Towards a glacial subdivision of the Ediacaran Period, with an example of the Boston Bay Group, Massachusetts

Abstract

After the Elatina glaciation of Snowball Earth, at least four distinct glacial advances and sea-level retreats punctuated Ediacaran time: Gaskiers glaciation (580 Ma), Fauquier glaciation (571 Ma), Bou-Azzer glaciation (566 Ma) and Hankalchough glaciation (551 Ma). Tillites or diamictites are commonly controversial, but periglacial paleosols with distinctive physical structure and degree of chemical weathering offer supporting evidence of glaciation and sea-level change useful for stratigraphic correlation. This paper reviews glacial advances of the Ediacaran stratotype and other sequences, and also reveals the value of paleosols and chemical index of alteration to understand the upper Squantum and Brookline members of the Roxbury Conglomerate near Boston, Massachusetts. The Boston Bay ice wedges are periglacial paleosols, and evidence of maritime glacial climate like that of modern coastal Greenland and Arctic Canada. Simple discoidal vendobiont fossils (Aspidella terranovica) in the Dorchester Member of the Roxbury Conglomerate and in the Cambridge Argillite are in heterolithic shale–siltstone facies that are interpreted as intertidal to shallow marine environments. Local marine transgressions and other paleosols showing significant chemical weathering represent temperate interglacial paleoclimates. Short glacial advances affecting climate and sea-level enable subdivision of the Ediacaran Period.

KEY POINTS

1. Four distinct glacial advances and sea-level retreats punctuated Ediacaran time: Gaskiers (580 Ma), Fauquier (571 Ma), Bou-Azzer (566 Ma), Hankalchough (551 Ma).

2. Paleosols with distinctive structures such as ice wedges were periglacial.

3. Squantum Member diamictites near Boston, Massachusetts are Gaskiers age.

Keywords:

• Paleosol
• ice wedge
• Ediacaran
• Squantum Member
• Boston Group

Acknowledgements

Richard Bailey organised a revealing excursion to Boston Bay in 2001, highlighting controversy concerning paleoenvironment of the Squantum Member, and leading to useful discussions with Nathan Sheldon, Bruce Runnegar, Hans Hofmann, Nicholas Christie-Blick, Ben Waggoner and Whitey Hagadorn. Meg Thompson provided stratigraphic data and much helpful discussion. Paul Wallace and John Donovan generously allowed use of photomicrographic equipment. Thoughtful reviews by Sandra Passchier, Henning Dypvik, Nora Noffke, A. Hope Jahren, Mark McMenamin and Vic Gostin greatly improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author.