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Ordovician Penobscot Formation

A cache by hollora Send Message to Owner Message this owner
Hidden : 10/22/2007
1 out of 5
1 out of 5

Size: Size: other (other)

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Geocache Description:

DUE TO ROAD CONSTUCTION - Access to view this formation should be done on foot only! Please use the parking coordinates and carefully walk down the road to view the formation. NOTE - The logging requirement of this Earthcache requires you to SEND me an email with answers! PLEASE EMAIL your answers don't just log it! The Penobscot Formation was defined by Smith, Bastin, and Brown (1907, page 4). Coordinates of the Earthcache are for the best viewing location.

Ordovician Penobscot Formation

The following formation about this Earthcache was taken from materials published on The Maine Geological Survey's webpage at www.Maine.Gov.

Geologic background - "The Penobscot formation is composed of metamorphosed shaly sediments which are typically developed along nearly the whole length of the western shore of Penobscot Bay. They occupy large areas west of the Penobscot Bay quadrangle, between the ocean and the Kennebec River. The most typical exposures of this formation occur in the northwestern part of the quadrangle, between Belfast Bay and Sandy Point.

The sedimentary character of the rocks is shown by the presence of distinct bedding at many localities. In color, the rocks of this formation vary from light gray through steel-gray and purplish gray to black, the darker grays being predominant. The weathered surfaces are usually rusty.” Keith incorporated much of the Smith, Bastin, and Brown (1907) work on his 1933 map. He did modify the extent of the Cambrian-Ordovician rocks mapped by Smith, Bastin and Brown as Penobscot Formation (sometimes correctly, sometimes not), but kept the defining "type area" along the western side of Penobscot Bay from Belfast through Searport and up the Penobscot River to Bucksport. The 1967 Preliminary Geologic Map of Maine (Hussey and others, 1967) grouped the older rocks of the Camden-Rockland area and Islesboro with the Penobscot Formation into a single map unit (SOp), and correlated them with rocks with similar lithologies to the east (Op) - as far east as Calais on the New Brunswick border. Recognizing that the upper portions of the sequence were everywhere in fault contact with younger Silurian-Devonian rocks and that intrusions that cut the sequence were Silurian or younger, the age assignment expanded to include the early Silurian.

It is not known why the Cambrian age assignment was dropped. The 1985 Geologic Map of Maine (Osberg and others, 1985) shows the (again) Ordovician-Cambrian Penobscot Formation as a separate entity extending as far south as Owls Head, through the type area (Belfast Bay to Sandy Point), and (interrupted by Devonian plutons) as far east as Beddington along the Air Line (State Route 9). Similar rocks further to the east are shown as Ordovician-Cambrian Cookson Formation (now the Cookson Group) defined in New Brunswick. Correlation with the fossil-bearing Cookson Group together with geochronologic work returned the age assignment to the Ordovician-Cambrian. Current thinking (Stewart, 1998, and references therein; Tucker and other, 2001) places the Penobscot Formation in the Ordovician-Cambrian. These rocks were deposited in relatively deep, oxygen-deficient waters along a continental slope concurrently with minor sea floor volcanism, and thrust northwest over Silurian rocks of the Fredericton Trough in the Silurian.

The rocks of the Penobscot Formation were complexly deformed and metamorphosed at this time, and possibly underwent regional metamorphism in the late Silurian during an early stage of the Acadian orogeny (West and others, 1995). Parts of the Penobscot Formation were metamorphosed in the contact aureoles of the post-Acadian Devonian plutons such as the Mount Waldo pluton (Stewart, 1998). Sedimentary bedding in the Penobscot Formation at the locality of this Earthcache is steeply dipping, striking generally north-northeast to south-southwest.

The center of the cut is looking almost directly at a bedding surface, along the axial plane of a broad, near-vertical fold. The dominant folds in the Penobscot Formation are northeast-trending isoclinal folds, so the broad fold in the roadcut is a later fold. At the southern end of the outcrop, late sub-horizontal folds also warp beds of quartzite. The most obvious features in the roadcut are the stripes of light-colored rock, felsic dikes, that intrude the Penobscot Formation. Inclusions of Penobscot Formation are readily visible in the dikes. The dikes are most likely Devonian in age and related to the Mount Waldo pluton. The contact with the pluton is less than 100 feet to the west, and exposures of the granite are present just to the south along Routes 1 and 3. Another prominent feature is the nearly continuous, apparently sub-horizontal fracture that traverses most of the roadcut. (The fracture only appears sub-horizontal along the face of the roadcut; it most likely has an orientation similar to the joints that dip to the northeast.) The fracture intersects felsic dikes at several places, but displacement of the dikes is minor. The fracture most likely formed due to stress relief during uplift and erosion of the area. While visible displacement of the felsic dikes is minor, slickensides produced by motion along a fracture surface are visible on joint surfaces at many places in the outcrop.

There is at times some seepage from fractures. One thing to note is the irregular pattern of the seepage when occurring . A planar fracture that appears continuous in 2-dimensions is not necessarily open in the 3rd dimension. Groundwater flow is through narrow channels along the fracture surface and visible seepage occurs where these channels exit the rock. These channels transmit groundwater to wells drilled in bedrock, the most common source of water for homes in rural Maine. On a larger scale, this irregularity in groundwater flow in the bedrock makes predicting yields in a bedrock well uncertain at best and folly at worst.

Accessing the Earthcache - To reach the above coordinates you should park in the observation turn out on Route 1 (parking coordinates) and carefully walk down the road to the North. There is a one lane road, to walk down (which is under construction near the end) just beyond the turn out. This little road goes past a home. Please respect the one home owner on this road and do not go near their home. To gather information to log the cache you do not need to go near the private home. The cache coordinates are the best location to view the formation and roadcut, however, the answer might be obtained by an observation made from other vantagepoints as well. Please use the coordinate for parking and do not park on the side of Route 1! This is a very busy road and your safety is of my utmost concern. As you look toward the roadcut you will be viewing the formation with a view towards the west; the entrance to Fort Knox State Park and the bridge observation tower is at the extreme right; Routes 1 and 3 southbound will be towards the left edge.

Logging the Earthcache To log this earthcache you must send the owner an email with the following information – Name and GC number of cache, number in your party and an estimate of the height of this formation overall. In addition it would be nice to post a photo of your GPSr with the cut in the background or you and your party.

Thank you for visiting the area. I hope you enjoy learning more about this formation.

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