|The Geology Page!|
|New Jersey, even for its comparatively small size, contains many different and complex geological regions. Four main geological regions are apparent in this state. They are the Ridge and Valley, the Highlands, the Piedmont, and the Coastal Plain. This last region is further subdivided into the Inner and Outer Coastal Plain. The formations that are exposed along the drainage basin of Big Brook are within the Inner Coastal Plain.
The Coastal Plain in New Jersey is a relatively flat area that can be considered to be the emerged portion of the former continental shelf of the Cretaceous and Tertiary Periods. This region is essentially the northern most extent of a larger region called the Atlantic Coastal Plain. This plain extends relatively uninterrupted from New Jersey down to Georgia and continues on westward as the Gulf Coastal Plain. Click here (pict. from Ref. 3) to see the shore line during the Late Cretaceous.
The Inner Coastal Plain is comprised of several formations that are made up of sands, silts, clays, and marls that were laid down mostly during the Late Cretaceous. These formations were deposited under many different types of conditions that varied from continental to marginal marine (estuary or lagoon) to full marine.
During the 40 million years of its formation, the Inner Coastal Plain experienced several oscillations of the rising (transgressions) and falling (regressions) of the sea level. This rising and falling cycle deposited the formations into three different shelf regions. These are the inner, middle, and outer shelf.
During the transgressive periods, the deep-water formations were deposited. These formations generally contain large amounts of a mineral called glauconite (a type of mica that forms in relatively deep water) and clay with some sand. During the regressive periods, the shallower formations were deposited which are characterized by large amounts of sand and mica flakes with some amounts of clay and feldspar.
Detailed studies of each formations' fauna in the Inner Coastal Plain have indicated that two main types of fauna occur in the two different types of environments. The two types of faunas are, for the most part, recurrent with the recurrent depositional environments. For example, mollusks that are found in shallow water are found in most of the shallow water formations. The shallow water fauna is called the Lucina fauna (named for a relatively uncommon bivalve characteristic of shallow water formations in New Jersey). The deep-water fauna is called the Cucullaea fauna (named for a fairly common bivalve genus characteristic of deep-water formations in New Jersey). The formations that were deposited under middle shelf conditions contain some mixture of these two faunas. It should be noted that some authors disagree with these alterations of faunas and point out that some deep-water elements occur in the relatively shallow environments and vise-versa.
|Three main formations occur along the Big Brook drainage basin (See "Formations of Big Brook" below). They are called, from oldest to youngest: the Mount Laurel, the Navesink, and the Red Bank. Additionally, there are Pleistocene deposits (the Cape May equivalent) that sparsely occur disconformably overlying the Cretaceous sediments. These deposits are minor and are not a continuous series. Also the Marshalltown is reported by some sources as occurring at Big Brook but, if this is true, it is exposed at its' extreme headwaters and would not be sufficiently exposed to merit discussion. Further, no distinctively Marshalltown fauna has been reported from this locale.
The Mount Laurel formation is made up of a dark to light massive to interbedded micaceous sand and clay. This formation may contain pebbles and limonite concretions (fossiliferous and nonfossiliferous) in certain layers. The presence of the mineral mica denotes some parts of this formation might have been deposited under middle to inner shelf conditions. When the Mt. Laurel was being deposited the sea was becoming shallower (regressive) and then deeper (transgressive) and would reach maximum depth (in this formation series) in the Navesink. The Mt. Laurel displays considerable diversity and variation in lithology (what makes up the formation and the way it's laid down). Much confusion has also surrounded the identity of this formation and its' characteristics in northern Monmouth County. It has been called the Winona, the Mt. Laurel and, finally, the Mt. Laurel / Winona undifferentiated. The authors propose to use the Mt. Laurel name for ease in description. It should be noted that even though the Mt. Laurel is used here the most accurate description would be the Mt. Laurel / Winona undifferentiated owing to the unique nature of the sediment in northern Monmouth County. It has been said to contain elements of both deep and shallow water faunas possibly due to the fact that some parts of the Mt. Laurel were deposited under middle shelf conditions and some under shallow water conditions.
The latest information seems to indicate that in northern Monmouth County the Mt. Laurel is somewhat different from the "classic" Mt. Laurel in more southerly exposures. The "classic" Mt. Laurel is lighter in color and contains more coarsely textured sand and less mica than its' Big Brook counterpart. Furthermore, the more southerly exposures contain prominent shell beds with fauna almost identical to that of the overlying Navesink. In fact further to the south in extreme southern New Jersey and Delaware, the Navesink and the Mt. Laurel are undifferentiatable and constitute one single formation. At Big Brook the Mt. Laurel is more similar to the Winona (the underlying formation in more southerly exposures) mostly in fauna. The Winona formation contains an assemblage of fossils somewhat like the "classic" Mt. Laurel and very unlike the Navesink. The Winona contains the shallow water Lucina fauna. At Big Brook the Mt. Laurel contains a Lucina fauna with some elements of the deep-water assemblage (Cucullaea fauna) near the top of the sequence. Also the major shell beds that are so prevalent in more southerly exposures are missing but are replaced by invertebrate layers that chiefly consist of soft molds and casts. All in all, one can see why so much confusion has surrounded these enigmatic surface exposures of the Mt. Laurel.
|The Mt. Laurel is very fossiliferous throughout its' 16 meters (about 50 feet) and contains most of the vertebrate fossils found at Big Brook. There is a distinctive invertebrate layer just below the Mt. Laurel / Navesink contact. The shell fossils are represented mostly by soft molds and casts that are difficult to remove intact and later preserve. There are, however, large concretions that locally form in this layer and preserve the delicate shells and other fossils that normally would not have made it into the fossil record. In addition to the invertebrate layers, there are several layers in the formation where the burrows from lobsters and ghost shrimps are preserved. The vertebrates, mostly shark and fish teeth, are found all throughout the formation but are concentrated in a few vertebrate layers (Bone Beds). Since the Mt. Laurel was closer to the shore, wood is frequently encountered but uncommonly in large pieces. Most of the dark matter seen in the formation is fossilized plant material. This closeness to the shore also preserved land animals, such as dinosaurs, and aquatic animals, such as turtles (Trionyx). There is an uncomfortable contact with the overlying Navesink formation and contains numerous pebbles and small concretions. Some mica may run over the contact for about a meter into the Navesink.|
|The Navesink formation is a thick greenish-grey sandy clay and is much more homogenous than the underlying Mt. Laurel. This formation gets its' greenish coloring from the mineral glauconite, which serves to identify it as being deposited under deep middle to outer shelf conditions. The Navesink is considered to be an outer shelf deposit based on fossil studies, as well as lithology and other factors. Some indicators point to a maximum water environment (about 2.6 meters above the Mt. Laurel / Navesink contact) at about 180 meters (about 594 feet) deep. This formation reaches a maximum thickness of 8 meters (about 26 feet) thick at Big Brook.
The Navesink is a very fossiliferous formation but mostly for invertebrates of the Cucullaea fauna. Fossils can occur anywhere in the formation but are concentrated into three main shell beds and one minor bed. The main shell beds, or "zones", can clearly be seen in weathered formation bank surfaces. These "zones" are called from lowest to highest: the Belemnitella "zone", the Exogyra "zone", the minor Agerostera / Choristothyris "zone", and the Pycnodonte "zone".
The Belemnitella "zone" is about 1.8 meters (about 6 feet) above stream level at the Hillsdale Road bridge. This "zone" mostly contains Belemnitella guards with some Exogyra and Pycnodonte shells as well as other minor members of the Cucullaea fauna. Interestingly, this layer seems to contain larger guards and smaller Exogyra and Pycnodonte shells than any other "zone". In fact, through the next two "zones" the size of the Exogyra and Pycnodonte shells gradually increase while the size of the Belemnitella guards generally decrease. This may indicate that the Belemnitella "zone" was deposited in the deepest water of the Navesink where the oxygen levels were low and kept most bivalves living on the seabed smaller in size (environmentally stunted). Belemnitella was more cosmopolitan and stayed close to the surface and far out to sea and thus would not have been affected by the low oxygen levels that the bottom organisms would have.
Some paleontologists have noted that some of the Belemnitella guards or, as the local people like to call them, "bullets" have a preferred orientation in situ. As the long and pointed guards fell to the sandy bottom, they would align themselves to the prevailing currents along the sea bottom perpendicular to the shore thus pointing the way to the ancient shore. This will only be noted for specimens that are collected in situ.
The Exogyra "zone" is about 5.5 meters (about 18 feet) above stream level at the Hillsdale Road bridge. Interestingly, in this layer most of the articulated larger Exogyra shells are found in situ with the smaller flattened valve down. Most of its' life, Exogyra laid on the sea floor with the convex half buried in the sand. When large storm events occurred, the shells would be uprooted and turned onto the more stable flattened valve. Smaller individuals could probably right themselves but the weight of the larger shells was probably too much for them and they died just as found.
|The minor Agerostera / Choristothyris "zone" is about 5.8 meters (about 19 feet) above stream level at the Hillsdale Road bridge. Most of the specimens of Agerostera / Choristothyris come from this "zone". This layer also contains molds and casts of bivalves and gastropods.
The Pycnodonte "zone" is about 6 meters (about 20 feet) above stream level at the Hillsdale Road bridge. This layer also contains larger Exogyra shells but fewer in numbers. This "zone" is also noted by the high concentration of shells drilled with holes by the boring sponge Cliona. A large majority of the shells are so bored with holes that it is probably the cause of their deaths.
Vertebrates are rare from the deeper waters of the Navesink formation except at the layer just above the Mt. Laurel / Navesink contact.
The last major formation exposed at Big Brook is the Red Bank. The Red Bank is comprised of two easily differentiated members, the lower Sandy Hook member and the upper Shrewsbury member.
The lower Sandy Hook member is a moderately fossiliferous dark grey micaceous silty feldspathic quartz sand. The contact with the underlying Navesink is gradational and contains some glauconite. The two can be segregated by the presence of mica in the Red Bank. The Sandy Hook member is interpreted as being deposited in the middle to inner shelf conditions as a slow regression of the sea started in the upper half of the Navesink. This member contains a diminutive number of elements of the recurring Lucina fauna. These fossils are usually smaller and in poorer condition than seen at other Lucina fauna formations along the Inner Coastal Plain. At Big Brook though, this member is not very fossiliferous and the fossils that are found are very fragmentary. Not more than 3 meters (about 10 feet) of the Sandy Hook member is apparent.
The upper Shrewsbury member is considerably thicker and much more distinct. This member is gradational from the lower Sandy Hook member and is a cross-bedded light orange to reddish brown micaceous fine to medium sized feldspathic quartz sand. This member is largely unfossiliferous but does contain some isolated small pockets of fossils.
Though no such pockets have been found recently at Big Brook, some adjacent exposures about one mile north have curiously yielded several specimens of Exogyra costata that have been completely replaced by limonite and/or siderite. This member seems to indicate a continued regression of the Late Cretaceous sea. Approximately 6 meters (about 20 feet) of the 36 meters (about 120 feet) of the Shrewsbury member is exposed along Big Brook.
|Overlying the Late Cretaceous marine sediments at Big Brook are some scanty occurrences of Pleistocene deposits. Most of the deposits are from the fluvial (sand and gravel deposited by the out-wash from the terminal moraine of a glacier) equivalent of the Cape May formation which was deposited during the last interglacial period before the Wisconsin glacial period. Paleontologists believed it to be deposited during the last encroachment of the Wisconsin ice sheet but this has been disproved. Fully one half of all reported Ice Age mammals in New Jersey have been recovered from these deposits at Big Brook and other area streams. Some of the mammals, such as the giant ground sloth and the giant beaver, were probably from warmer interglacial periods and have been washed out from older Pleistocene deposits. The cold loving animals, such as the Mastodon and the Caribou, have been presumably washed out from the fluvial equivalent of the Cape May formation. These deposits, though scanty and diminutive, have provided much information to paleontologists about the life in New Jersey during this last great episode at Big Brook.|
|The Geology of Big Brook!|
|Cape May (equivalent) - Fluvial deposition. Up to two meters thick. Not always present. Cross-bedded light to dark sand and gravel. May sit on to top of the Red Bank, Navesink, or the Mt. Laurel. May have some Ice Age mammal fossils, as well as fossils that have been reworked from the other formations.|
|Mount Laurel - Inner to middle shelf deposition. This formation is very fossiliferous in certain layers and most of the vertebrate fossils found at Big Brook are from this formation. Four different lithologies present.
Bioturbated pebble sands and micaceous clays containing
abundant glauconite filled organic burrows. Molds and
casts of various invertebrates and some vertebrate teeth
and bones occur near the contact.
Massive light colored sand bed; some horizons locally
contain empty casts of Belemnitella.
Thinly bedded dark clays and light cross-bedded sands
interbedded with massive light colored sands
Massive, gray, poorly sorted clays and silty micaceous
sands. Localized phosphaic bone beds.
|Red Bank (Shrewsbury member) - Inner shelf deposition. Cross-bedded orange to reddish-brown, micaceous, fine to medium feldspathic quartz sand and may contain large amounts of limonite/siderite. Generally unfossiliferous.
(Sandy Hook member) - Middle to inner shelf deposition. Moderately fossiliferous, dark gray, micaceous, silty, feldspathic quartz sand.
|Navesink - Middle to outer shelf deposition. Thick greenish-gray, clayey glauconitic sand. The most fossiliferous formation at Big Brook. It contains mostly invertebrates in a least 3 main shell beds. These shell beds contain mostly original shell with some molds and casts. Vertebrates are rare but sometimes occur at the base of this formation.|
|Scale: 1 cm to 1.5 meters|
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|Aerial View Of Where Formations Outcrop Along Big Brook|