Holocene Evolution of Boston Inner Harbor, Massachusetts

Peter S. Rosen, Benno M. Brenninkmeyer, Lynn M. Maybury

Abstract


Recent building excavations in the previously landfilled Back Bay section of Boston, Massachusetts, permitted direct observation of the Holocene estuarine stratigraphic sequence. The environmental changes that occurred during this period were a concomitant decreasing submergence rate and an increasing energy level. While the diminishing submergence rate is well-documented in the regional stratigraphy, it is believed that this sequence reflects the growth of tidal resonance in the Gulf of Maine system, resulting in a change from microtidal to mesotidal conditions. Three major estuarine depositional units define the changing conditions at this site. The Lower Unit reflects rapid deepening and low energy depositional conditions; the Middle Unit marks sufficient tidal prism to lead to the formation of tidal channels; while the Upper Unit, with coarser material and tidalities, reflects higher energy during deposition.

 

The 5 m thick estuarine sequence unconformably overlies a weathered surface of the Boston Blue Clay, a glacio-marine meltwater deposit (approximately 14,000 yr BP). The Lower Unit (5,630 to 3,570 yr BP) defines the beginning of the Holocene submergence of Boston Inner Harbor with a sequence grading from intertidal saltmarsh to subtidal laminated muds. Average accumulation rates of 1 mm/yr roughly correspond to the thickness of laminations, suggesting an undisturbed annual deposit. The Middle Unit (3,570 to 3,090 yr BP) is interpreted as channel deposits based on numerous channel-bottom shell-lag layers. Initial average accumulation rates of 11.7 mm/yr diminish to 1.3 mm/yr at the top of the unit. The Upper Unit is a well-mixed sandy silt with occasional interfingered tidalities. Initial accumulation was 3 mm/yr, and diminished to 0.5 mm/yr at the top of the sequence. This is interpreted as a tidal flat environment, corresponding to historical records showing tidal flats and salt marsh in the area.

 

The stratigraphy concurs with archaeological observations at the Boylston Street Fishweir, a 4,400 yr old series of structures built and used over a several-hundred year period. The exposure of the weir is located in the Lower Unit of this sequence. Despite the collection of over 65,000 wooden stakes (parts of the weir) since it was discovered in 1913, no other artifacts have been found associated with it. Since modern fishweirs in New England, which have similar construction, are maintained by foot at low tide, this appeared inconsistent. However, as this was a microtidal setting at that time, the fishweir was probably a subtidal device maintained by canoe rather than on foot, as in the Chesapeake Bay during historic times.

 


Keywords


Estuarine sedimentation; Boylston Street Fishweir; laminated muds stratigraphy

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