RATE AND ORIGIN OF CAVERNOUS "HONEYCOMB"
WEATHERING OF QAYTBEI FORT, ALEXANDRIA EGYPT.
GAMAL M. KAMH
Geology Dept., Faculty of Science, Menoufiya Univ, Egypt. |
Qaytbei Fort is located on the Mediterranean Sea coast at the extreme Western part of Alexandria City. It was built in 1499 on the pharos ruins, then mostly destroyed and re-built in 1884 using oolitic limestone as the original material used for its building then re-built again in 1984 and finally reconstructed in 2000 to 2003. This fort shows cavernous weathering that differs in intensity with respect to wall side orientation as well as height along these walls.
This paper aims to examine the origin of honeycomb noted on the outside walls of this fort and computing rate of honeycomb growth on these walls to indicate weathering intensity at this area on such construction rock.
Field data including measuring depth of honeycomb, rock sampling from the weathered parts of the fort's walls, and laboratory analyses to check rock petrography, mineralogy, petrophysical, geotechnical and salt content have been done.
It was found that the 1884 limestone used in reconstruction is pel-oo-sparite in composition, while that used in 1984 and 2003 has biomicrite composition. The reconstruction rock used in 1884 has low to medium geotechnical properties while that used in 1984 and 2003 has medium geotechnical properties. Salt weathering is the main process of honeycomb formation acting through salt crystallization and/or salt hydration mechanism. The rate of honeycomb growth is ranging from 0.76 mma-1 to 2.9 mma-1 with maximum rate on the North facing wall side at height zone 11 - 14 m above sea level (sea spray zone).
This pole position falls close to the
Carboniferous poles of Africa suggesting that the Mn-Fe ore could
have been deposited during the Carboniferous time. Furthermore, the
paleolatitude determined for these rocks, based on the mean
paleomagnetic inclination, suggests that the ore was deposited where
the area of study was situated at latitude 10oS of the
equator (i.e. at tropical weather conditions).
The minerals
and chemistry show that the ore is enriched with Na, K, Ca, Mg, Sr,
Co and Ni indicating that the ore is probably of sedimentary marine
origin. Whereas the enrichment of ore samples with hydrothermal
elements (As, Zn, Cu, Pb, V, W, and F) is restricted to the faulted
areas indicating a later phase of hydrothermal activity affected the
ore.
Abu Thora sandstone gave a mean paleomagnetic direction
of
Dec. = 326o, Inc. = 42o, K = 19.3;
 95 =
17.9o which yields a paleomagnetic pole position
at
Lat. = 59o; Long. = 302o (Dp = 13.5, Dm
= 22).
This magnetization is significantly different from
that of the Mn-Fe ore and hosting rocks. It is believed that this
remnant magnetization is related to the red pigmentation of the
rock. A Cretaceous age is assigned for this remnance. It is
suggested that this porous sandstone acquired its magnetization
later after its deposition, more probably by diagenetic chemical
processes.
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