This field trip was done as part of the 2008 Keck Geology Consortium Symposium, held at Smith College. This trip went to six localities in the Deerfield basin, a Triassic-Jurassic rift valley in west-central Massachusetts. Stop 1, shown below, is to Mt. Sugarloaf, a large, steep hill made of Triassic fluvial conglomerate and sandstone.

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This photo shows a small stream channel filled with conglomerate, cut into somewhat finer-grained conglomerate below it.

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This is a photo of coarse conglomerate, taken of a pavement outcrop at the summit. Cobbles and pebbles are of vein quartz, granite, gneiss, shist, and phyllite.

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View from the top of Mt. Sugarloaf, looking south toward the Holyoke Range, a set of east- to south-dipping cuestas made of the Holyoke Basalt flow, which is up to 200 m thick. The Connecticut River is to the left. The flat valley floor is covered with glacial lake sediments, and so is very flat, fertile, and free of rocks.

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Small scale cross bedding in coarse sandstone.

 

Stop 2 is to the east of North Sugarloaf Mtn., at an outcrop of some of the varved glacial lake sediments. This outcrop has spectacular, thick varves. These represent annual layers deposited by: 1) sand from swift-flowing spring runoff waters, 2) silt deposited during the melting season, and 3) clay deposited during the winter.

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Here the varve structure is clearly seen visible. There is one complete varve, the one the finger is touching. A thin layer of sand is on the bottom, followed by silt that has a series of vertical cracks where the trowel cut through it to smooth it off. The clay layer is grayer than the slightly browner silt layer, and was cut smoothly by the trowel.

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A more distant view showing several complete varves. Notice that not all of the varves have sand at the bottom, but they do all have silt/clay couplets.

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Near the top of the outcrop is an overturned fold made of varved sediments. This is presumably from a down-slope slump. To the lower right are flat-lying, undisturbed varves. Above and to the left are steeply dipping varves on one limb of the fold. The fold hinge is very tight, and is located at the contact between deformed and flat-lying varves, unfortunately obscured by debris.

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The clay is quite sticky, and it can pull your boots off if you're not careful. It makes nice modeling clay, though I understand it is no good for firing. The rocks under the glacial sediments are Jurassic here, certainly a good time for dinosaurs.

 

Stop 3 is on top of the Sunderland Delta, a sand and gravel delta that was deposited by glacial meltwater streams into the eastern margin of Glacial Lake Hitchcock. Naturally there are lots of sand and gravel pits in such deltas.

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Wide view of the pit operation. The lake was to the right, so the river water that made this deposit flowed from left to right.

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Closer view of some of the foreset beds, and even a glimpse of topset beds on the southeast margin of the pit.

 

Stop 4 is in the coarse depositional breccia at Roaring Brook, just east of Mt. Toby and immediately west of the Connecticut Valley eastern border fault, the Mesozoic lystric normal fault along which most movement occurred that dropped down the Connecticut Valley basin, of which the Deerfield basin is a small part. The breccia is probably talus or proximal alluvial fan material.

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Coarse, angular breccia exposed in an overhang. Luckily, a fallen block exposes a nice little cave.

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Closeup of some angular clasts in the breccia. Most seem to have been transported at least a short distance, so I would guess that this is a proximal alluvial fan deposit rather than talus. The clasts are mostly phyllite and low-grade schist, with some quartzite and granite clasts.

 

Stop 5 is at Barton Cove, a small park on the west side of the Connecticut River in Turners Falls. The bedrock ridge that makes up the park was a temporary glacial meltwater spillway, so there are deep plunge pools on the downstream side. The photos below show breccias and folding in the dipping at 30° but otherwise undisturbed Jurassic redbeds. There is considerable question as to the origin of this deformation. The main ideas are: 1) earthquake-initiated slides of relatively soft, wet, gently dipping fluvial and lacustrine sediments; 2) dewatering structures where water escaping from compacting sediments deep down disrupted the layering in pipe-like or sill-like structures; and 3) small thrust faults associated with compression or transpression late in the rifting episode that made the Connecticut Valley basin. No conclusion was reached, but there are people working on it. All photos below were taken from a small area ~50 m across, uphill from the northeasternmost plunge pool. All the photographed areas are within sight of one another.

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Local breccia zone where sandy layers are disrupted in a more shaley matrix. The fragments are quite coherent and do not really resemble deformed soft sediment.

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Folded fine-grained sandstone.

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Kink folds in shaly sediments.

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Another fold in fine-grained sediments.

 

Stop 6 is at a road cut along Rt. 2 in Turners Falls. The primary rock exposed here is the Deerfield basalt, composed of two lava flows having a combined thickness of 55 m. I think the two flows are from the same eruptive episode. Jurassic red beds are exposed above and below the lavas. The base of the flow is made of pillow lavas in a fragmental matrix, probably formed as the lava flowed into one of the lakes that periodically occupied the rift valley floor. The top of the flow is highly vescicular, and the contact between the two flows is highlighted by abundant vesicles.

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Pillow lavas in a fragmental matrix at the base of the Deerfield basalt flow. The contact with the underlying redbeds are in the notch to the lower right.

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Some of the pillow lavas in a fragmental matrix ~5 m above the base of the lava.

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Upper contact of the lava with Jurassic red beds. The lava is the rounded surface at bottom center, where the tiny white specs are filled vesicles. The red beds to the upper right are layered, dipping here at ~40° to the right.

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Closeup of the vescicular basalt near the top of the flow. Vescicles are filled with minerals, including green material (chlorite?), white low solubility material (zeolite?), and carbonate (calcite?). Many of the vescicles on the surface have had their filling weathered out.

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Fresh surface, where the vesicles are mostly filled with greenish material. The petrology of this flow, here at least, is a complete mess. Don't expect to find much in the way of the original minerals.

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Successive fracture steps that formed as this joint surface extended along the cooling front during cooling of the lava flow. Nice columnar jointing, however,is not present at this outcrop.

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The upper part of this flow has several nicely exposed coarse compaction sills. Here you can see the fine-grained rock (below) and coarse-grained rock (above) in the compaction sill. As I recall from publications by Tony Philpotts, these form after the lava becomes a weak, non-convecting mush by the touching of growing crystals. The crystalline mush is denser than the interstitial liquid, so it tends to sink as the interstitial liquid rises. The mush fractures typically along subhorizontal surfaces, and these fill with the rising interstitial liquid. The initially crystal-free, incompatible element-rich, volatile-"rich" liquid filling the sill can then grow relatively large crystals because of few nuclei.