Allostratigraphic interpretation of a modern coarse clastic barrier complex: depositional facies, processes and relative sea level relationships

Davis, Laurence H. M. (2003) Allostratigraphic interpretation of a modern coarse clastic barrier complex: depositional facies, processes and relative sea level relationships. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis presents an allostratigraphic and facies interpretation of the modern coarse clastic Flat Island barrier complex, situated on the tectonically-structured southern margin of St. George's Bay, western Newfoundland, Canada. The study is based on an integrated geological, geophysical and oceanographic dataset, including aerial photography, multibeam sonar bathymetry data, shallow reflection seismic data, seabed samples, seabed video, wave and current measurements, cores, and pit and outcrop observations. -- The Flat Island barrier complex evolved under the influence of a cyclic relative sea level regime associated with post-glacial eustatic sea level rise and superimposed isostatic uplift. The stratal architecture of the barrier complex reflects the interrelationships of eustatic rise, isostatic uplift, basin physiography, and local sediment supply variations. The barrier complex comprises a four-part stratigraphic succession (Units A - D) delineated by key bounding discontinuities (BD1 - BD5). -- The lower-most marine allostratigraphic unit (A) consists of aggradational, fine-grained glaciomarine sediments deposited during late-glacial ice recession, which was accompanied by marine onlap of isostatically depressed terrain. Unit A is bounded below and above by "initial" and "maximum" transgressive surfaces, respectively (bounding discontinuities BD1 and BD2). -- Maximum coastal onlap (~13.5 kyBP) was followed by forced regression, as isostatic uplift ensued and exceeded the rate of ongoing eustatic sea level rise. Forced regression led to the development of a subaerial unconformity (BD3) as fluvial channel systems sought progressively lower base levels, locally incising the maximum transgressive surface (BD2). -- Isolated delta bodies (Unit B), graded to (present-day) elevations of approximately +26m to -25m, are interpreted as accretionary forced regression deposits. Delta growth occurred during periods of slow forced regression (possibly related to eustatic pulses), accompanied by high rates of glacio-fluvial sediment supply. The subaerial unconformity (BD3) truncates and incises the top of Unit B delta deposits, which downlap the marine maximum transgressive surface (BD2). -- A "lowstand-stillstand" occurred as the rate of isostatic rebound diminished and became equal to the rate of eustatic sea level rise (~9.5 kyBP). The subaerial unconformity (and correlative conformity) at lowest base level are overlain by aggraded fluvial channel fill and delta front deposits (Unit C). Unit C displays stratigraphically climbing delta front clinoforms, consistent with normal regression during stable to rising relative sea level. The Unit C deposits are interpreted to mark the turnover from relative sea level "fall" to relative "rise". The top of Unit C defines a maximum regressive surface (BD4). -- A regional transgressive surface (BD5) developed as Holocene eustatic sea level rise became dominant, and is characterized by widespread shoreline and wave base ravinement, with erosion of up to 20m of vertical section. Below present sea level, preserved elements of the subaerial unconformity (BD3) are restricted to deeply incised channel systems. -- High rates of littoral sediment supply combined with favourable basin margin physiography promoted the formation and seaward progradation of the modem barrier - shoreface complex (Unit D) during regional relative sea level rise. Shoreface progradation beyond the (pre-existing) slope margin results in ongoing, episodic retrogressive slumping and turbidity flows. Hydrodynamic monitoring and modeling indicate that sediment transport on the barrier shoreface is storm-driven, and dominated by high magnitude, low frequency events occurring at quasi-decadal time scales. -- Conglomeratic barrier deposits overlying the sandy shoreface sediments consist of discordant sets of aggradational to progradational beach ridges with intervening tidal swales, and local washovers. The barrier fronts an estuarine embayment that is being progressively infilled by overwash and bayhead delta sediments. Flat Island barrier is responding to ongoing regional transgression through processes of episodic shoreline erosion, lateral accretion, washover, and in-place drowning. -- This thesis research was motivated by the challenges faced by academic and industry researchers when attempting to interpret the genetic origins and depositional environments of ancient marginal marine coarse clastics. The study provides valuable insights regarding coarse clastic facies architecture, depositional processes, and relative sea level relationships.

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