Florida, Massachusetts: Rift to Slope Deposits (Field Trip 2)

Figure 2.1: Though the Stanford Granite Gneiss is basement rock, the conglomerate and meta-arkose seen
in this field trip are derived from slope rise deposits.

Stop 1: Weakly Foliated Stanford Granite Gneiss

Figure 2.2: Weakly foliated "Rapakivi"gneiss includes

squashed feldspars surrounded by thin rims of plagioclase.

Pen photographed for scale.

The granite contains plagioclase, K-feldspars, bluish quartz, garnets, epidote and clusters of biotite.  The granite is termed "Rapakivi" granite, meaning that there are large phenocrysts of K-feldspar surrounded by thin rinds of plagioclase.  Quartz veins, not weakly foliated like the surrounding rock, must have been deposited fairly late.  Some of the feldspars are squished into eye-shapes, so texturally the rock can be classified as augen gneiss.  Strike and dip are approximately 320, 14.
     A gently plunging anticline (not pictured)  exhibits the least deformation at its core.




Stop 2: Strongly Foliated Stanford Granite Gneiss

Figure 2.3: Strongly foliated Stanford granite gneiss, in

which quartz grains are  prominent and feldspars are 

flattened significantly.

Compared to the first outcrop, the gneiss here is more strongly foliated with fewer grain size variations.  Quartz is more dominant, feldspars are more elongated, and folding here is strong enough to obliterate the Rapakivi texture.  Overall, it appears that stresses here, at the boundary of the Stanford granite gneiss, were stronger than at the formation's center.  Strike and dip are about 260, 16.
     In both of the first two stops, the granite protolith of the gneiss is Precambrian.






Stop 3: Dalton Formation

Figure 2.4: Dalton conglomerate, in which many lithic clasts

are flattened parallel to bedding almost to the extent of being 

reabsorbed by the matrix.  Quartz grains, however, are largely

undeformed.  Pen is photographed for scale.

The conglomerate here includes grains of various sizes and levels of deformation including large grains flattened almost to the point of being reabsorbed by the matrix. Quartz, unlike other minerals, is not stretched, indicating the sediment did not reach an extremely high temperature during metamorphism.  Flattening occurs in a direction almost parallel to bedding, which is still visible.  The sediment was deposited roughly at the end of the Precambrian.  Strike and dip are 285, 5.

Figure 2. 5: Note that in this conglomerate, there is more

deformation in the bed below the drawn line than in that

above it.  This suggests either different lithologies or

different stresses.

Stop 4: Conglomerate with Magnetite
This conglomerate is similar to that from stop 3, and is of the same age, but is more deformed and includes octahedral magnetite grains.  ~40 cm thick bedding is still visible.  The outcrop is cut through by a fault, through which quartz precipitates.  Strike and dip are 280, 12.


   







Stop 5: Meta-arkose/ Conglomerate
Arkose and conglomerate are interbeded.  Bedding is thicker than at either stops 3 or 4.  Grain size is smaller than at the previous stop.  Strike and dip are 305, 16.  The outcrop is of late Precambrian age.

Stop 6: Age equivalent of Cheshire Quartzite
This  rock is a garnetiferous schist.  Compared to the other outcrops of this field trip, this rock was formed in deeper water, though was still part of the continental shelf.  Strike an dip are roughly 260, 17. Additional Tectonic Context

• The stratigraphic rift to drift transition lies between a less stable sedimentary layer of synrift basin rocks and a more uniform and stable layer of sediment (Williams and Hiscott, 1987).  Synrift deposits can be recognized by various characteristics, including substantial and rapid thickness changes, boundary by listric faults defining basin highs, and the presence of large quantities of volcanic rocks.  
• The creation of the Iapetus Ocean, at about 550 Ma, marks the rift-drift transition (Williams and Hiscott, 1987), though precision in dating might be limited by the presence of lock-zones (see previous post).
• According to Withjack et. al. (1998), the transition from rift to drift was diachronous (began earlier in southern North America) and was marked by a switched from northwest-southeast extension to shortening at the same time as sills and dikes intruded the continental crust.  Normal faulting at this time was replaced by reverse faulting because the maximum stress before rifting and the minimum stress after rifting were subvertical.   

Figure 2.6: Adapted from Withjack et. al. (1998)  Extensional forces on either side of the rift acted on the lithosphere (left panel), allowing asthenosphere to upwell from below.  The asthenosphere intruded through the weakened crust (right panel), forming a volcanic wedge.  These intrusions exerted force away from the ridge at a rate faster than extension, which necessitated compensational forces as a sort of rebound effect.