced, smooth hills about 1
km across. The area between the hills is similar in appearance to the
smooth plains (unit ps), but for mapping convenience this area has been
included in the Odin Formation. The distribution pattern of the Odin is
highly dependent on resolution and lighting but in general appears
similar to that of the Alpes Formation of the Imbrium Basin on the Moon.
The Odin, like the Alpes, occurs in broad lobes (unit co) such as those
in Odin Planitia beyond the main basin scarp. As unit com, the Odin also
mantles the intercrater plains (unit pi) and cl and c2 crater materials
out to a distance of 1200 km from the main Caloris scarp. The Odin
Formation is interpreted as part of the Caloris ejecta sequence, but its
mode of origin is less clear than those of certain other Caloris
formations. The unit may consist of late-arriving, blocky, coherent
ejecta from deep within the Caloris cavity, later partly buried by
smooth plains (unit ps).

The [Van Eyck Formation]{.underline} (units cvl and cvs), which is the
most distinctive of the circum-Caloris stratigraphic units, was called
the Caloris lineated terrain by Trask and Guest (1975). The inner
boundary of the Van Eyck is generally coincident with the weak outer
Caloris scarp. The unit forms radial ridges and grooves that are
extensively embayed by smooth plains (unit ps). The Van Eyck is similar
in morphology but somewhat more degraded than the Fra Mauro Formation
around the Imbrium Basin on the Moon; secondary cratering and ballistic
deposition of

ejecta from Caloris undoubtedly played an important role in its
emplacement. It is difficult to define individual secondary craters
within the Van Eyck, but at a distance of about one basin diameter,
numerous clusters and chains of moderately well preserved craters occur
that are interpreted as far-flung Caloris secondary craters. These
craters have been included in a separate facies (unit cvs) of the Van
Eyck Formation because of their regional stratigraphic significance.

**YOUNGER PLAINS MATERIALS**

The Caloris floor plains material (unit cfp) is a special problem and is
not included in the Caloris Group. The plains have some features in
common with the Maunder Formation in the floor of Orientale on the Moon
(McCauley, 1977; Scott and others, 1977) but do not show the radial and
circumferential ridges characteristic of the Maunder that led to its
interpretation as a basin floor unit. The Caloris floor plains have a
more open, coarser fracture pattern than does the Maunder. In addition,
the Caloris ridges and the fractures cutting them have a crude rhombic
pattern that led Strom and others (1975) to conclude that the plains
materials subsided and then were gently uplifted to produce the open
tension fractures observed. The ridges in the floor of Caloris lack the
crenulated crests that are common on lunar ridges. Regardless of the
origin and tectonic history of these plains, it seems clear that they
represent a deep basin fill that obscures the original floor of the
Caloris Basin.

The largest single expanse of the smooth plains material (unit ps)
surrounds the Caloris Basin---mostly in Tir and Budh Planitiae---but
many smaller patches occur in crater floors and other topographic
depressions within the heavily cratered terrain in the southeastern part
of the quadrangle. The plains are characterized by a relatively sparse
crater density and an abundance of mare-type wrinkle ridges; overlap
relations indicate that the plains are younger than the more densely
cratered units. The plains also embay the Caloris Formation and account
in particular for the skeletal map pattern of the Van Eyck Formation
(unit cvl). The ubiquitous distribution of unit ps in topographically
low regions supports the hypothesis that these materials were deposited
in a fluid or semifluid state as basin ejecta or volcanic flows. The
plains are thought to be slightly younger but dose to the same age as
the Caloris Basin materials (Trask and Guest, 1975); thus parts of the
plains are probably Caloris ejecta, either impact melt or very fluid
debris flows. No obvious secondary craters from Caloris have been
recognized on the smooth plains. The presence of large patches of smooth
plains in the floor of the Tolstoj Basin and in irregular depressions in
the extreme southeastern part of the map indicates that at least some of
these materials may be volcanic (Schultz, 1977). However, the absence of
unequivocal lava-flow fronts and well-defined volcanic vents such as
those of the lunar maria prevents a firm conclusion regarding volcanic
origin.

Small patches of very smooth plains material (unit pvs) occur in the
floors of many of the youngest craters. The patches may consist of
fallback and impact melt related to the formation of individual craters
and therefore may not represent late-stage volcanic fill or volcanic
modification of the more youthful mercurian craters. Schultz (1977)
suggested compositional differences or endogenic modification as
possible causes of the color contrasts among the floor, wall, and rim
areas of the dark-halo craters Zeami (120 km diameter), Tyagaraja (100
km diameter), and Balzac (80 km diameter). The dark ejecta and floors
plains of these craters are distinctly redder than the surrounding
plains, whereas their anomalously bright floor patches, central peaks,
and wall areas are distinctly bluer. None of these dark-halo craters has
associated bright rays, although secondary craters are well preserved.
Compositional implications of contrasting color differences for
mercurian crater and plains materials have been discussed by Hapke and
others (1975).

**CRATER MATERIALS**

The mapping employs a fivefold division of craters on the basis of their
apparent freshness (McCauley and others, 1980); craters less than about
30 km diameter were not mapped. A random distribution of c4 and c5
craters occurs throughout the quadrangle, superposed on all other units.
The Caloris event appears to have occurred late in c3 time, and Tolstoj
appears to be an early c1 event (McCauley and others, 1980).

Cumulative crater-size frequency distribution curves for the smooth
plains (unit ps) and heavily cratered terrain (units c1 to c5 and unit
pi) are shown in figures 1 and 2. The plains have a crater density
similar to that of the Cayley Plains Formation on the Moon and, if their
cratering history is about the same, have a similar age (3.9 to 3.8
b.y.). Significant disagreement exists, however, concerning the
cratering rate on Mercury relative to the Moon. The crater curves for
the heavily cratered terrain (fig. 2) show a general superposition on
the 10-percent saturation line (Gault, 1970) for crater diameters
between 50 and 90 km, but a rapid falloff from this line below and above
these extreme diameters. The relation of crater distribution between
lunar and mercurian heavily cratered terrain was described by Strom
(1977). Schaber and others (1977) discussed the reduction in numbers of
craters and basins per unit area on Mercury relative to the Moon.

**STRUCTURE**

The circumbasin scarps around Caloris, Tolstoj, and Mozart are the most
prominent structural features in the quadrangle. The main Caloris Montes
scarp is thought to approximate the edge of the basin of excavation of
Caloris and is probably a structural and stratigraphic counterpart of
the Montes Rook scarp around the Orientale Basin on the Moon (McCauley,
1977). A subdued outer scarp is present around most of the visible part
of Caloris, better seen in the Shakespeare quadrangle to the north. This
scarp is generally coincident with the transition between the massifs of
the Caloris Montes Formation (cm) and the lineated facies of the Van
Eyck Formation (unit cvl). The roughly rectilinear outlines of massifs
within the Caloris Montes suggest structural control by a prebasin
fracture pattern. The much lower, discontinuous outer scarp is
considered to be the feeble equivalent of the Montes Cordillera scarp
around Orientale. Like the Cordillera, it probably lies outside the
limit of the crater of excavation. Its poor development and spacing much
closer to the edge of the basin may be due to the greater mercurian
gravity, as described by Gault and others (1975). The Van Eyck Formation
is characterized by an extensive radial ridge-and-valley system with
minor concentric scarps and lineaments. These features are considered
for the most part as gouges and depositional plumes from secondary
cratering within the Van Eyck; the remarkably straight ridges and steep
walls, however, suggest formation by fracturing.

Only a small part of the ridge and fracture system that characterizes
the floor of Caloris is within the quadrangle. The ridges in the floor
of Caloris, which are like those within the smooth plains (unit ps), do
not appear to be as complex as lunar mare ridges and are cut by numerous
open grabenlike gashes. This area and its antipode in the Discovery
quadrangle are the only two on Mercury where tensional forces can now be
seen to have shaped the surface (Strom and others, 1975).

The Tolstoj Basin is encompassed by parts of at least three ragged and
discontinuous inward-facing scarps. Lineated ejecta is best developed in
the vicinity of and beyond the outer scarp, whereas blocky materials
occur between the inner and outer scarps. These relations are similar to
those around Caloris, although Tolstoj is less than half its size and is
much more severely degraded by later impact cratering.

The sharpness of the single rim-scarp of Mozart reflects the youth
(younger than unit ps) of this large impact. The position of Mozart at
the west terminator of the

Mariner 10 image data precludes visibility of its floor and thus hides
any evidence of a possible central uplift or inner structural ring.

Lobate scarps or ridges, which are best seen within the smooth plains
material (unit ps) and vary locally within the intercrater plains
material (unit pi), are generally steep on one side and gently dipping
on the other. Some, like the lunar mare ridges, appear to mark the
outlines of subjacent craters. Most workers, particularly Strom and
others (1975), Melosh (1977), and Melosh and Dzurisin (1978), have
ascribed these ridges to compression and a slight shortening of the
crust of Mercury after formation of most of the present surface. Some
ridges, however, may represent flow fronts, but their estimated heights
of several hundred meters would require formation by extraordinarily
viscous lavas.

Numerous faint lineaments are visible within the quadrangle, especially
in the area between the Tolstoj Basin and the large crater Zeami to the
northeast (fig. 3). Many of these lineaments may be faint
secondary-crater chains or gouges; others may represent traces of an
ancestral structural pattern that partly controlled the excavation of
the craters and basin. The lineaments may have been enhanced or
preserved by the gentle upwarping of this region of Tolstoj ejecta
discussed above. The largest lineament, which marks the northwest limit
of recognizable Tolstoj ejecta, is a subdued scarp some 450 km long.
Rejuvenation of earlier faults or fractures by subsequent impacts
probably occurred throughout the history of the planet. Thus, except for
the lobate compressional scarps, it is difficult to separate internally
produced structures from those of the complex impact history of Mercury.
The azimuthal trends of all lineaments mapped within the quadrangle are,
however, dominantly northwest (315°) and northeast (35°--40°) (fig. 3).
A minor, almost north-south trend is also observed. This situation is
reminiscent of the so-called lunar grid on the Moon, which is generally
ascribed to planetwide internal causes.

**GEOLOGIC HISTORY**

The interpretable geologic history within the Tolstoj quadrangle begins
with the period of formation of the intercrater plains (unit pi), which
persisted until shortly after impact of the asteroid that created the
Tolstoj Basin. After this event was a period of only slightly less
intense bombardment when the c1 and largest c2 craters were superposed
on the intercrater plains. This period was followed by impact of the
asteroid that created the Caloris Basin and deposits of the Caloris
Group. Although the intermediate plains (unit psi) were resurfaced at
the time of the Caloris impact, their formation actually extended from
the end of the intercrater plains-forming period through the end of
formation of c3 craters. At about the time the last c3 craters and the
first c4 craters were being formed, the upper surface of the smooth
plains (unit ps) and Caloris floor plains (unit cfp) were being
emplaced. Part of the smooth plains and Caloris floor plains materials
may have been deposited during or immediately after the Caloris event.

After the emplacement of most of the smooth plains, some late c3 craters
and all c4 and c5 craters, including the large crater Mozart, were
superposed on all previous deposits. The recognizable geologic history
of the quadrangle ends with these events probably several billion years
ago A summary of the generalized geology history of Mercury was given by
Guest and O'Donnell (1977) and Davies and others (1978).

**REFERENCES**

Davies, M. E., Dwornik, S. E., Gault, D. E., and Strom, R. G., 1978,
Atlas of Mercury: National Aeronautics and Space Administration Special
Publication SP-423, 128 p.

Gault, D. E., 1970, Saturation and equilibrium conditions for impact
cratering on the lunar surface: Criteria and implications: Radio
Science, v. 5, no. 2, p. 273--291.

Gault, D. E., Guest, J. E., Murray, J. B., Dzurisin, Daniel, and Malin,
M. C., 1975, Some comparisons of impact craters on Mercury and the Moon:
Journal of Geophysical Research, v. 80, no. 17, p. 2444--2460.

Guest, J. E., and O'Donnell, W. P., 1977, Surface history of Mercury: A
review: Vistas in Astronomy, v. 20, p. 273--300.

Hackman, R. J., 1966, Geologic map of the Montes Apenninus region of the
Moon: U.S. Geological Survey Miscellaneous Investigations Series Map
I-463, scale 1:1,000,000.

Hapke, Bruce, Danielson, G. E., Jr., Klaasen, Kenneth, and Wilson,
Lionel, 1975, Photometric observations of Mercury from Mariner 10, 1975:
Journal of Geophysical Research v. 80, no. 17, p. 2431--2443.

McCauley, J. F., 1977, Orientale and Caloris: Physics of the Earth and
Planetary Interiors, v. 15, no. 2--3, p. 220--250.

McCauley, J. F., Guest, J. E., Schaber, G. G., Trask, N. J., and
Greeley, Ronald, 1980, Stratigraphy of the Caloris Basin, Mercury:
Icarus, (in press).

Malin, M. C., 1976, Observations of intercrater plains on Mercury:
Geophysical Research Letter, v. 3, no. 10, p. 581--584.

Melosh, H. J., 1977, Global tectonics of a despun planet: Icarus, v. 31,
p. 221--243.

Melosh, H. J., and Dzurisin, Daniel, 1978, Mercurian global tectonics: A
consequence of tidal despinning?: Icarus, v. 35, p 227--236.

Schaber, G. G., Boyce, J. M., and Trask, N. J., 1977, Moon-Mercury:
Large impact structures, isostasy, and average crustal viscosity:
Physics of the Earth and Planetary Interiors, v. 15, no. 2--3, p.
189--201.

Schultz, P. H., 1977, Endogenic modification of impact craters on
Mercury: Physics of the Earth and Planetary Interiors, v. 15, no. 2--3,
p. 202--219.

Scott, D. H., McCauley, J. F., and West, M. N., 1977, Geologic map of
the west side of the Moon: U.S. Geological Survey Miscellaneous
Investigations Series Map I-1034, scale 1:5,000,000.

Shoemaker, E. M., and Hackman, R. J., 1962, Stratigraphic basis for a
lunar time scale, *in* Kopal, Zdenek, and Mikhailov, Z. K., eds., The
Moon: International Astronomical Union Symposium, 14th, Leningrad,
U.S.S.R., 1960: London, Academic Press, p. 289--300.

Strom, R. G., 1977, Origin and relative age of lunar and mercurian
intercrater plains: Physics of the Earth and Planetary Interiors, v. 15,
no. 2--3, p. 156--172.

Strom, R. G., Trask, N. J., and Guest, J. E., 1975, Tectonism and
volcanism on Mercury: Journal of Geophysical Research, v. 80, no. 17, p.
2478--2507.

Trask, N. J., and Guest, J. E., 1975, Preliminary geologic terrain map
of Mercury: Journal of Geophysical Research, v. 80, no. 17, p.
2461--2477.

**NOTES ON BASE**

This map sheet is one of a series covering that part of the surface of
Mercury that was illuminated during the Mariner 10 encounters (Davies
and Batson, 1975). The source of map data was the Mariner 10 television
experiment (Murray, 1975).

ADOPTED FIGURE

The map projections are based on a sphere with a radius of 2439 km.

PROJECTION

The Mercator projection is used for this sheet, with a scale of
1:5,000,000 at the equator. Latitudes are based on the assumption that
the spin axis of Mercury is perpendicular to the plane of the orbit.
Longitudes are positive westwards in accordance with the usage of the
International Astronomical Union (IAU, 1971). Meridians are numbered so
that a reference crater named Hun Kal (lat --0.6°) is centered on long
20° (Murray and others, 1974; Davies and Batson, 1975).

CONTROL

Planimetric control is provided by photogrammetric triangulation using
Mariner 10 pictures (Davies and Batson, 1975). Discrepancies between
images in the base mosaic and computed control point positions appear to
be less than 10 km.

MAPPING TECHNIQUES

Mapping techniques are similar to those described by Batson (1973a,
1973b). A mosaic was made with the pictures that had been digitally
transformed to the Mercator projection. Shaded relief was copied from
the mosaics and portrayed with uniform illumination with the sun to the
west. Many Mariner 10 pictures besides those in the base mosaic were
examined to improve the portrayal. The shading is not generalized, and
may be interpreted with nearly photographic reliability (Inge, 1972;
Inge and Bridges, 1976).

Shaded relief analysis and representation were made by Patricia M.
Bridges.

NOMEMCLATURE

All names on this sheet are approved by the International Astronomical
Union (IAU, 1977).

H-8: Abbreviation for Mercury (Hermes) sheet number 8.

H 5M 0/180 G: Abbreviation for Mercury (Hermes) 1:5,000,000 series;
center of sheet, lat 0°, long 180°; geologic map, G.

REFERENCES

Batson, R. M., 1973a, Cartographic products from the Mariner 9 mission:
Journal of Geophysical Research, v. 78, no. 20, p. 4424--4435.

\_\_\_\_\_1973b, Television cartography: U. S. Geological Survey
open-file report, no. 1960, 35 p.

Davies, M. E., and Batson, R. M., 1975, Surface coordinates and
cartography of Mercury: Journal of Geophysical Research, v. 80, no. 17,
p. 2417--2430.

Inge, J. L., 1972, Principles of lunar illustration: Aeronautical Chart
and Information Center Reference Publication RP-72-1, 60 p.

Inge, J. L., and Bridges, P. M., 1976, Applied photointerpretation for
airbrush cartography: Photogrammetric Engineering and Remote Sensing, v.
42, no. 6, p. 749--760.

International Astronomical Union, Commission 16, 1971, Physical study of
planets and satellites, *in* Proceedings 14th General Assembly 1970:
International Astronomical Union Transactions, v. 14B, p. 105--108.

\_\_\_\_\_1977, Physical study of planets and satellites, *in*
Proceedings 16th General Assembly 1976, International Astronomical Union
Transactions, v. 16B, p. 325, 331--336, 355--362.

Murray, B. C., 1975, The Mariner 10 pictures of Mercury: An overview:
Journal of Geophysical Research, v. 80, no. 17, p. 2342--2344.

Murray, B. C., Belton, M. J. S., Danielson, G. E., Davies, M. E., Gault,
D. E., Hapke, Bruce, O'Leary, Brian, Strom, R. G., Soumi, Verner, and
Trask, Newell, 1974, Mercury's surface: Preliminary description and
interpretation from Mariner 10 pictures: Science, v. 185, no. 4146, p.
169--179.