Bennington, Vermont: Rift to Shelf Deposits (Field Trip 1)

Figure 1.1:Rift Clastics seen at these outcrops include Cheshire quartzite, Monkton quartzite,
Winooski dolomite, and Bascom marble.

Stop 1: Mount Holly Complex

Figure 1.2 Banded gneiss at this roadside outcrop contains
mafic and felsic bands, some of which are truncated.  The
greenish color is primarily due to the alteration of feldspar.
Arrow points to bedding truncation.  Pencil photographed
for scale.

This outcrop consists primarily of gneiss with bands of pegmatite.  The pegmatite is close to granite in composition.  Smaller bands of quartz, biotite, and epidote can also be seen.  In some areas, banding is truncated.  These truncations, as well as foliations in the gneiss can be seen in the photo at right.
     From argon dating, we know that this outcrop is roughly 1.1 billion years old, significant in that it tells us that the Taconic orogeny did not generate enough heat in this area to reset the argon clock.  The sediments were originally deposited in shallow water   The strike and dip are 220, 66.  In the photo at right, foliations in the gneiss can be seen.

Figure 1. 3: An anticline in this outcrop is its most distinctive structural feature;
line drawn in is parallel to bedding.

Stop 2: Cheshire quartzite

Figure 1.4: From closer up, cross-bedding can faintly be
observed in the fairly homogenous quartzite.  The cross-
bedding, asymptotically truncated, can be used as a "way up"
structure.  Here, up is up. 

The direction of truncation of the cross-bedding demonstrates that the rocks are currently right side up.  The quartzite at this stop is fairly homogeneous, probably having formed from very pure quartz sand from when the formation was part of a beach.  An alternative explanation for the homogeneity of this quartzite is that during diagenesis fluid might have weathered out other minerals such as feldspar; however, this would require a substantial amount of fluid.  Like in stop 1, the features of this outcrop are indicative of a wet climate, and show that chemical weathering is dominant.  Out west, in contrast, where the climate is drier, physical weathering is dominant.

Between the Mount Holly Complex and the Cheshire quartzite, we bypassed the Dalton Formation, also part of the rift phase of the breakup of Rodinia.   

Stop 3: Monkton quartzite thrust over Winooski Dolomite

Figure 1.5: The contact between the older Monkton

Quartzite and younger Winooski Dolomite is thrust fault.

In the Monkton, the rusty looking rock is the quartzite,

 while interbedding of phyllite and marble accounts for the other colors.

At this stop, an acid test confirmed the presence of dolomite.  Quartz veins pervade throughout the dolomite matrix. Bedding in the dolomite of the lower formation is continuous.The Monkton is composed of phyllite, quartzite, and dolomitic marble.  Truncations in both formations indicate that the contact between them is a fault.

     Though the Monkton quartzite is older than the Winooski dolomite, they are both from Cambrian sedimentary deposits.  Both were deposited in a shallow marine setting.

Stop 4: Scolithus tubes

Figure 1.6: The dark, vertical linear features are the trace
fossils from burrowing.  Pen photographed for scale.

 Due to bioturbation, this outcrop of Cheshire Quartzite has not retained sedimentary structures.  The fact that bioturbation didn't destroy its own evidence completely indicates that sedimentation was rapid.

Stop 5: Bascom Marble

Layers of folded grey calcite marble alternate with darker beds of marble.  Bedding thickness ranges from about 5-40 cm.  The white calcite veins parallel to bedding are not part of the depositional sedimentary history; they precipitated from fluid along bedding planes later on.

     Slip is going toward the core of the fold, where cleavage is at the highest angle to bedding.  

     The calcite beds are of Ordovician age, and were likely deformed during the Taconic orogeny.

Figure 1.7: This syncline includes layers of clay-rich dark calcite marble as well as lighter-colored calcite marble. The folding caused stress to overpower the strength of the rock, inducing a fault.

Additional Tectonic Context

• The unconformity between Mt. Holly Gneiss basement and rift sediments suggest that the Laurentian margin at the breakup of Rodinia was a lower-plate margin (Lister 1986).  The uplift and subsidence of the lower plate resulted in high rates of deposition of the adjacent trough.  

Figure 1.8: Adapted from Lister et. al., 1986.  The Laurentian margin was the lower plate margin associated with the detachment faulting when Rodinia broke up.  When the crust bowed down due to removal of asthenosphere below, a basin formed, where the Cheshire quartzite,  Dalton quartzite, Monkton quartzite, and Winooski dolomite were then deposited. 

• Courtillot and Vink (1983) discuss how parts of rifted continents can be matched back together by examining the pattern of magnetic anomalies associated with rifting, which are interrupted by the edges of continents.  This does not work, however, if the rift has propagated.  Propagation occurs because the.rifting crust is not uniformly weak; the strongest sections remain attached longest, creating locked zones.  As volcanic material intrudes through the weak zones of crust, it eventually stretches and breaks apart the locked zones.  
• During rifting, clastic deposition of clastic volcanic rocks occurred at the Laurentian margin, and were overlain by carbonate shelf deposits once the margin was passive (Allen et. al., 2010).  Sandstones that were metamorphosed into the Cheshire quartzite make up part of the rift zone.  In Western Vermont and Massachusetts, these sandstones overlie the Grenville basement directly because teh Pinnacle Formation, which can be observed further north, pinched out completely.  The Pinnacle Formation is composed of synrift volcanic clastics.  Since the Cheshire Formation lies between volcanic clastic deposits/basement and carbonates, it marks the transition from rift deposits to passive margin deposits.  
• Allen et. al. (2010) further divides the carbonate deposits into a shallow to deep water succession, ending in the Winooski dolomite.  That the shelf carbonates interfinger with slope deposits suggest rapid subsidence during rifting, as Lister et. al. (1986) suggest, due to the instability of the lower-plate margin during and immediately following rifting.