Volcanism in the Valley:  A Brief Examination of Trimble Knob, Monterey, Virginia

Regional Setting and History

Early Phanerozoic to Present

The history of the Valley and Ridge province of Virginia is quite complicated.  By about 540 million years ago (mya) during the Late Cambrian period, a divergent continental margin had formed between what is now the Allegheny Plateau and Blue Ridge provinces of North America (Figure 1).  This margin resulted from rifting of the Rodinia supercontinent into its North American and African segments and subsequent opening of the Iapetus Ocean, which had begun some 100 million years earlier.  The quiescent continental margin thus formed deposited thick beds of limestone and dolostone.  Today, these sediments are preserved in the ubiquitous lower carbonate rocks of the Valley and Ridge province such as the Elbrook, Beekmantown, and New Market Formations.

However, this divergent continental margin was not to last forever.  Beginning about 450 mya during the mid-Ordivician period, an island arc terrane called the Chopawamsic/Arvonia collided with eastern North America precipitating the Taconic orogeny.  As a result of this orogeny, a deep flysch basin formed in what is now the Valley and Ridge province.  Into this basin flowed voluminous sediments from the Taconic Mountains to the east.  Formations such as the Massanutten Sandstone, Martinsburg Shale, and Oswego Sandstone testify clearly to this mountain building event. 

After an approximately 80 million year period of inter-orogenic calm during which limestone deposition predominated in the aforementioned flysch basin (Tonolaway Limestone and Helderburg Group representative), a second terrane, called Avalon, collided with eastern North America.  Known as the Acadian orogeny (Early Devonian period), this mountain building event deepened the flysch basin resulting in the deposition of green, gray, and red sandstones and shales.  Collectively, these sediments are identified today as the Catskill Clastic Wedge.  The Needmore, Millboro, Brallier, Greenland Gap, Hampshire, and Price/Pocono Formations are all members of the Catskill.

Following the Acadian orogeny was another period of inter-orogenic calm lasting roughly 30 million years.  During this calm period, the then-shallow flysch basin (having been filled with the Catskill Clastic Wedge) precipitated fossiliferous limestone sediments.  These are represented presently by the Greenbrier, Bluefield, and Princeton Formations.  This calm period, however, was interrupted by yet another mountain building event, namely, the colossal Alleghenian orogeny (Permian period).  The largest of the orogenies, the Alleghenian resulted from the collision of North America and Africa and the closing of the Iapetus Ocean.  The supercontinent thus formed is known today by the name Pangaea.  Extensive folding and thrust faulting characterized this mountain building event, and Himalaya-sized mountains were formed.  The distinctive folds of the Valley and Ridge province and the beautiful Blue Ridge Mountains are testament to the Alleghenian orogeny.

The final, major tectonic event to strike Virginia occurred in the Late Triassic to Early Jurassic periods approximately 200-230 mya.  This, of course, was the rifting of Pangaea into Africa and North America and the creation of what is now the Atlantic Ocean.   By the Cenozoic era roughly 70 mya, the east coast of North America had become a mature, divergent continental margin marked by quiet marine deposition.  This margin persists to the present day (Fichter 1997; Fichter & Baedke, 2000). 

Eocene Intrusions

As a result of the aforementioned tectonic activity, many igneous intrusions can be found throughout Virginia.  Some are associated with the three major orogenies mentioned; however, most of these igneous bodies were emplaced during the Triassic/Jurassic rifting of Pangaea.  The famous Triassic Basins of the Virginia Piedmont (Figure 1) are the largest and most studied of these intrusions.   Nevertheless, smaller, more isolated bodies abound.  In the vicinity of Highland County, Virginia, northwest-southeast trending dikes are quite common.  Augusta, Rockingham, and Pendleton Counties evidence many of these (Figure 3).  When geologists first recognized the igneous intrusions of Highland County in the 19th century, they assumed emplacement concomitant to rifting.  This was a natural assumption since these intrusions cross-cut both the sedimentary rocks described earlier and the extensive folds of the Valley and Ridge.  Until the mid-20th century there was little question that the igneous rocks of Highland County were Triassic/Jurassic in age (Tso et al., 2004; Tso & Surber, 2006). 

However, in 1969 two geologists named P. D. Fullager and M. L. Bottino came to a surprising conclusion.  Using K-Ar and Rb-Sr isotopic dating techniques, Fullager and Bottino found that most of the Highland County intrusions are not Triassic/Jurassic in age; rather, they were emplaced during the Eocene epoch approximately 47 mya, well after the divergent continental margin had formed (Tso & Surber, 2006).  Geologists considered this finding significant for two reasons.  First, it made the Highland County rocks the youngest igneous rocks in the eastern United States (Tso et al., 2004).  Secondly, and more importantly, it begged the question, "How and why did these intrusions form?"  Divergent continental margins are typically not conducive to igneous activity.  Naturally, renewed interest in these igneous intrusions ensued.  Subsequent study uncovered Eocene intrusions in Pendleton and Rockingham Counties as well, Ugly Mountain and Mole Hill being respective examples (Figure 3).  Moreover, isotopic dating has since determined that Trimble Knob is the youngest of all of these intrusions.  Dated at 35 million years old (ma), Trimble Knob stands as the youngest known igneous body east of the Mississippi River (Tso et al., 2004).

Lithology

The Eocene rocks of Highland County are bimodal in composition, containing both "mafic (basalt, picrobasalt, and basanite) and felsic (trachyte, trachydacite, and rhyolite) members" (Tso et al., 2004) (Figure 4).  These rocks locally cross-cut Lower Ordivician (Beekmantown Dolomite) to Middle Devonian (Millboro Shale) sedimentary rocks.  Trimble Knob itself is basaltic in composition and is found locally enclosed by the Millboro Shale (Rader et al., 1986).  However, Trimble Knob is unique among the Eocene intrusives with respect to its size, shape, and texture.  Roughly circular in horizontal cross-section, Trimble Knob has a diameter of approximately 150 m (500 ft), making it the largest of the Highland County instrusives.  Combined with its conical geomorphology, these facts might lead one to believe that Trimble Knob is a volcanic cinder cone similar to those found in the western United States; however, the absence of cinders, ash beds, and lava flows adjacent to the body preclude such an estimate (Tso et al., 2004). 

Presently, geologists consider Trimble Knob to be the erosional remnant of a volcanic neck or plug.  Nevertheless, complete understanding of Trimble Knob can be attained only through study of its distinctive texture.  The basalts of Trimble Knob have a porphyritic, sub-ophitic texture containing phenocrysts of olivine, augite, and hypersthene.  What is more, in the outer one-third of the intrusion are found xenoliths of shale, limestone, and sandstone derived from the surrounding country rock, principally the Millboro Shale and underlying Helderburg Group (Late Silurian/Early Devonian carbonates).  This region of the intrusion has been described as a breccia and varies in xenolith content from 50 to 90 percent of the breccia mass (Rader et al., 1986).  The breccia of Trimble Knob is key to understanding its genesis.