Title

Mapping of Holocene Reefs in Southeast Florida (USA) by Remotely Sensed Optical and Acoustic Data Together with in situ Techniques

Event Name/Location

2002 Ocean Sciences Meeting

Document Type

Conference Proceeding

Publication Date

2002

Keywords

Data Processing, Remote Sensing, Seafloor Morphology, Seafloor Geology, Seafloor Geophysics, North America

Abstract

Using a combination of laser based (LIDAR, LADS) and multibeam sonar bathymetry, acoustic and optical remote-sensing, together with in situ observations, the Southeast Florida (Broward County, USA) subtidal was investigated to over 200 m depth. Five well developed and sequentially deeper coast parallel ridges were mapped. An inshore ridge at approximately 5 m depth may be a lithified shoreface or tidal bar. The next two at average depth of 10m and 20m respectively have their genesis as Holocene reef lines with Acropora palmata frameworks which terminated at roughly 5ky and 7ky uncorrected for radiocarbon age. Two deeper ridges were mapped for the first time, one at 45m depth the other at 85m depth. These may be drowned reefs formed during the Holocene transgression. The upper three reefs are separated by sandy area and, in some regions, wide rubble beds.

Both laser and multibeam bathymetry were used to delineate the hardground and sandy areas. Single beam acoustic seafloor classification using QTCView 5 based on a 50 kHz signal was used to differentiate further between sand, solid rock, and rubble areas. Ground-truthing of remote-sensed interfaces of sand/rubble/rock ("reef edges") employed divers on scooters that towed a WAAS DGPS in a surface buoy allowing the divers to mark waypoints.

Results indicated that: (1) aerial imagery was not suitable for bottom classification due to depth, shading effects, and turbidity; (2) a combination of laser bathymetry and acoustic bottom classification differentiated sand from reef from rubble; (3) continuous, Holocene barrier reef systems exists along the entire length of Broward County; (4) rubble beds are discontinous and likely represent a leeward halo of reef-derived debris formed while the reef grew at or near sea-level; (5) inshore hardgrounds are often repeatedly buried and uncovered by sand; (6) the sand/high-relief-reef interfaces are relatively stable; (7) two deeper unknown hardground ridges, likely drowned Holocene reefs, were mapped for the first time.

Comments

Abstract #OS02-OS21F-116

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