to be a glass-bottomed boat sailing through a Late Cretaceous sea busy with life.”
|Big Brook at a slightly high water level as seen from the Hillsdale Road bridge facing east|
A WINDOW INTO A LATE CRETACEOUS CONTINENTAL SHELF
Step into the 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.
A PALEONTOLOGICAL GRAB BAG
|A Late Cretaceous terrestrial fauna similar in some respects to eastern North America|
From National Geographic
|The Late Cretaceous marine ecosystem teemed with life. |
In addition, abundant macro-invertebrate remains include brachiopods, bryozoans and molluscs (bivalves such as oysters, snails, belemnites and ammonites) and disarticulated arthropod carapaces and claws (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.|
Modified from Roadside Geology of New Jersey
BIG PICTURE TECTONIC STUFF
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. But the geological story of the plain 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.
Birth of Pangaea...
In a parade of orogenic events, they accreted to Laurentia's 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 had been ravaged by erosion.
Demise of Pangaea...
In the Late Triassic, Pangaea’s fragmentation began. As the Atlantic Ocean began to open within the schism, the remnants of Pangaea's central mountainous spine began to fragment as well. A portion remained astride the Atlantic Coast in newly-formed 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.
Subsidence and sedimentation of the Atlantic margin...
Beginning probably during Jurassic time, lithospheric cooling of North America's newly-formed 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 geo-indentation that would become a portion of the Coastal Plain - the Raritan embayment - influenced sedimentation in New Jersey between Staten Island at the western end of Long Island and Jersey's northern Coastal Plain.
Formation of the Atlantic Coastal Plain...
Concomitant with crustal cooling and subsidence, 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.
Of course, the flood waters of the Cretaceous have receded exposing the broad Coastal Plain on the eastern seaboard. Although global seas continued to vacillate, erosion became the dominant geological process through the Tertiary. Ice Age glaciers made it to northern New Jersey but not to the south, while the Coastal Plain continued to receive a thin and varied veneer of colluvial and alluvial Quaternary and Holocene debris. Today, the modern shoreline is 10 miles to the east of Big Brook as it lethargically dissects its way to the sea through Late Cretaceous sediments. As for Monmouth County, the Jersey Shore and the entire East Coast, they await the sea's return, like it always does.
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 provinces of New Jersey.
|The geomorphic provinces of Northeastern and Mid-Atlantic North America with New Jersey encircled. |
Modified from the USGS Tapestry of Time bedrock map located 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 multitude of orogens that formed eastern North America, New Jersey’s main geological subdivisions or provinces are the Valley and Ridge, the Highlands (equivalent to the familiar Blue Ridge down south), the Piedmont and the Coastal Plain.
By definition, the four geomorphic or physiographic regions are each unique as to relief, landforms and geology. Being inherited subsequent to the tectonic collisions that occurred throughout the Paleozoic, 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 Newark Basin (green and red), which lies interposed within the Piedmont. It's 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.
THE COASTAL PLAIN
The Coastal Plain Province is relatively featureless save a few gently undulating hills and overlapping rocks of the Piedmont Province to the west. 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.
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 Outer 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 cedar swamps and pine-scrub oak of the 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 Monmouth Group, the state's youngest Cretaceous package, was laid down - a unit that is compacted but unlithified. New Jersey's Monmouth Group includes the basal Mount Laurel sand (5-60 feet thick), the transgressive marl of the Navesink (25-60 feet thick), the regressive silt and sand of the Redbank Formation (thin film to 100 feet thick) and the Tinton Formation's 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.
|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
|The bucolic Hillsdale Road bridge over Big Brook facing south|
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.
|The short path through the woods to Big Brook|
|The Hillsdale Road bridge seen from Big Brook. |
The water level is somewhat high here, so the mudflats would be the best option for fossil-foraging.
|Seen here at high water, the brook's bed, gravel bars and mudflats are less accessible for foraging.|
|Geologic bedrock map of Late Cretaceous and Paleogene Formations of New Jersey's Inner Coastal Plain|
Modified from Zehdra Allen-Lafayette
|Standing directly in Big Brook's stream, this cut bank is within the Navesink Formation with a portion of the overlying Red Bank Formation. The voids are where large clusters of bivalves have avulsed (or were excavated) from the glauconitic matrix.|
|Close-up of a heavily bioturbinated and water-saturated Navesink bank with an iron-rich mineral seep.|
|A gravel bar alongside the streambed of Big Brook|
|A 35 mm long Scapanorhynchus anterior tooth|
|Shark teeth from Archaeolamna-Cretolamna (?) and Squalicorax|
|Top Row: Squalicorax, Odontaspis, Archaeolamna and Scapanorhynchus. |
Bottom Row: Two Squalicorax, Four unidentified and Enchodus.
|Vertebral centra from a shark and a ray|
|Calcitic rostra from belemnites|
|Diagram of a belemnite from ukfossils.co.uk|
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 Cliona cretacica created trace holes by boring into Pycnodonte's shell (lower left). The shell on the right is the lower (or "right") valve of Exogyra.
|Top row are artifacts; bottom row are remnants of crustacean carapaces and claws.|
|Note the morphological similarities of Ophiomorpha from the Upper Cretaceous Blackhawk Formation of Utah to the burrows found at Big Brook.|
|A heavily bioturbinated and deer-trampled mudflat alongside Big Brook|
Most assuredly, there's a lot to experience and 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, Department of Environmental Protection, Geological Survey, 1999.
• Greensand and Greensand Soils of New Jersey: A Review by J.C.F. Tedrow, 2002.
• New York Paleontological Society. Established in 1970, individual and family memberships are open to all, regardless of education or previous experience, It's a fantastic way to visit Big Brook and many other fossil-collecting localities in the Northeast with an enthusiastic and well-informed group of amateurs and professionals. Meetings are held in the American Museum of Natural History in New York City and includes an outstanding newsletter. Visit them here to join. Yes, I am a member.
• Paleocommunities and Depositional Environments of the Upper Cretaceous Navesink Formation by J. Bret Bennington et al, Department of Geology, Hofstra University, 1999.
• Paleontology and Sequence Stratigraphy of the Upper Cretaceous Navesink Formation, New Jersey by J. Bret Bennington, Hofstra University, Long Island Geologists Field Trip, 2003.
• Pictorial Guide to Fossils by Gerard R. Case, 1992.
• Roadside Geology of New Jersey by David P. Garper, Mountain Press Publishing Company, 2013.
• Shell Color and Predation in the Cretaceous Oyster Pycnodonte Convexa from New Jersey by J. Bret Bennington. Hofstra University.
• Summary of Lithostratigraphy and Biostratigraphy of the Atlantic Coastal Plain by Richard K. Ollson, Rutgers University.
• Uppermost Campanian-Maestrichian Strontium Isotopic, Biostratigraphic and Sequence Stratigraphic Framework of the New Jersey Coastal Plain by Peter J. Sugarman et al, GSA Bulletin, 1995.