The sedimentology of the Bloomington fan complex: an element of the Oak Ridges Moraine, Southern Ontario

摘要

The Oak Ridges Moraine is a major physiographic feature of south-centralOntario, extending from Rice Lake westward to the Niagara Escarpment. Whilemuch previous work has largely postulated a relatively simple the origin of themoraine, recent investigations have concentrated on delineating the discernibleglacigenic deposits (or landform architectural elements) which comprise thecomplex mosaic of the Oak Ridges Moraine. This study investigates thesedimentology of the Bloomington fan complex, one of the oldest elements of theOak Ridges Moraine.The main sediment body of the Bloomington fan complex was depositedduring early stages of the formation of the Oak Ridges Moraine, when the icesubdivided, and formed a confined, interlobate lake basin between the northern andsouthern lobes. Deposition from several conduits produced a fan complexcharacterized by multiple, laterally overlapping, fan bodies.It appears that the fans were active sequentially in an eastward direction,until the formation of the Bloomington fan complex was dominated by the largestfan fed by a conduit near the northeastern margin of the deposit. Followingdeposition of the fan complex, the northern and southern ice margins continued toretreat, opening drainage outlets to the west and causing water levels to drop in thelake basin. Glaciofluvial sediment was deposited at this time, cutting into theunderlying fan complex. Re-advancing northern ice then closed westerly outlets,and caused water levels to increase, initiating the re-advance of the southern ice. Asthe southern ice approached the Bloomington fan, it deposited an ice-marginalsediment complex consisting of glacigenic sediment gravity flows, andglaciolacustrine and glaciofluvial sediments exhibiting north and northwesterlypaleocurrents. Continued advance of the southern ice, overriding the fan complex,iiproduced large-scale glaciotectonic deformation structures, and deposited theHalton Till.The subaqueous fan depositional model that is postulated for theBloomington fan complex differs from published models due to the complex faciesassociations produced by the multiple conduit sources of sediment feeding the fans.The fluctuating northern and southern ice margins, which moved across the studyarea in opposite directions, controlled the water level in the interlobate basin andcaused major changes in depositional environments. The influence of these twolobes also caused deposition from two distinct source directions. Finally, erosion,deposition, and deformation of the deposit with the readvance of the southern icecontributed further to the complexity of the Bloomington fan complex.