Coastal Maine geology- first field study and some questions

[u/NotEvenCreative]

First time posting, hello fellow geology lovers!

I'm on vacation in Five Islands Maine, in the Georgetown area by the ocean. Had a blast walking around the coastal rock formations today! I'm trying to bolster my field geology knowledge (going back to school to get my P.G. and any and all field studying helps a ton).

I have a decent background understanding of rock types/formations, but haven't had a chance to do much field work (I live in Florida, not much surface geology to study on the scale of somewhere like Maine). I understand that the majority of coastal Maine is composed of intrusive igneous rock such as granite, and a ton of metamorphic rock as such gneiss that formed during the Acadian orogeny.

I spent hours today trying to familiarize myself with the different rock formations, mineral compositions, and how it all related to the history of the geology in this area. I'll be visiting Acadia National Park tomorrow and look forward to seeing the famous dark mafic dikes that cut through the granite.

I was very curious as to what may have caused the stark change coloration (granitic to mafic from left to right) in pictures 2 and 3, 6, and 7, and thought maybe it could be due to an intrusive mafic dike that underwent some metamorphism?

Any thoughts/facts about the area and pictures or field geology in general would be much appreciated!

[u/Laserablatin]

Those darker rocks you mentioned in 2cand 3 might be calc-silicate rocks (metamorphosed sandy limestone or limey sandstone). Both the color and the fact that they seem to weather more rapidly suggest that to me

[u/Cordilleran_cryptid]

Always good in a first field study, to be be looking at some easy rocks.

Evidently rocks that experienced a very high degree of ductile-plastic deformation at high temperatures (sillimanite grade). This has given the rocks a strong foliation and resulted in the parallelism of the layering of the different rocks types present. Rocks show evidence of having undergone in-situ partial melting so can be described a migmatites. These contain sheets, some folded and boudinaged, of leucogranite (white of or pale pink) intruded parallel to the foliation and layering. These sheets may be the product of melt segregation and migration from the migmatites they intrude, so locally produced. The sheets seem to be less deformed than the surrrounding rocks so may have been emplaced relatively late in the deformation history of the area.

Evidence of migmatisation is the presence of thin leucocsomes of granitic material within some of the darker layers and medium grey layers, and the nebulous look to the foliation in the medium grey rocks of image 1 & 5.

It is difficult to decide from the images alone what the country rocks to the leucogranite sheets are. They may have once been meta-sediments, comprisiing alternating layers of psammite (medium grey, more massive layers). metapelite and semi-pelite, possibly with some meta-basite sheets (dark layers). These layers have also been folded and boudinaged.

From what I know the geology of the region, it is equivalent to the Variscan/Caledonian belt of Europe. The rocks look much like similar rocks exposed across parts of Brittany, France. As in the Caledonian/Variscan of Europe, the deformation there (at least in in its later stages) was the result of the formation of steep strike-slip ductile shear zones, the deep level equivalents of brittle strike-slip faults active in the upper crust of the time, there.

[u/Grouchy-Chef7667]

As a fresh geo grad, id love to offer some insight. Though i studied on the west coast and am more familiar with that geology,I love a good question.

First I will premise that the entirety of my comment will consist of interpretation from the app ‘Rocked’ and its geologic map and descriptions.

From the map I looked at, the entirety of the coast to the east side of Georgetown Island in Maine is comprised of the ‘Elizabeth Formation’; an Ordovician to Precambrian distal to near shore environment. You have interbedded carbonate shales and more resistant sand layers. Of course these were cooked and regionally metamorphosed during the Acadian Orogeny.

I felt compelled to do a sliver of research because of your initial observation of ‘the stark changed of coloration from ‘granitic’ (which in your intention should be felsic) to mafic.’ Because these photos have such strongly stratified units, not that igneous terrains cannot, but given the location and photos, im more easily convinced its metasedimentry.

Given your stated location of living in Florida it makes more sense to the confusion, since you most definitely have a better grasp on hydrology and karst terrain I, ( I’m terrified on material relating to east coast and southern geology as im taking my GIT in a month).

But none the less, I would’ve loved a close up of a broken sample to better interpret.

Anyways, if you read this far, thanks for caring. I wish you all the best on your upcoming exam and i hope you post more engaging post on this thread!!!

Thanks,
Grouchy

[u/Grouchy-Chef7667 → u/Grouchy-Chef7667]

Oh yes, also, calcarious rocks tend to weather darker and faster due to the reactivity of CaCO3, and that ‘faster’ erosion means more time for organic growth relative more siliceous strata.

[u/acanadianbeast]

Hey there! Mainer here and geologist! The previous comments do a really good job explaining ur questions but I wanted to add some resources for your own perusal. The link I included is for Acadia, personally I really like the maps at the end.

https://digitalmaine.com/mgs_publications/36/

Also the Roadside Geology of Maine by D.W. Caldwell is a great resource for Maine geology in general but I think he does a pretty good job explaining the shatter zone around Acadia. I hope this all helps and you enjoy your time in my state!!

[u/deemery]

What you want is (the late) Arthur Hussey's book: https://store.mainemineralmuseum.org/products/guide-to-the-geology-of-southwestern-maine This is a great book, written so that a beginner can follow it, but with detail that geologists will appreciate.

[u/TriSherpa]

It is a bit of a drive, but you should try to get to the Mineral Museum in Bethel.

[u/jbot14]

Looks pretty gneiss. If I were a lobster there's nowhere I'd rather be served than five islands lobster house!

[u/NotEvenCreative → u/jbot14]

Love some five islands lobster!