postglacial period, and relative sea- level fell to at least -40 m sometime 

 between 9000 and 11,500 years ago. 



During lowered sea-level, the ancestral Penobscot River flowed across 

 the subaerially exposed head of the bay and debouched into Middle Passage. 

 Organic-matter-rich mud from the river was deposited rapidly in remnant, gla- 

 cially scoured depressions in the lower reaches of Middle and West Passages 

 behind a shallow (<20 m water depth) bedrock sill across the bay mouth. East 

 Passage was isolated from the rest of the bay system and received only small 

 amounts of locally derived fine-grained sediments. 



During the Holocene transgression that accompanied the eustatic rise of 

 sea- level, the locus of sedimentation shifted to the head of the bay. Here, 

 heterogeneous fluvial deposits filled the ancestral valley of the Penobscot 

 River as base level rose, and the migrating surf zone created a gently dipping 

 erosional unconformity, marked by a thin (<2 m) lag deposit of coarse sand and 

 gravel. As sea- level continued to rise, a thin (<9 m) layer of acoustically 

 transparent muddy sediments acciomulated over a shallow platform in the eastern 

 half of the bay head. Graded sediments within this stratum began to accumu- 

 late early in the transgression, and they record both the decrease in energy 

 conditions and the waning influence of the Penobscot River at the head of the 

 bay. In contrast, relatively thick (up to 25 m) silty clays accumulated 

 within a subbottom trough in the western half of the bay head. This deposit 

 apparently developed late in the transgression after sea- level had reached 

 -20 m and after the westward transport of fine-grained sediments from the 

 Penobscot River had been established. 



During and since the late Holocene transgression of sea- level, waves and 

 currents have eroded, reworked, and redistributed Holocene sediments: 

 (1) atop the shallow margins; (2) within constricted channels; (3) around top- 

 ographic highs; and (4) over the shallow bedrock sill at the bay mouth. 



The variable distribution, characteristics, and thickness (0 to more 

 than 30 m) of Holocene deposits in Penobscot Bay primarily reflect: (1) the 

 irregular glacially eroded bedrock topography beneath the bay; (2) the paleo- 

 geography of the bay during the sea-level lowstand; (3) the postglacial loca- 

 tion of the ancestral Penobscot River; and (4) the wave and current regime 

 during and since the Holocene sea-level transgression. (Author). 



171 KOLLA, v., and MACURDA, D. B. , JR. "Sea-Level Changes and Timing of 

 Turbidity-Current Events in Deep-sea Fan Systems," Sea-Level Changes: An 

 Integerated Approach . Special Publication No. 42, Society of Economic 

 Paleontologists and Mineralogists, Houston, TX, pp 381-392. 



Although lowstands of sea- level greatly favor the development of deep- 

 sea fan systems, the timing and type of turbidite events in these systems may 

 depend not only on the sea- level changes but also on the nature of available 

 sediments, tectonic setting, size, and gradients of the basins. Thus, in 

 basins with steep gradients, located on continental or transitional crust or 

 along active margins close to sedimentary sources that could supply coarse- 

 grained sediments, unchannelled turbidite sand lobes and channel levee 

 complexes might have been deposited during sea- level rises in these basins. 

 In contrast, large fan systems, such as the Indus Fan, located off passive 

 margins in oceanic basins with flat gradients, distant sedimentary sources, 

 and a predominantly fine-grained sediment supply have channel -attached lobes 



80 



