Paper No. 229-15
Presentation Time: 9:00 AM-6:30 PM
REANALYSIS OF DEPOSITIONAL MECHANISMS, MICROFACIES SEQUENCES, AND EVENT STRATIGRAPHY OF SQUANTUM DIAMICTITES AND ASSOCIATED STRATA IN THE EDIACARAN BOSTON BAY GROUP
Reanalysis of the Neoproterozoic (Ediacaran) Boston Bay Group on the Squantum Penninsula (Quincy, MA) with a particular focus on the spectacular coastal outcrops of Squantum Head is based on new high resolution mapping combined with detailed measurement and description of stratigraphic sections. Bedding characteristics, sedimentary structures, and microfacies architecture is displayed in digitally rendered stratigraphic panels. Careful mapping and evaluation of faults that have been omitted or overlooked in most prior published stratigraphic analyses permitted adjustment and proper measurement of stratigraphic sections. Interpretation of sedimentologic data indicates submarine gravity mass transport was the primary mechanism of formation of coarse clastic facies. These processes resulted in a spectrum of interbedded matrix and clast supported conglomerates. Intercalated within the conglomerates are graded pebbly grannulestones and sandstones transported by turbidity currents or as other sorts of cohesionless mass flows. Most of the fine sediment in the matrix of diamictites was introduced by admixing slope or basinal deposits during transport of proximally derived extrabasinal clast supported gravels and very coarse conglomerates. Evidence of this mixing is well displayed at the basal contacts of diamictites. The two main diamictite horizons are underlain and separated by thick successions of very thinly laminated sand-silt turbidites with slump folded intervals up to 2m thick. One debrite within slope or basinal turbidites contains deformed slope derived intraclasts and olistoliths up to 4m in size. We interpret the main diamictite sequences to represent re-deposited proximal coarse clastics formed during sea level lowstands. Thick sequences of thinly laminated mudstone to sandstone and interbedded slumped strata and intraclast-rich debrites represent deposition during sea level highstands.