Abstract: Regressive Sedimentation Model for James Bay Lowlands, Ontario, Canada

I. Peter Martini

The Ontario portion of the James Bay Lowlands is characterized by a vast extension of wetlands and wide flat coasts. The land has been subjected to active rebound for the last 8,000 years, since deglaciation, and it still is rising at an approximate rate of 1 m per 100 years. Rebound, shallow tidal flats, promontories, and bays that have developed as a consequence of bedrock structures and differential glacial erosion and deposition, coupled with anticlockwise marine currents, strong storms, ice rafting, and ice pushing have determined a rather unique sediment distribution along recent and ancient coastlines. Furthermore, the shallow waters of the bay are brackish, and essentially fresh near estuaries of the entrenching rivers that drain the lowlands (no delta develops al ng this coast), and the fauna of the tidal flats are characterized by a relatively large but restricted molluscan population.

The marine sediments of James Bay are characterized by a substratum of fossiliferous blue clay that was deposited in an early postglacial sea (the Tyrrell sea), absent only on local Paleozoic bedrock or Pleistocene high. The distribution of more recent sediments is characterized by areas of bypass where thin (order of a few centimeters), very poorly sorted, pebbly, silty clay overlies bedrock of Tyrrell sea clay; tidal-flat areas where abundant sand deposition occurs which is characterized by rather uniform texture, a variety of ripple marks, and shallow (order of 20 cm) and wide (order of 10 m) sand waves; and storm deposits that form beach ridges ranging from simple sand and gravelly sets of bars migrating inland in the upper parts of the flats, to gravelly more stable composite lon itudinal ridges and transverse ridges. The upper tidal flats and marshes are characterized by thin (few centimeters and millimeters) clay and silt drapes stabilized by either algal mats or grasses, over middle-flat sediments. Peat covers inland areas and thickens progressively (up to 2 to 2.50 m) in older parts of the lowlands.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma