ARCHITECTURE, STRATIGRAPHY AND SEDIMENTOLOGY OF A FJORD-HEAD DELTA, ZEBALLOS, BRITISH COLUMBIA

We conducted an integrated geophysical and geological investigation of a fjord-head delta at Zeballos, approximately 300 km northwest of Victoria, British Columbia. The objective of the study was to improve understanding of sedimentation in high-energy fjord-head deltas. A ground penetrating radar survey revealed the presence of two deltas, one inset within the other, which formed as sea level fell during and after deglaciation. Both deltas comprise topset, foreset and bottomset facies. An electromagnetic survey, drill hole data, and gravel pit exposures further show that the delta deposits coarsen upwards, from silt in the bottomset beds to sand and gravel in the foreset and topset beds.

The uppermost metre of the seaward part of the modern delta, which is less than 500 years old, is divisible into five units; these are, from bottom to top: 1) distributary channel gravel; 2) distributary channel sand; 3) interbedded organic-rich silt and sand; 4) middle intertidal, organic-rich sandy silt; and 5) upper intertidal peat. The vertical succession of sedimentary units in section is repeated in the present-day marsh, landward to the upper limit of tides. Sequential airphoto analysis indicates that the distribution of units 1, 2, and 3 in the marsh is related, in part, to human-induced changes to the main channel of Zeballos River.

An unusual layer of silt and sand is present throughout the Zeballos marsh, directly below sediments that have Cs¹³⁷ activity. The unit ranges from a few millimetres to eight centimetres thick, and from a single homogeneous layer to five silt-sand couplets. It increases in thickness away from river channels and in a landward direction to the upper limit of the marsh. The sediments contain elevated levels of gold, silver, mercury, arsenic, lead, and zinc, as well as freshwater diatoms, and are interpreted as mine tailings discharged into Zeballos River between 1934 and 1947.