Lmst

A bizarre #sedimentary feature

Some layers in the #Neoproterozoic #Squantum Member of the #Roxbury #Conglomerate have been rolled up like a pie crust, into a tube. If this were a #tillite, as it once was thought to be, then it could have been caused by the drag of a #Gaskiers #glacier over the till. These rocks are mostly interpreted as deposited by submarine debris flows these days, in which case they could’ve been deformed during a slump and flow. Site of a field trip for #UMassBoston.

The circular form at the center of this color digital photograph is a bit of rolled up sandstone, with relict layering expressed like the layers of an onion in this exposure. Blue key ring usb drive and key at top right for scale.

A small #outcrop of the #Squantum Member of the #Roxbury #Conglomerate

These #rocks are in the #BostonBay Group, which is currently dated to #Neoproterozoic. The Squantum Member was interpreted as a #tillite, possibly associated with the #Gaskiers #glaciation, although there a scant signs like scratched pebbles or faceted cobbles. It is currently commonly interpreted as a #debrite (debris flow). Details in captions.
#geology #glacier #turbidite #Quincy #Boston #Massachusetts #NewEngland

Google Eart view of Squantum, Quincy, immediately southeast of Boston, Massachusetts USA. The small outcrop examined here is near the red pin. The “classic” Squantum cliff and beach exposures are near the pin for Myles Standish Cairn.

My photo of a pice of float, not attached to bedrock. I interpret depositional up is at the bottom of this photo, and it is reverse graded as a debris pulse flowed over fine-grained sediments deposited in quieter conditions. Cell phone for scale.

My photo of a piece of float, probably Squantum Member. There is compressive metamorphic foliation expressed as squashed clasts; compression is perpendicular to the pencil.

Abrupt change in grain size in Squantum Member, depositional up is interpreted to be top right.

Anomalous pebble in layered #sedimentary #rock

An angular pebble is conformably surrounded by argillaceous and sandy layers in this #photograph of the #Neoproterozoic #Squantum Member of the #Roxbury Conglomerate. The wide range of clast sizes led to an interpretation as a #glacial #tillite thru the 1970s, but the Squantum Member is now mostly interpreted as a #debrisFlow (#debrite or #turbidite). My #photo.

#Boston #fieldTrip #UMassBoston

In this color photo, a greenish circa 2 centimeter pebble at the center is bounded by distorted quasi parallel bedding below and above. Image height is about 10 cm. White balance card at right has an inch scale visible. My photo.

Deformed bedding in meta#sedimentary #rocks

The horizontal bedding of the #Squantum Member of the #Roxbury Conglomerate (#Neoproterozoic, #Boston Bay Group) has been disturbed locally.

In the #glacial #tillite interpretation of the Squantum (favored through at least the 1970s), the distortion might be caused by glaciotectonic drag at the ice base. The current interpretation as a #debrite or #turbidite would probably attribute the deformations to synsedimentary slumping. My #photo. #fieldTrip

This color photo show a circa 12 cm vertical extent of sedimentary rocks. In the upper part of the image, beds are inclined upward to the left. Near the center of the image, a bed has been rolled over itself. These features could have been caused by sediment slumping leftward, possibly in a debris flow. There is a white balance card with inch and centimeter scales at the bottom left. My photo.

Rapid changes in #clast size in #sedimentary #rocks

The #Neoproterozoic #Squantum Member of the #Roxbury Conglomerate (#Boston Bay Group) transitions from sandstone, to almost-clast-supported conglomerate, to laminated argillite…in about 25 cm. Fifty years ago, it was interpreted as a #glacial deposit and called the Squantum #Tillite, presumably associated with the #Gaskiers Glaciation. Paucity of evidence (e.g., few scratches or faceted clasts) suggests a debris flow. My #photo.

A circa 25 cm high image shows a sandstone at bottom, conglomerate with angular clasts in the middle, and laminate argillite at the top. A white balance card with cm and inch scales is at the bottom left. My photo.

The #glacier went that-a-way

The #striations on this face of the #Ediacaran (ca. 560 Ma) #Squantum Member of the #Roxbury #Conglomerate were probably left by the #Pleistocene #Laurentide ice sheet, which retreated ca. 12 ka. The striations have the right orientation, from northwest to southeast.

The Squantum Member was once interpreted as a (#glacial) #tillite. The most common contemporary model is a peri-#Gondwana #debrite (#turbidite).

My #photo. #Boston #geology #NewEngland #fieldTrip

#tillite

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