MALLOWS BAY

GEOLOGY

MARYLAND U.S.A

 

Mallows Bay is located in a Semiconsolidated sand and gravel aquifer, the sediment expands in a wedge that drops and thickens coastward to the Atlantic Ocean. The aquifer is associated with the Atlantic Coastal Plain which expands in depth from the Appalachian Piedmont physiographic Province to the continental shelf.  

The aquifers formed from, deltaic, fluvial, and shallow marine deposits, over a few million years. The aquifer systems expand for 50,000 square miles along the Northern section of the Atlantic Coast, this section is called the Northern Atlantic Coast Aquifer System. 

During the Paleocene‐Eocene Thermal Maximum (PETM) the Atlantic Ocean stretched inland up against the Piedmont physiographic Province, the planet heated up by 5 to 8 °C during that time which is why the sea level increased inland so drastically. The Aquia Formations last million years of deposition was during the thermal maximum 56 million years ago. 

  

ROCK LAYERS

All the Formations present within Mallows Bay are bordered by unconformities above and below. The oldest formation is the Aquia Formation (Upper Paleocene) and the youngest is Holocene Deposits (10,000 years to current).  

AQUIA FORMATION

The Aquia Formation was deposited during the Upper Paleocene

(59-56 million years ago). The formation is comprised of glauconitic (Green iron-potassium Phyllosilicate, has a low weathering resistance in the mica group) to fine and coarse-grained quartz that's olive-black to olive-grey and tends to fracture in sheet-like structures. Beds of sand and olive-grey silty clay and very fine sandy silt are also present in the formation. The fine to medium-grained sand has most likely been cemented by Calcite and/or Dolomite and has moderate to poor sorting. 

The above Marlboro Clay is represented by a sharp lithologic boundary, although the above clay layer isn't visible in Mallows Bay as it's been weathered out and replaced with newer Holocene sediment. At its base well-rounded quartz, sand grains, and pebbles are visible and most likely originated from the below Potomac Group during the transgression of the Aquia Sea as the Atlantic reached inland towards the Piedmont physiographic province during the Eocene. The formation itself was deposited within a middle to shallow shelf during the Paleocene‐Eocene Thermal Maximum (PETM).

The formation thickens from the Potomac River 30 feet (9 meters) to 200 feet (61 meters) at its highest point within the Waldorf vicinity, the formations texture is more coarse as it gets thicker towards the northeast as the formation also dips in a southeastward direction at 15-20 feet per mile. 

 

The formation is split into two members (Virginia only) 

 

Piscataway Member

. The lower Piscataway member is Paleocene in age and has poorer sorting and more visible bedding. 

Paspotansa Member

. The above Paleocene Paspotansa Member is more calcareous and contains zones of nannofossils, the formation contains missing zones which are reflected in a bed with sand at its base. The formation crops out along the Potomac River, and both members correlate with each other through biostratigraphy. The upper Paspotansa member is what's most likely visible at Mallows Bay.   

 

MARYLAND POINT FORMATION

The Upper Pleistocene (126,000 to12,000 years in age) Maryland Point Formation is comprised of fine-to coarse-grained sand, well-sorted to poorly sorted grey-orange sand within the upper third of the formation. The lower section of the formation is poorly sorted silty grey-olive clay, at its base its olive-grey with pebbly sand. The clay contains plant fragments. Majority of the sediment has been reworked from adjacent highlands and has been dated to 70,000 years in age.

 

The formation contains oyster beds within a sandy-mud matrix at 2-10 feet (1-4 meters) above the sea level around bluffs east and west of Maryland Point. Areas with wavy-bedded white and orange sand extend kilometres between Nanjemoy Creek and the Prince George county line, these areas are often broken by slumping. Its base is an unconformity with Miocene sediments as more olive-grey pebbly sand is visible which marks a gap in time.

 

The formation is usually 25-40 feet (8-14 meters) thick, the upper contact lies at 30 feet (9 meters) and its lowest contact is at 0-10 feet (0-3 meters) and sits below sea level.      

HOLOCENE DEPOSITS

The youngest deposits in the Mallow's Bay area are comprised of loose, non-cemented sediment that lies adjacent to modern wetlands and waterways. Poorly sorted sand and gravel, well-sorted sand, silt, and clay along with coarser sediment which are tan in colour, dark green, and black due to locally reworked glauconite. In areas where the deposits are undercut local flora will collapse from the top of the cliffs, this makes the areas very dangerous.

These deposits were formed in tidal marshes, rivers, and swamps, Thickness ranges from 2-20 feet (1-6.5 meters) but might possibly be thicker in other unmapped channels.   

 

Several locations along the Potomac and Wicomico waterways also contain brackish water clamshells and locally abundant lenses of shells which have been reworked from older Paleocene deposits. 

 

RIVER GRAVEL SYSTEMS 

River Gravel is deposited when the water's velocity slows down, rocks and pebbles are the first to settle on the floor of the river as it's the heaviest. Much later in the process the lighter silt and clay get deposited as the water needs to be near still for that sediment to settle.

The gravels will most likely cement into conglomerate in a few million years from now as more layers of pebbles and rocks will accumulate, sections of the Aquia Formation are also collapsing into the shallow water and will most likely cement mixed in with the conglomerate.    

REFEreNCES

Lucy McCartan, Department of Natural Resources, & Maryland Geologic Survey. (n.d.). National Geologic Map Database: Charles County (1989). United States Geologic Survey (USGS). 

USGS Groundwater Information. (n.d.). Semiconsolidated Sand Aquifers. 

USGS Groundwater Information. (n.d.). Northern Atlantic Coastal Plain aquifer system extent. 

Jean M., Marci M. Robinson, Timothy J. Bralower, Jocelyn A. Sessa, Elizabeth A. Hajek, Lee R. Kump, Sheila M. Trampush, Debra A. Willard, Lucy E. Edwards, David S. Powars, & Gregory A. Wandless. (2017). Shallow marine response to global climate change during the Paleocene‐Eocene Thermal Maximum, Salisbury Embayment, USA. Paleoceanography and Paleoclimatology, 32(7), 710–728. 

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