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Unformatted text preview: ilt and clay that were eroded from higher areas
along an ancient continental margin and carried downslope by
density currents to deeper parts of the ocean. Sediments deposited
in this manner are called turbidites. Each turbidite may contain
five different layers and each layer is defined by sediment grain
size and sedimentary structures (fig. 3). The layers range from
several inches to more than ten feet thick. In New Jersey, turbidite
sequences generally contain only the upper three finer grained
layers (fig. 3, layers C, D, and
E) because the lower two were
eroded or deposited farther up
the continental slope.
The Martinsburg is divided
into the Bushkill, Ramseyburg,
and the High Point Members.
The Bushkill Member is the
oldest, consisting of about
C Sand1,500 feet of black shale that
was formed from clay deposited
in the deepest parts of the ocean
basin. Overlying the Bushkill
Member and younger than it
are the interlayered shales,
siltstones and minor sandstones
of the Ramseyburg Member. This
layering and overall increase
in grain size shows that these
sediments were laid down in a
Figure 3. This diagram portrays the
five parts of a classic turbidite. Not all slightly shallower ocean closer to
parts are present every time. The most their source. The Ramseyburg
common turbidite in this part of New ranges from 2,400 to 5,000 feet
Jersey lacks part A and sometimes B. in thickness in New Jersey and
The remaining parts, C, D, and E have a
is about 3,500 feet thick near the
combined thickness of several inches.
park. The High Point Member is
4 as much as 4,500 feet thick, and is the youngest and only member
in the park. It consists of alternating thin sandstone, siltstone,
and shale. The sandstone layers are typically coarse at their base
and finer-grained near their top, a feature called graded bedding.
These layers are as much as 6 feet thick and make up the coarsest
part of the turbidite sequence (fig. 3). Pebbles of shale in their base
show that the currents that carried them were powerful enough t...
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