News from the Geoblogosphere
New from Snet:
, a new tool to create lithological/sedimentological logs online..
Blog post recommendation
Big Brook - New Jersey's Classic Late Cretaceous, Fossil-Collecting Locality
"At first it may seem to be a piddly little dribble through the farmlands and forests of rural New Jersey, but careful observation shows Monmouth County's Big Brook
to be a glass-bottomed boat sailing through a Late Cretaceous sea busy with life.” From the
New York Paleontological Society Field Guide
Big Brook at somewhat high water as seen from the Hillsdale Road bridge facing east
A WINDOW INTO A LATE CRETACEOUS CONTINENTAL SHELF
Step into the unassuming waters of this very ordinary-looking brook, and you’ll go back in time to North America’s continental shelf only a few million years before the Great Extinction that ended the Age of the Dinosaurs. In its lazy and short course to the Atlantic, Big Brook has carved a shallow, curvy trough through New Jersey's Inner Coastal Plain through the upper sediments of a Late Cretaceous paleoshelf. In so doing, weathering of the streambank provides a steady supply of fossils that are washed into the streambed.
Big Brook is one of the East Coast’s classic fossil-collecting localities with both amateurs and professionals alike. As early as 1863, the Smithsonian Institute in New York sent an expedition to explore and gather fossils from the brook and others within Monmouth County. Less widely appreciated amongst amateurs is that the strata through which Big Brook transects preserves an outstanding sedimentological record of the transition from an inner to an outer shelf environment during the Late Cretaceous as sea level rose.
PALEONTOLOGICAL GRAB BAG
Big Brook is an eclectic fossiliferous grab bag. Although rare, be on the lookout for fragmented, water-worn dinosaur bones and teeth - hadrosaurs, theropods, ankylosaurs and ornithomimids - from the Late Cretaceous terrestrial shoreline to the west. Add to the mix, Pleistocene mammalian remains of mastodon, sloth, beaver and horse. From the Holocene, there's even an occasional Native American Lenni-Lenape arrowhead from the surrounding countryside and a coral from Paleozoic tropical seas entrapped within the Appalachian orogen that was transported to the area fluvially or via glacial outwash.
A Late Cretaceous terrestrial fauna similar in some respects to eastern North America
From National Geographic
But by far, the big attraction is teeth and some jaw and vertebral remains from Late Cretaceous osteichthyans (fish), chondrichthyans (shark, rays and skates) and, less often, large marine reptiles (mosasaurs, turtles and plesiosaurs).
The Late Cretaceous marine ecosystem teemed with life.
In addition, abundant macro-invertebrate remains include brachiopods, bryozoans and molluscs (bivalves such as oysters, gastropod snails, belemnites and ammonites) and disarticulated arthropod carapaces (lobster, crab and shrimp) - all from the Late Cretaceous shelf ecosystem.
The shelf's benthic and demersal zone was rich and diverse with brachiopods, bryozoans, molluscs and arthropods.
From Matthew McCullough on Flickr and license here.
WHERE ARE WE?
Big Brook is barely 50 miles south of New York City via the Garden State Parkway off exit 109 west. The brook winds its way through the rural New Jersey hamlets of Colts Neck and Marlboro to the Navesink River near the borough of Red Bank, and ultimately the Atlantic Ocean. Besides Big Brook, other nearby fossil-bearing tributaries include Poricy Brook in Middletown Township, Ramanessin Brook in Holmdel and Shark River (Eocene and Miocene) in Neptune and Wall Townships.
They're all located in gentrified and well-healed Monmouth County, which is in the top 1.2% of counties by wealth in the United States. The County's website advertises itself as the "Gateway to the Jersey Shore", while locals know it as Springsteen country (“…sprung from cages on highway nine…”).
The arrow points to the location of Big Brook within Monmouth County.
Roadside Geology of New Jersey
BIG PICTURE TECTONIC STUFF
How did the landform through which Big Brook flows originate? Some sixty-seven million years ago, this lazy, oak-shaded stream and the surrounding countryside were a tiny submerged section of the newly-formed, Atlantic continental shelf. During the Late Cretaceous, global high seas drowned the shelf that now represents the broad, low relief of the Atlantic Coastal Plain through which Big Brook flows. The modern shoreline is 10 miles due east of Big Brook. But the geological story of Big Brook begins well before the Cretaceous.
Paleozoic tectonic convergence...
Beginning in the early Paleozoic, Laurentia - the rifted megacontinental sibling of the Late Proterozoic supercontinent of Rodinia - was converged upon by a procession of magmatic arcs, micro-continents and megacontinents and their intervening ocean basins. In what is referred to as a supercontinental cycle, a fragmented Rodinia was beginning to reassemble albeit repackaged in a new configuration called Pangaea.
Birth of Pangaea...
In a parade of orogenic events, the landmasses and ocean basins accreted to Laurentia's ever-growing ancestral core - building mountains and adding crust with each collision. By the Pennsylvanian Period of the Paleozoic (below), their cumulative convergence had constructed the supercontinent of Pangaea and built a massive, centrally-located mountainous spine. By the Late Permian at the close of the Paleozoic, the mountains were ravaged by erosion.
Subsequent to the collision of Gondwana (the other megacontinental sibling of Rodinia) with equatorially-positioned Laurentia in the Middle Pennsylvanian, Monmouth County is nestled somewhere within the lofty peaks of the Appalachians. The supercontinent of Pangaea is fully formed and awaits its imminent fragmentation.
Demise of Pangaea...
In the Late Triassic, as did Rodinia in the Late Proterozoic, Pangaea’s fragmentation began. As the Atlantic Ocean began to form within the schism, the worn remnants of Pangaea's central mountainous spine began to fragment as well. A portion remained astride the Atlantic Coast in eastern North America - today's Appalachians - while other remnants were carried across the globe on the backs of Pangaea's rifted siblings. Pangaea's breakup also endowed North America (and of course New Jersey) with a new passive shoreline characterized by seismic and volcanic inactivity, and most importantly, subsidence and sedimentation.
Marginal subsidence and sedimentation of the Atlantic margin...
Beginning probably during Jurassic time, lithospheric cooling of North America's newly-forming, passive margin - in concert with the weight of voluminous sedimentation -
promoted rapid subsidence and provided a vast accommodation space for the accumulation of clastic erosive products.
With subsidence, the Atlantic margin was broken into a series of faulted-blocks, which experienced differential movements. Downward movement created embayments - deep indentations of the ancient shoreline in which sediments accumulated in greater thicknesses in greater water depths. Upward movement created structural highs, arches or uplifts - with thinner sequences and even the absence of deposition. One such coastal indentation - the
influenced sedimentation in New Jersey between Staten Island at the western end of Long Island and Jersey's northern Coastal Plain.
Map showing the outline of the Atlantic Coastal Plain and major structural elements that persist on North America's modern coast line. In particular, the Raritan Embayment between is encircled.
Summary of Lithostratigraphy and Biostratigraphy of the Atlantic Coast
Formation of the Atlantic Coastal Plain...
Concomitant with crustal cooling and subsiding, deposition in the coastal plain began in earnest in the Early Cretaceous with fluvial sedimentation from the highlands of the Appalachians. However, in the Late Cretaceous (below), marine incursions representative of global high seas flooded low-lying regions of the world that included the newly-formed, low-lying Atlantic coast.
Progressing from the shoreline seaward, gravel and sand on the inner continental shelf gave way to silt and clay, and, in progressively deeper water, glauconitic sand and silt. The deposits record a progressive but discontinuous and fluctuating rise in sea level - perhaps four in Late Cretaceous time and three in the Cenozoic. Thus, the landform of the Atlantic Coastal Plain gradually developed, representative of some 150 million years of sedimentation.
With Pangaea fragmented apart, the Late Cretaceous witnessed the initiation of the development of the ecosystem of Monmouth County (red dot) on the submerged Atlantic Coastal Plain (light blue). The Atlantic Ocean has opened between the north and south, and is actively spreading. Note the Mid-Atlantic spreading center (light blue) along the line of tectonic divergence. The Western Interior Seaway in central North America is about to become confluent between the waters of the Arctic and the Gulf of Mexico.
A colorful Tapestry of Terrain and Time...
The Paleozoic collisions that assembled North America formed geomorphic provinces that are seen in the colorful mosaic of the Tapestry of Terrain and Time map by the USGS here. The yellow and tan Cretaceous and Cenozoic deposits of New Jersey (within the ellipse) illustrates the extent of the Coastal Plain within the state, which is continuous with that of the entire Atlantic and Gulf Coasts from Cape Cod and Long Island, through northern Jersey at Sandy Hook to southern New Jersey at Cape May, and down to Florida and around to the Gulf of Mexico. Let's take a closer look at the geomorphic provinces of New Jersey.
Northeastern and Mid-Atlantic North America from the USGS Tapestry of Time bedrock map
New Jersey is encircled. The main website and map are found here.
NEW JERSEY’S GEOMORPHIC PROVINCES
For an area its size, New Jersey has a diverse geological history. From west to east, from the mountains to the sea and across the orogens that formed eastern North America, New Jersey’s main geological subdivisions or provinces are the
Valley and Ridge
(equivalent to the familiar Blue Ridge down south), the
By definition, the four geomorphic or physiographic regions are each unique as to relief, landforms and geology. They all were inherited subsequent to the tectonic collisions that occurred throughout the Paleozoic, hence they're on strike from northeast to southwest in accordance with the direction of tectonic convergence. A fifth smaller province - in accordance with tectonic divergence - is the
(green and red), which lies interposed within the Piedmont. It is a sediment-filled rift-basin, one of many along the east coast that formed during the initial stages of Atlantic opening in the Late Triassic and Jurassic.
Geologic Bedrock Map and Physiographic Provinces of New Jersey
The region of Big Brook within the Atlantic's Inner Coastal Plain is located at the red dot.
Modified from the Department of Environmental Protection, Division of Science, Geological Survey, 1999
THE COASTAL PLAIN
The Coastal Plain Province is relatively featureless, save a few gently undulating hills, and overlaps the rocks of the Piedmont Province. It covers the entire lower half of New Jersey (see above map), dipping seaward from 10 to 60 feet per mile to the the southeast and extending beneath the Atlantic Ocean to the edge of the Continental Shelf at the Baltimore Canyon Trough. Its unconsolidated and compacted (but not cemented) sediments range in age from the Cretaceous to the Miocene. The composition of its bedrock and fossils confirms that it was submerged by Late Cretaceous high seas.
This west to east cross-section through the modern Coastal Plain and continental shelf illustrates the increasingly deep seaward-facing wedge of sediments that extends from a feather edge at the Fall Line of the Piedmont to the sea, where it's over a mile thick.
From the USGS and the
Roadside Geology of New Jersey
THE INNER AND OUTER REGIONS OF THE COASTAL PLAIN
The Coastal Plain is further subdivided into two regions. Because it was uplifted, weathered and dissected, the
Inner Coastal Plain
is higher in altitude than the Outer Plain but not by much. Its composition is largely a mix of quartz sand, glauconitic sand, silt and clay. This fertile agricultural zone gave rise to New Jersey’s nickname as the Garden State. It's also the location of Big Brook within Monmouth County. The
Plain is a region of lower altitude where low-relief terraces are bounded by subtle erosional scarps. It consists of Tertiary and Quaternary sand, and being acidic and less fertile, is the location of Jersey’s heavily forested Pine barrens.
THE MONMOUTH GROUP
As mentioned, in the Early Cretaceous the Coastal Plain region of New Jersey received deltaic and floodplain-derived (non-marine) sediments from the Appalachian highlands to the west. In the Late Cretaceous and into the early Paleocene, sea levels rose and flooded the coastal region in a series of transgressions over land and regressions back again. Layer after layer, sequence after sequence (packages of strata deposited during a single cycle of sea level rise and fall), sediments of the sea were laid down beginning with the earliest Late Cretaceous Raritan Bass River Formation upon the latest Early Cretaceous fluvial Potomac Formation (chart below).
In the late Late Cretaceous - from the Campanian into the Maastrichtian - the
, the state's youngest Cretaceous package, was laid down - a unit that is compacted but unlithified. New Jersey's Monmouth Group includes the basal
sand (5-60 feet thick), the transgressive marl of the
(25-60 feet thick), the regressive
silt-sand (thin film to 100 feet thick) and
coarse quartzose and glaucontic sand (20-40 feet thick).
As the sea transgressed and regressed, shorelines moved accordingly. Existing sediments were eroded, reworked and redeposited, leaving behind unconformities. Breaks between sequences were punctuated by lag deposits or "shell beds." The great majority of fossils at Big Brook such as within the Navesink Formation, which we will visit, were eroded from lag deposits and released into the streambed. They were deposited within the neritic zone - the relatively shallow waters of the ocean from the littoral zone (closest to the shore) to the drop-off at the edge of the continental shelf.
STRATIGRAPHY OF BIG BROOK
Big Brook's journey to the sea, has excavated a channel into the Inner Coastal Plain. It cut through the
Red Bank Formation's Sandy Hook Member (Krsh)
, through the
Navesink Formation (Kns)
Mount Laurel Formation (Kml)
, and in some areas, into the
Wenonah Formation (Kw)
- all deposits of the vacillating Late Cretaceous sea. You can find the complete map of the Freehold and Marlboro Quadrangles here. For orientation, the red arrow marks the location of the car park on the north side of the Hillsdale Road bridge.
This map depicts the channel of Big Brook. The red arrow points to the car park on the north side of the Hillsdale Road bridge.
Modified from the Bedrock Geologic Map of the Freehold and Marlboro Quadrangles, New Jersey, 1996.
TWO PORTALS TO BIG BROOK
Two unmarked bridges are your portals to Big Brook- one on Boundary Road and the other on neighboring Hillsdale Road just east. On the north side of the Hillsdale Road bridge is a designated car park, while parking is on the street just south of the Boundary Road bridge (as of this writing). The photographs taken and fossils displayed in this post were observed and collected from four visits to Big Brook on the east side of the Hillsdale Road Bridge.
The countryside through which Big Brook flows is peppered with residential settlements, horse farms and wineries that are either private or post no trespassing. Big Brook's banks are private as well and off limits to excavation. They're dangerous too, since they are unconsolidated and slump and collapse with little provocation, especially by overzealous excavators. But the streambed is fair game. The only caveat is that on a nice summer day you may have to share it with a paleontologist, a geologist, a scout troop and a fossil club - all after the same piece of time.
The bucolic Hillsdale Road bridge over Big Brook facing south
YOUR FOSSIL-FORAGING ARMAMENTARIUM
To extract the fossil bounty at Big Brook Preserve, no geological hammers and chisels are necessary owing to the unconsolidated nature of the bedrock. In fact, they're prohibited by a posted Colts Neck Township ordinance in order to preserve the resource and minimize over-collecting. All you'll need are a pair of Wellies or suitable waders (there's some broken glass so don't go barefoot), and equip yourself with a garden trowel (with a maximum blade length of 6") and a small, homemade sifting-screen (no greater than 18” x 18”). I borrowed a kitchen colander from home.
OFFICIAL RULES AND POLICIES
The Department of Recreation and Parks of the Township of Colts Neck has determined that "there is an increasing need for the preservation of the many natural resources located within Big Brook Preserve. It has been observed that natural resources such as fossils have been taken from the park in large quantities. It has also been observed that certain other dangerous conditions continue to threaten the natural beauty, assets and environmental resources within Big Brook Park."
Therefore, "Fossil extraction is prohibited from the walls of the streambed above the stream waterline", and "No person may harvest more than five fossils per day." With all that in mind, you’re ready to “beachcomb” at Big Brook, panning and sifting for treasures buried within the streambed.
INTO THE WOODS
After parking your car in the designated area, assemble your regalia, and follow the short footpath through the woods on Late Cretaceous Navesink soil. You can make out the brook's shallow, shady trough running from right to left. I've been to Big Brook many times over the years and haven't seen one mosquito or tick. Having said that, come prepared!
Path through the woods to Big Brook
STEP INTO THE BROOK AND GO BACK IN TIME
Slide down the vegetated slope to the brook, and step back in time into the continental shelf. The deposition rate of the Navesink has been estimated to be about a meter in a million years, so from the footpath to the streamline, you're back about 2 million years. You can wade through the brook upstream (west) to the Boundary Road bridge about a half-mile or go downstream for a quarter-mile or so.
The Hillsdale Road bridge seen from Big Brook.
Water level is somewhat high here, so the mudflat would be a good option for fossil-foraging.
TIME TO GET WET AND MUDDY
Within the brook's oak-shaded world, things become quiet and peaceful with gurgling waters, chirping birds, occasional hawks cruising overhead and gentle breezes wafting down from above. Only the occasional car flying across the bridge will remind you of the civilization that surrounds you.
Many collectors choose to visit Big Brook in the Spring or after a heavy rain, thinking that new runoff refreshes the fossils that weather in from the banks. Others say it makes no difference. Cobbles and fossils tend to aggregate in horizons, so many collectors focus on sieving there.
If the brook is at high water and the bed is totally flooded, it's safest not to enter, besides, hunting for fossils will be extremely difficult and unproductive. Trees and large limbs that have fallen across the brook add a measure of challenge to negotiating the stream, especially if the current is swift and you're forced to the center of the channel.
Seen here at slight high water, the brook's bed, gravel bars and mudflats are somewhat less accessible.
Wade and trudge around until you've found a "good" spot, be it a mudflat or gravel bar, or simply excavate directly into the streambed. Just don't disturb off-limit streambanks. Although highly fossiliferous and tempting, once again, they are unstable. Groundwater near the base of exposures is under artesian pressure and continually discharging from small seeps and springs that undermine the cliff-face provoked by the slightest excavation - historically a potentially fatal mistake.
STREAMCUTS IN THE NAVESINK
The stretch of brook east of the Hillsdale Road bridge is largely within the Navesink Formation, while some portions of the bed are Mount Laurel and deeper Wenonah (according to the bedrock map). The age of the Navesink has been estimated to range from about 70 million years at the base of the formation to about 66 million years at the top, almost at the end of the Mesozoic.
Directly above chronologically, the K-T boundary between the end of the Cretaceous and the beginning of the Cenozoic ("T" stands for Tertiary) has been identified in test borings buried beneath the Outer Coastal Plain and at Inversand Mine in the town of Sewell within the Inner Coastal Plain on the western part of the state across the Delaware River from Philadelphia.
The Navesink is a transgressive interval in the last of six depositional cycles of changing sea levels coupled with subsidence that includes the overlying Red Bank formation. An idealized cycle includes a basal glauconitic unit (of massive flooding and maxiumum faunal diversity), a superjacent clay or silt surface (representing the highstand tract deposited in shallower water than the previous tract), and a sandy unit )that may contain a lowstand tract at the top). The sequence of lower glauconite sand, middle clay-silt and an upper quartz sand was repeated some four or five times on the plain's inner to mid-shelf in the Late Cretaceous.
Glauconite sands of New Jersey...
The Navesink is a massively bedded, olive-gray, olive-black and dark greenish-black clayey, glauconite sand unit - also called greensand or marl - that is compacted but unlithified. Glauconite is an iron-rich mica (iron potassium phyllosilicate) that forms diagenetically at the sediment-water interface on the continental shelf from clay minerals during prolonged intervals of sediment starvation. Glauconite is not confined solely to the Navesink but is found in most of the Late Cretaceous formations within the Inner Coastal Plain.
Geologic bedrock map of Late Cretaceous and Paleogene Formations of New Jersey's Inner Coastal Plain
Modified from Zehdra Allen-Lafayette
Beds of glauconite are not only highly fossiliferous within the Atlantic coastal plain, but were widely mined as fertilizer in the 19th century due to their nutrient-rich composition and their ability to hold water within its micro-pore spaces. In fact, the duck-billed dinosaur
was discovered in an old marl, fertilizer pit in Haddonfield, New Jersey, in 1858. It was the first almost complete dinosaur skeleton discovered in the United States and is now the New Jersey state fossil. The hadrosaur, being terrestrial, was thought to have been carried to the Late Cretaceous marine environment via a "bloat and bloat" or fluvial-flood scenario. The time-frame and marine burial are astoundingly reminiscent of Nothronychus graffami in Big Water, Utah. You can read about it in my post here.
Highly fossiliferous and bioturbinated...
The streamcut provides an excellent opportunity to inspect a portion of the Navesink in cross-section down to the streamline. Above the Navesink are fossil-depauperate sands of the Red Bank Formation, stained red by iron oxide.
Within the Navesink are quartz-rich sand layers and sand-filled burrows containing granules, black phosphate pebbles and small lignite fragments. Unseen are planktonic microfossils (such as
), and, to the unaided eye, disarticulated macrofossil horizons of bivalves.
The latter form lag deposits exposed within the streamcut that are traceable for some distance. Lag deposits are common in the Late Cretaceous of North America and represent complex taphonomic histories that include multiple episodes of exhumation and reburial associated with sea level cyclicity. Although a work in progress, four or more distinct facies within the Navesink have been identified using these litho- and biofacies horizons that correlate to sequence boundaries and unconformities.
Standing directly in Big Brook's stream, this cut bank is within the Navesink Formation. A lag deposit is about two feet above stream level with a sandy facies above it. The voids are where large bivalves have avulsed (or were excavated) from the matrix.
Iron-rich mineral seeps help to identify bedding interfaces. Note the myriad of overlapping vertical and horizontal burrows exposed within the bank. The extensive infaunal bioturbination is very evident. Not one visitor to Big Brook that I've seen has taken the time to study the streamcuts through the Navesink. There's a great story to be told in the banks, not just by what's washed into the bed!
FROM STREAMBANK TO STREAMBED - THE FOSSILS OF BIG BROOK
The fossil fauna of Big Brook is in keeping with a thriving shelf environment. The following is a small sample of its bounty discovered on four visits to the brook east of the Hillsdale Bridge.
It's easy to overlook small fossils from the brook's mud and gravel bed, especially tiny shark and fish teeth, some of which are a barely 2 mm in diameter. The screen affords an opportunity to patiently inspect your excavated sample. Note the camouflaged remnants of a Gyrphaed scallop shell, a Cephalopal belemnite rostrum and a Squalicorax shark tooth below.
Unquestionably, the major attraction at Big Brook is shark teeth, and there are many varieties to be found. To the geologically-uninitiated, they appear incongruous to the modern landscape. Their abundance is a testimony to the richness of the Late Cretaceous ecosystem.
Shark teeth are well preserved, whereas their skeletal remains being cartilaginous are not, with the exception of an occasional vertebral centrum. All the dental specimens, particularly the radicular structures, being less calcified and more porous, are stained by iron derived from the host sediment. Some permineralization of the teeth has occurred during burial, fossilization and diagenesis (alteration induced by chemical and physical processes mediated by water and stopping short of metamorphism).
Scapanorhynchus ("spade-snout") is an extinct genus of shark from the Cretaceous. Often referred to as the anatomically similar goblin shark, which is distinct enough to have been placed within its own genus, Scapanorhychus had an elongated and flattened snout with awl-shaped teeth suited for tearing flesh and seizing fish.
is the species that frequented the Atlantic shelf in the Late Cretaceous. The 35 mm long anterior tooth (below) is sigmoidal in shape viewed from a lateral aspect with prominent striations running from the root to the apex of the crown. It has a bulbous lingual cingulum (facing the viewer) between the furcation of the two roots, which have a prominent length. Two small, opposing cusplets are variably found on anteriors at the cemento-enamel junction (where the crown meets the root), while posterior teeth exhibit heterodontic variability, although shark teeth are generally homodontic - of the same or similar morphology.
ARCHAEOLAMNA - CRETOLAMNA (?) AND SQUALICORAX
Some degree of difficulty can exist in identifying shark teeth from
Cretlamna appendiculata (
lower left below). Likely the latter, both genera have teeth that are robust with heavy, triangular side cusps and a thick, bi-lobate root with a deep U-shaped furcation.
Squalicorax (lower right) is a genus of an extinct Cretaceous lamniform shark related to the Great White and Goblin sharks. Its blade-like teeth possess a distinctively curved and serrated crown with a prominent notch on the mesial aspect. Its bi-lobate roots are separated by a shallow furca. Squalicorax ("crow-shark") was both a coastal predator and scavanger, as evidenced by teeth having been found embedded within the metatarsal of a hadrosaur, obviously non-marine indigenous. The species is likely
Here are a few more examples of the many shark and fish teeth found at Big Brook. The abbreviated radicular length assists in the exfoliation of teeth under stress, which are replaced within a week by a seemingly endless supply of unerupted teeth. With the assumedly large number of sharks feeding on the shelf in the Late Cretaceous, this accounts for the large number and diversity of teeth recovered from Big Brook. Many of the bones recovered from the seafloor show fossil evidence of shark predation and scavenging and bear the distinctive teeth marks of Squalicorax's serrations. Likewise, many serrations and cusp tips of shark teeth exhibit signs of wear and chipping related to lifestyle and behavior.
Top Row: Squalicorax, Odontaspis, Archaeolamna and Scapanorhynchus.
Bottom Row: Two Squalicorax, Four unidentified and Enchodus.
On the left is one-third of a vertebral centrum of a shark with its characteristic concentric rings and saucer-like depressed center. On the right is possibly a vertebral centrum of a ray, also a chondrichthyan.
Enchodus (upper right tooth above) is an extinct genus of small to medium-size bony fish in the Late Cretaceous. Thought to be a highly predatory species, it possessed fang-like teeth in the anterior, more conventional posterior teeth and a compliment of palatine teeth as well.
First appearing in the Jurassic, belemnites are Mesozoic molluscs and members of an extinct order of the class Cephalopoda ("head-foot") that superficially appear squid-like. They possessed 10 equi-length arms studded with small inward-curving hooks used for grasping prey but lacked the pair of specialized tentacles present in modern squid. Uniquely, they possessed hard internal skeletons (below) - not hydroxylapatite of phosphatic bone - composed of calcium carbonate (calcite) in the form of a bullet-shaped rostrum or "guard."
Located on the posterior aspect and often mistakenly assumed to be anterior for propulsion through water, the rostrum (diagram) was attached to a chambered, conical shell called a phragmocone, and that to the tentacular head of the cephalopod. Based on the behavior of extant lifeforms, it is assumed that they were powerful swimmers and active predators.
The rostra found at Big Brook are plentiful and easy to spot but generally fragmented. Close inspection of a rostrum in cross-section shows its internal structure to be of non-uniform, radiating concentric crystallites that are interpreted as growth rings. Early colonists suspected they formed when lighting bolts struck the ground, hence they are referred to as "thunderbolts." Locals refer to them as "bullets."
Exogyra is an extinct genus of saltwater oyster, a common marine bivalve mollusc, that lived in great abundance within the benthic zone (just above, at and below the sediment surface) of the warm Cretaceous sea. Five species have been reported from New Jersey (
C. cancellata, C. costata, C. erraticostata, C. spinifera and C. ponderosa
), some of which are found at Big Brook. Exogyra and pycnodonte oysters are preserved in great numbers within the Navesink Formation's muddy glauconitic sands of Big Brook, typical of an outer shelf environment. Assemblages of the bivalves Exogyra, Pycnodonte and Agerostrea form biofacies horizons within the Navesink.
PYCNODONTE MUTABILIS WITH CLIONA CRETACICA BORINGS AND EXOGYRA
Another extinct Cretaceous saltwater oyster in the same family as exogyra, Pycnodonte is also a bivalve that is well represented at Big Brook. Many of the upper valves (referred to as "left") preserve the original shell coloration in the form of reddish brown radial bands, which are often discontinuous or offset indicating growth lines. The upper valve is strongly convex with concentric growth rings. Pycnodonte can reach up to 10 cm across.
Many modern oysters fall victim to predation from crustaceans such as lobsters and crabs, and gastropods. The oyster might survive the invasive attempts by continually accreting new shell layers. Back in the Cretaceous, the predatory sponge
created trace holes by boring into Pycnodonte's shell (lower left). The shell on the right is the lower (or "right") valve of Exogyra.
Also an extinct genus of Late Cretaceous fossil oyster, this bivalve was prominent within the Navesink beds. It's semilunar shape and highly recognizable scalloped edge are characteristic. Along with Pycnodonte, it served as a biofacies assemblage horizon.
Inoceramus ("strong-pot") is also an extinct species of bivalve, a saltwater clam that resembles an extant oyster. It had a worldwide distribution during the Cretaceous that included the Western Interior Sea way as well as coastal regions, the Atlantic included. Its prisms of calcite confirmed it with its typical pearly luster. Inoceramus, along with the bivalves previously mentioned, are found in lag deposits that weather into the brook and provide biostratigraphic facies recognition.
CARAPACES, PINCERS AND CLAWS
Common to the Late Cretaceous shelf's ecosystem were various arthropodal crustaceans - lobsters, crabs and shrimp - that left fragmented remains of their dorsal exoskeletal carapaces, pincers and claws. Many of the specimens may be molts rather than the remains of the parent lifeform.
The claws of the callianassid crustacean Callianassa are commonly preserved within infaunal burrows and, to the astute observer, can occasionally be spotted in situ within the stream banks. The following specimens were collected from Big Brook's mudflats. Many coprolites (fecal pellets) bear a striking resemblance to exoskeletal remains in terms of glossiness, black color and similarity in shape. Exoskeletons often possess a marked symmetry, have sutures between fused segments and have surface rugosities distinctive of arthropods (bottom row, far right).
Top row are artifacts; bottom row are remnants of crustacean carapaces and claws.
Typical of a coastal shelf ecosystem, many invertebrates such as clams, oysters, shrimp and worms plied the seafloor and burrowed into the glauconitic sand and mud seeking food and protection from predators. The formations of the Monmouth Group have been extensively burrowed by various worms, digging bivalves and shrimp. Callianassa is a genus of "mud" or "ghost" shrimp common to the shelf fauna that reinforced their burrows with fecal pellets to prevent collapse. In time, the burrows filled in with sand and became iron-cemented, which is what I believe are demonstrated below. The pointed specimen at the right is a belemnite guard.
Trace fossils such as this - also called ichnofossils - are geological records of biological activity. They are impressions created on and borings tunneled into the substrate of the seafloor.
is a trace fossil classification or ichnotaxon of a burrowing organism in a near-shore environment. Callianassa is considered to be the best-known modern analog for this burrow. Trace fossils also include the organic digestive fecal remains or coprolites left behind by lifeforms, which are also found at Big Brook.
I suspect that the following specimen, displayed from three perspectives, may be a cross-section of a small sand-lined burrow - a remnant of a marine organism that lived on and within the seafloor. On the left, a small circular entry on the seafloor leads to the burrow; the middle photo shows the lobate burrow from below; and on the right, the sandy substrate and burrow are visible from a lateral perspective. Other interpretations of this specimen are welcomed.
Artifact or ichnofossil? The following specimens were extracted from Big Brook's streambed. The specimen on the left appears to contain an anastomosing network of iron-cemented burrows enveloping an oyster shell. On the right, a small section of cemented quart sand is enveloped by a similar burrow with extensive, irregular branching. Other interpretations?
Note the similarities of ophiomorpha from the Upper Cretaceous Blackhawk Formation of Utah
There are many specimens or artifacts at Big Brook that defy any attempt at identification. Iron, as contained within the Navesink, can cement the clayey and sandy substrate together into strangely shaped concretions. Often the result is a "fossil" that is totally inexplicable, many with curious, highly symmetrical holes running entirely through them. Some resemble vertebral centra with small foramina, and others appear almost as if man made. Again, any other interpretations?
Here's the belemnite rostrum (pictured above) that has begun to acquire a surface coating of gravelly iron-cemented material. One can envision, that once totally encrusted, it might defy identification unless visualized in cross-section.
CONTEMPORARY SUBSTRATE BURROWING
Unnoticed by a passing deer (and most of the visitors that come to the area), the mudflats of Big Brook are typically criss-crossed by a maze of horizontal burrows, reminiscent of the coastal shelf some 70 million years ago.
Burrowing of marine substrates was not new in the Cretaceous. Horizontal and vertical burrowing has been going on throughout the Phanerozoic beginning in the Cambrian with the Burgess Shale type-biota. In the latest Precambrian, largely horizontal mining of benthic surfaces has been identified amongst the Ediacaran biota. It is believed that vertical subsurface excavation (for protection or to feed) in the Cambrian reworked the seafloor to the extent that it disrupted the cyanobacterial mat to which the Ediacara biota attached and thrived. With the coming of the "substrate" or "agronomic revolution", it is thought that might have led to their demise.
Most assuredly, there's a lot to experience and yet to comprehend in the "piddly little dribble" of Big Brook.
OUTSTANDING SOURCES OF GEOLOGICAL AND PALEONTOLOGICAL INFORMATION
Bedrock Geologic Map of Central and Southern New Jersey
by James P. Owens et al, 1998.
Bedrock Geologic Map of of the Freehold and Marlboro Quadrangles, Middlesex and Monmouth Counties, New Jersey
by Peter J. Sugarman and James P. Owens, 1996.
• Big and Ramanessin Brooks
by the New York Paleontological Society, Field Trip 2002.
Callainassid, Burrowing Bivalve, and Gryphaeid Oyster Biofacies in the Upper Cretaceous Navesink Formation, Central New Jersey: Paleoecological Implications and Sedimentological Implications
by J.B. Bennington et al, Department of Biology, Hofstra University.
• Cretaceous Fossils of New Jersey - Part I
by Horace G. Richards et al, 1958.
Cretaceous Stratigraphy of the Atlantic Coastal Plain, Atlantic Highlands of New Jersey
by Richard L. Ollson, Department of Geological Sciences, Rutgers University, GSA Centenial Field Guide-Northeastern Section, 1987.
Geology Map of New Jersey