VEIN FORMATION IN NON-MARINE MUDSTONES OF THE
SIMONSON, Bruce M., Geology Dept., Oberlin College,
Oberlin, OH 44074
Mudstones are the dominant lithology in the non-marine sedimentary
fill of the Newark basin. Most of these mudstones were deposited in
paleoenvironments that were at least intermittently subaerial and
contained depositional macroporosity in the form of air bubbles
(vesicles), desiccation cracks and (or) root tubules. Macropores
typically disappear from marine mudstones during compaction, but
those of the Newark mudstones are still there, albeit filled with
coarsely crystalline authigenic minerals (cements). The principal
cement phases (analcime polyhedra, albite laths, blocky K-feldspar
crystals, dolomite rhombs, and anhedral crystals of calcite, gypsum,
anhydrite and/or barytocelesite) are organized into three paragenetic
sequences that occur in successive and mutually exclusive
stratigraphic zone that are each ca. 5,000' (1.5 km) thick (Simonson
and Smoot, 1995). Van Houten (1962) first described the cements
and interpreted them as syndepositional precipitates from surface to
near-surface saline lake waters, but 40Ar/39Ar age dates of the
authigenic K-feldspars indicate closure ca. 20 million years after the
host mudstones were deposited (Kunk et al., 1995). Given the well-
constrained stratigraphy, this implies reactive fluids continued were
circulating at depths of ca. 6,500' (2 km) in the Newark Basin.
Veins filled with mineral assemblages similar to those filling
the depositional macropores are widespread in research cores drilled
in the Newark basin (Olsen et al., 1996). The veins range up to at
least 4 cm in width, are mostly either bedding-parallel or bedding-
normal, and vary in character. Many veins are fibrous and display
crack-seal textures indicating incremental opening and filling
(Ramsey, 1980), whereas others are coarsely crystalline and display
competitive-growth textures indicative of open void-filling
precipitation. Still others are banded, indicating multiple stages of
filling. In addition, previously precipitated crystals (e.g., coarse
euhedra of analcime) were fractured after initial growth, creating
room for additional mineral precipitation in some veins. Finally,
veins in the lower part of the mudstone succession (where the organic-
rich deep lake deposits are thickest and most abundant) contain
bitumen that was also fractured after emplacement.
The existence and diverse character of the veins in the Newark
mudstones has important implications. 1) Many features of the veins
indicate they represent fractures that were reopened and filled
repeatedly. This implies a long and complex history of fluid flow
extending into late diagenesis. 2) The fact that the cements in the
veins and the depositional macropores in the host mudstones have
similar parageneses suggests they were made possible by the late
diagenetic creation of fracture permeability. The cause(s) of
fracturing and the driving force for fluid flow have yet to be
pinpointed, but will be discussed. The apparent contradiction between
large-scale, late-diagenetic fluid flow and the stratabound character of
the vein fillings has likewise yet to be resolved.
Kunk, M.J., Simonson, B.M., and Smoot, J.P., 1995, 40Ar/39Ar
constraints on the age of K-feldspar cementation in non-marine
sediments of the Newark, Gettysburg, and Culpeper basins:
Geological Society of America Abstracts with Program, v. 27, n.
1, p. 62.
Olsen, P.E, Kent, D.V., Cornet, B., Witte, W.K., and Schlische, R.W.,
1996, High resolution stratigraphy of the Newark rift basin
(early Mesozoic, eastern North America): Geological Society of
America Bulletin, v. 108, p. 40-77.
Ramsey, J.G., 1980, The crack-seal mechanism of rock deformation:
Nature, v. 284, p. 135-139.
Simonson, B.M., and Smoot, J.P., 1994, Distribution and origin of
macropore-filling cements in non-marine mudstones, Early
Mesozoic Newark Basin, New Jersey And Pennsylvania:
Geological Society of America Abstracts with Program, v. 26, p.
Van Houten, F.B., 1962, Cyclic sedimentation and the origin of
analcime-rich Upper Triassic Lockatong Formation, west-central
New Jersey and adjacent Pennsylvania: American Journal of
Science, v. 260, p. 561-576.
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