Recent sedimentological onterpretations in the Avalon terrane of the Boston Basin, Massachusetts

The depositional history of the Boston Basin remains somewhat enigmatic and controversial despite nearly over a century of research. Resolution of the basin's geologic history has remained formidable and complex due. In part, to attempts to work within the existing stratigraphy which often carried with it historic interpretations, many of which went uncontested. More importantly however, our knowledge of depositional systems and mechanisms has Increased significantly over this span of time. Reinterpretatlons In light of these advances were not only Inevitable but neccessary. As a consequence of the circumstances described above, our reinvestigation into the depositional history of the Boston Basin succession, principally the Boston Bay Group, has centered on recognizing, measuring, and coding a suite of facies whose textures In many Instances can be closely linked to specific depositional mechanisms responsible for the emplacement of the facies in question. From this facies analysis a preliminary depositional model for the Boston Basin has begun to emerge. It appears that the Boston Basin originated as a rifted successor, or arc-related basin either just prior to the closure of the Cadomlan Ocean or during the opening of Iapetus. The early history of the basin Is marked by the presence of a suite of bimodal volcanics In the form of water-lain tuffs, dikes, sills, and flows, and coarse debris flows. Geochemlcal markers suggest that the basin was In contact with the open ocean very early In its history. The next stage In the evolution of the Boston Basin was marked by the development of a rapidly prograding submarine elope/fan succession. The slope/fan Betting appears to have been characterized by the deposition of ice frontal and/or ice-rafted dlamictons which periodically overloaded the slope Initiating sediment gravity flows which reworked and redistributed sediment downslope. During periods of more equable climate and/or tectonic quiescence, blankets of fine sand and mud were deposited. There Is no evidence of a shelf or shelf-type deposits during this time. The last stage in the evolution of the basin was the appearance of shelf sediments such as quartz and calcareous sands, and muds with abundant organic matter and shelled organisms. There Is also evidence for climatic warming which is suggested to have been brought about by a tectonlcally-forced rise In eustatic sea-level. RESUME L'hlstoire sedlmentalre du Bassln de Boston Bouleve encore quelques enigmes et controversy et, cela, apres pres d'un slecle de travaux. Dans une certain mesure, la resolution de l'histoire geologlque du bassln est demeuree formidable et complexe par suite des tentatlves de travalller au sein de la stratigraphie actuelle qui. elle, comportalt souvent des interpretations hlstoriques rareaent mises en doute. Or, plus important est le bond en avant saislssant qu'a fait notre connalssance des systemes et mecanlsmes de depot durant cette periode. Il devint done non seuleoent inevitable. mala bien necessaire de reconsiderer lea conceptions traditionnelles a la lumiere de ces acquis. Pour ce falre, notre reexamen de l'hlstoire sedimentalre de la succession du Bassln de Boston, et surtout du Groupe de Boston Bay, s'est axe sur la reconnaissance, la mesure et le codage d'une suite de facies dont les textures sont souvent reliees de facon intime aux mecanismes de depot distincts responsables de la raise en place desdits facies. Cette analyse faclologique commence a engendrer un modele preliminaire du depot dans le Bassln de Boston. Il semble que le Bassin de Boston alt debute sous forme d'un rift successeur ou d'un bassln allie a un arc solt juste avant le serrage de l’Ocean Cadomien, solt lors de l’ouverture de L'Iapetus. L'hlstoire du bassin est d'abord marquee par la presence d'une suite de volcanites blmodales sous formes de tufa aquatiques, dykes, filons-couches. coulees et coulees de debris. Les marqueurs geochimiques suggerent que le bassin etait ouvert sur le large des l’amorce de son hlstoire. Le stade suivant dans l’evolution du Bassln de Boston fut marque par le developpement d'une sequence de pente et cone sous-marlns a progradation rapide. L'environnement de pente et cone semble avoir ete carscterise par le depot de diamictons glaciaires frontaux et/ou supraglaciares qui ont surcharge periodiquetoent la pente, declencbant alnsi des ecoulements gravitalres qui ont repris les sediments et les ont redistribue en bas de pente. Les periodes de climat plus uniforme et/ou de stase tectonique donnerent lieu a des epandages de sable fin et de boue. On ne trouve pas trace d'une plate-forme ou de depots typiques d'une plate-forme lors de cette periode. La derniere phase evolutive du bassin correspond a L’apparltlon de sediments de plate-forme tels que des sables quartzeux et calcaires alnsi que des boues riches en raatiere organique et en organlsmes coquilllers. Certains Indices temoignent d'un rechauffement climatlque que l'on croit du a une elevation eustatlque du niveau marin en reponse aux sollicitations de la tectonique. [Traduit par le journal]