麻豆传媒

Located on the east coast of Labrador, Makkovik Bay is a low relief, 35 km-long, complex fjord. Oceanographically, it is a nonstagnant, well stratified, partially-mixed-type estuary during the summer, probably with moderate sea water exchange. The degree of stratification appears to be related to fluvial flux and tidal mixing which is attenuated by sills and bay mouth constrictions. Low values of suspended sediment and high rates of sediment accumulation by sediment traps suggest that the contemporary environment is wave-dominated, resuspending marginal sediments with deposition in the deeper basins.

An original technique using morphology and character of reflection is capable of synthesizing 3.5 kHz acoustic analogue data. Three major acoustic units are recognizable within Makkovik Bay: lower basin-fill, conformable cover and upper basin-fill units. The lower basin-fill unit underlies the conformable cover unit in the deeper, quieter basins and is thought to be composed of coarse to muddy clastics deposited under relatively high energy conditions, probably during stages of postglacial transgression. The age of the oldest sediments is extrapolated to be approximately 19,000 years B.P.

The conformable cover unit is an acoustically well stratified unit found mantling the entire bay. It outcrops along topographic highs and flanks of high energy bays and along the nearshore in moderate energy bays. Short cores and grab samples penetrate an upper unit of stiff, homogeneous, olive gray, clayey silt and a lower unit of alternating brownish gray and olive gray beds with thin basal silt laminae. The distribution of benthonic foraminifera suggests a high rate of sedimentation in a cold, open bay type environment. Detailed analyses of the clay and clay-size minerals show prominent differential sedimentation, concentrating illite approximately and feldspar distally. The conformable cover unit is interpreted to have had runoff-dominated deposition, occurring during the last major transgression and terminating at approximately 10,000 years B.P. Glacial detritus was transported fluvially northwards along the Makkovik Brook Valley and distributed across Makkovik Bay within the upper fresher water layer of a well stratified two-layer type estuary. Rapid deposition occurred through the lower, more saline water layer.

The upper basin-fill unit is characterized by poor acoustic reflectivity and a ponded morphology with a variable degree of flank-onlap which is inversely related to wave energy. The surficial sediments include a nearshore coarse veneer (unit I), a basinal, organic rich, grayish-olive clayey silt (unit II), a lower-flank, coarse grayish-olive mud (unit III) and a stiff, gray clayey silt (unit IV) which corresponds to the conformable cover unit. Two piston cores penetrate 4 major facies that represent a transition from runoff-dominated to contemporary wave-dominated deposition during regression and decreasing fluvial flux. The lithologies change upwards from alternating olive gray and yellow brown through gray brown to dark grayish olive coloured muds. The sedimentation rate decreases from approximately 0.3 to 0.07 cm/year. The brackish to open marine type foraminifera are followed by agglutinated, cold water estuarine types. The dominant clay trend shows an upward decreasing illite-feldspar ratio suggesting further nearshore concentration of illite during resuspension. The transition to wave-dominated deposition occurs earlier in the Approaches than the Outer Bay; it is controlled primarily by runoff and relative sea level.

An erosional unconformity occurs above the base of the upper basin-fill unit, approximately at 6,000 years B.P. This event is chronologically correlatable with a resurgence in runoff.

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Pages: 282
Supervisor: D. J. W. Piper / H. B. S. Cooke