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The role of igneous sills in shaping the Martian uplandsRelations among geologic units and landforms suggest that igneous sills lie beneath much of the intercrater and intracrater terrain of the Martian uplands. The igneous rocks crop out along the upland-lowland front and in crater floors and other depressions that are low enough to intersect the sill's intrusion horizons. It is suggested that heat from the cooling sills melted some of the ice contained in overlying fragmental deposits, creating valley networks by subsurface flow of the meltwater. Terrains with undulatory, smooth surfaces and softened traces of valleys were created by more direct contact with the sills. Widespread subsidence following emplacement of the sills deformed both them and the nonvolcanic deposits that overlie them, accounting for the many structures that continue from ridged plains into the hilly uplands. Crater counts show that the deposit that became valleyed, softened, and ridged probably began to form (and to acquire interstitial ice) during or shortly after the Middle Noachian Epoch, and continued to form as late as the Early Hesperian Epoch. The upper layers of this deposit, many of the visible valleys, and the ridged plains and postulated sills all have similar Early Hesperian ages. Continued formation of valleys is indicated by their incision of fresh-appearing crater ejecta. The dependence of valley formation on internal processes implies that Mars did not necessarily have a dense early atmosphere or warm climate.
Document ID
19890049144
Document Type
Conference Paper
Authors
Wilhelms, D. E. (San Jose State University CA, United States)
Baldwin, R. J. (USGS Menlo Park, CA, United States)
Date Acquired
August 14, 2013
Publication Date
January 1, 1989
Subject Category
LUNAR AND PLANETARY EXPLORATION
Meeting Information
Lunar and Planetary Science Conference(Houston, TX)
Funding Number(s)
CONTRACT_GRANT: NAGW-1052
CONTRACT_GRANT: NASA ORDER W-15814
Distribution Limits
Public
Copyright
Other