RIFT BASINS, THAUMASITE AND LAZARD CAHN

The last Post described two minerals that had Lazard Cahn (Honorary Life President of Colorado Springs Mineralogical Society) labels accompanying the specimens.  However, I have a third Cahn specimen that lacks a label other than what is written on the outside of a small box with lid: L. Cahn, Thaumasite (white), Berger’s Quarry, Patterson, N.J.  The inside of the lid states: Thaumasite, CHY, biotite.  I am unable to interpret CHY.  The "specimen" is a residue of several hundred fragments of various minerals as noted in the microphotographs near the end of the Post. This box was gifted to me from the family of Willard Wulff, a Charter Member of CSMS and a student of Lazard Cahn. 

Burger’s Quarry (not Berger's; AKA Upper New Street Quarry) is one of several quarries near Paterson, New Jersey, that have excavated and produced “Trap Rock” for decades. Geology.com defines trap rock “as a name used in the construction industry for any dark-colored igneous rock that is used to produce crushed stone.  Basalt, gabbro, diabase, and peridotite are the most common rock types referred to as trap rock.”

The traprock exposed at Burger’s Quarry are part of several igneous formations exposed in various Mesozoic rift basins that extend from Nova Scotia, Canada to North Carolina and even further north and south in the subsurface and to the east buried by sediments of the Continental Shelf.

The Eastern North American Rift Basins along the Atlantic coast associated with the early Mesozoic breakup of the supercontinent Pangaea.  Map courtesy of  Roy Schlische and the Structural Geology and Tectonics Group at Rutgers University.

These rift basins are associated with the breakup of the supercontinent Pangaea around the Late Triassic--Early Jurassic (either side of the 200 Ma absolute age boundary) that resulted in the formation of the proto-Atlantic Ocean as the continents pulled apart.  Today we look at the Mid-Atlantic Ridge and see these same rifting processes working to further separate North America from Europe and Africa.

View of supercontinent Pangaea.  Map courtesy of Institute of Geophysics at The University of Texas Austin.

Although the Atlantic Ocean represents “full” and continuing rifting, there were several basins in North America where rifting was aborted. These basins are structurally known as half grabens.  A graben is where two normal faults bound each side of the basin—as easily noted in the Basin and Range Physiographic Province.  In a half graben there is only a single bounding normal fault. One of the best-known rift basins in the eastern U.S. is the Newark Basin located in northeastern New Jersey (and the site of Burger’s Quarry). 

Location of Newark Basin outlined in white.  The green represents Triassic and Jurassic sedimentary rocks.  The red arc in the northeast are the Wachung Mountains.  Precambrian rocks bound the western edge of the Basin.  Beyond those are the early Paleozoic rocks associated with the late Paleozoic Appalachian Orogeny.  The yellow and orange are Cenozoic Coastal Plain sediments or sedimentary rocks.  Map is Public Domain and courtesy of the U.S. Geological Survey.

Early on, as the Basin was subsiding, massive unsorted sediments from the surrounding highlands were pouring into the lowland and later consolidated to form conglomerates, sandstones and mudrocks (shale etc.) known today as the Newark Supergroup. These sedimentary rocks are usually red or orange due to the oxidation of iron oxide minerals that were in the original sediments. There are also several lacustrine formations that represent large lake systems within the Basin.

The Palisades Sill (intrusive)is along the east side of the Newark Basin and extend north along the Hudson River.  The Watchung Mountains are exposed flood basalts (extrusive) cropping out along the eastern side of the Basin.  Top map courtesy of Columbia University.  The lower drawing of the Newark Basin half graben is Public Domain and courtesy of the U.S. Geological Survey. 

The sedimentary rocks of the Newark Supergroup are well known for their enclosed fossils including dinosaurs, plants, “fish” and various invertebrates.  Perhaps the most famous tracks are those of various dinosaurs discovered in “redbeds” of the Newark.   In fact, Eubrontes, a three toed dinosaur only known from their tracks, was designated the State Fossil of Connecticut in 1991.  In the same state, Dinosaur State Park protects hundreds of tracks discovered in 1966.

Tracks named Eubrontes.  Paleontologists believe that a three-toed dinosaur created these trackways in the rift basin sedimentary rocks.  Since dinosaurs fossils cannot be positively "connected" to the tracks these impressions are termed ichnofossils. Public Domain photo courtesy of Dinosaur State Park.

The tensions associated with the pull-apart basins allowed continued faulting and tilting contemporaneous with the eruption of flood basalts and the emplacement of subsurface dikes and sills due to decompression melting of rocks of the earth’s mantle as they travel upward along thermal plumes.

Sills are igneous rocks that are sheet-like and tabular in design and intrude older rocks parallel to bedding or foliations planes (concordant).  They are usually fed by dikes, igneous structures that intrude across existing bedding planes (discordant).  Magma forming sills cools at a slower rate since the structures are intrusive into preexisting rocks.  This allows for crystals to grow larger than those associated with extrusive lava flows, usually basalt.

Cartoon showing the formation of sills that are intruded parallel to bedding plains (Palisades Sill) while dikes (dykes) intrude across bedding planes and may flow on the surface (flood basalts such as the Watchung Mountains).  Courtesy of Differencebetween.net

The best-known igneous structure associated with the basins is the Palisades Sill that trends along the Hudson River for about 50 miles north of New York City. The layered rocks form massive cliffs along the River and are a well-known landform.  The Sill is composed of diorite, a magmatic rock with larger crystals than the extrusive basalt and was the first igneous rock to appear in the Basin, perhaps around 200 Ma in the earliest Jurassic. 

Still in the early Jurassic, volcanic rocks broke to the surface in the form of eruptions and flood basalts and formed the parent rock of the Watchung Mountains in the northeastern part of New Jersey.  Olson (1980) described these episodes as three separate flood basalts that may have filled the entire basin with each eruption.  After each major volcanic episode, the basin continued sinking and was again filled with sedimentary rocks with the end result being alternating layers of basalt and red sedimentary rocks.  With aborted rifting of the basin, deposition and volcanism ceased and erosion became dominant.

Highlands of the Watchung Mountains with Paterson, New Jersey in the far background. Public Domain photo.

Today the Watchung Mountains are three (plus some smaller remnants) parallel ridges of exposed basalt that are about 400-500 feet higher than the surrounding landscape.  The First, Second and Third Watchung (as the exposures are known) are major landforms and conservation groups fight to preserve their unique plant and animal life as well as the exposures of columnar jointed traprock.

Perhaps more than any other group of minerals found in the Watchung Mountains, the best known are the zeolites (microporous aluminosilicate minerals) and their companions.  One cannot attend a rock/ mineral show without noting Watchung specimens of analcime, stilbite, apopolite, chabazite, datolite, heulandite, stilbite and especially of pectolite (sodium calcium silicate) and prehnite (calcium aluminum silicate): both silicates often appear with zeolites.  It appears that at Burger’s Quarry the extrusive magma cooled in a lake and the basalt forming pillow lava.  Many of the fine zeolite specimens crystalized in the spaces between the pillows.

The Cahn specimen in my collection, thaumasite, looks like (in my opinion) a zeolite.  However, it is a sulfate [Ca₃Si(OH)₆(CO₃)(SO₄)-12H₂O], but one that often appears with zeolites.  In the Watchungs thaumasite is formed when the basalt is heated by geothermal action.  It is usually colorless, but may be white, and forms prismatic hexagonal crystals (like apatite).  Thaumasite is quite soft (3.5 Mohs), leaves a white streak, and is translucent to transparent.  The clear, nice hexagonal crystals have a vitreous luster while the smaller fibrous crystals have a silky luster.  Thaumasite is not a common mineral and outside of the rift basin basalts is sometimes found with other calc-silicate minerals (as at Crestmore, California) or in hydrothermal copper deposits.

Photomicrographs of microcrystals of mostly transparent thaumasite mixed with biotite and others!  The longest crystal of thaumasite in the top photo (near the center) is ~ 1 mm in length.  All of the other thaumasite crystals are less than 1 mm in length.  The dark sheet mineral is evidently a  mica of the biotite family.  The greenish-brown fragments are unknown---perhaps smoky quartz sine many fragments show conchoidal fracture.   Notice the .5 mm crystal near the center of the lower photo "perched" on unknown fragments of another dark (although not shiny) mineral. Perhaps the crystal is titanite, or perhaps not.

I continue to search dusty drawers in small rock shops for additional Cahn specimens or mineral labels.  If any reader has a Cahn or Wulff label for thaumasite, please contact me.

REFERENCES CITED

Olsen, P. E., 1980, Triassic and Jurassic Formations of the Newark Basin in Manspeizer, W., ed., Field studies of New Jersey geology and guide to field trips: New York State Geological Association, 52nd Annual Meeting, Newark, New Jersey, Rutgers University.