The ancient Enjefa beaches EarthCache

Introduction

At the end of the last ice age some 10000 years ago the global sea levels started to rise due to melting of the ice, and the present Persian Gulf, which lay dry was flooded. From 7000 years ago on the rise in global sea level continued though not as fast as before. Here at Enjefa Beach you can find outcrops that were formed during that time. However they show that the sea level was actually falling here and not rising.

Depositional environment

Depositional environment, seen from above.

Enjefa Beach is a long stretch of beach south of Kuwait City. Along the beach you can find outcrops with various sedimentary structures. Not all of them are visible everywhere across the beach, but generally you will see (a) horizontally laminated sandstone at the bottom, followed by (b,c) cross-bedded sandstone and a (d) bioturbated sandstone at the top. These deposits tell us what the coastline here looked like some 7000 years ago: there was a beach or barrier island here with a shallow water sand flat behind. A tidal channel cut through the beach and brought water onto the sand flat. Several smaller channels branched off from this main channel and distributed water across the flat.

Sedimentary structures

Sedimentary structures

a) Horizontally laminated sandstone: When you look at this rock you will see very thin horizontal layers. If you look closely you can see that the grain size increases upwards within most laminae. There are several of these small cycles present. At the top of some cycles you can find a white band of shells. There are several places all across the Gulf region where you find beaches with masses of tiny shells, and also here each band of shells represents the top of an ancient beach or barrier island. Each of these cycles from fine-grained sand to coarse grained sand and shells represents a small change in sea level: the bigger the grains, the larger the wave energy needed to deposit these grains. Thus the smaller the grains, the deeper the water, while largest grains are found closest to the beach or on the beach. These deposits tell us the sea became shallower and deeper again several times.

b) Trough cross-bedding: These trough cross-beds in the middle of this outcrop were deposited in a tidal channel or tidal inlet that cuts through the beach and transports water to the area behind the beach during high tide. Trough cross-bedding is formed when the water is deeper and the grain size relatively large due to fast flowing water. The orientation of trough cross-beds can give an indication of the direction the water flowed. Here they indicate the currents flowed roughly NW, nearly perpendicular to the coastline. This sandstone was deposited closer to the coast than the laminated sandstones (a).

c) Planar cross-bedding: Those deposits formed in the shallower part of the channel. These deposits are closer to the coast than the laminated sandstones (a), though the distance to the sea might be the same as the trough cross-bedding (b). Depositional environments not only change with rising or falling sea-level, they can also shift sideways, for example when a tidal channel meanders sideways or forms a completely new channel bed. This planar cross-bedding indicates a dominant current towards the north, parallel with the sea. It is quite possible that this was a smaller channel that branched off from the main channel (b).

d) Bioturbated sandstone: At the top of the outcrop you can see there are hardly any sedimentary structures apart from thick round tubes criss-crossing through the rock. Those are burrows left behind by worms and other animals that lived in this sediment, burrowed through it in search for food and destroyed the layering (bioturbation). Those worms lived on sand flats behind the beach that might have been dry during low tide and flooded through tidal channels (b, c) during high tide. Normally, sand flats are found behind the beach but can be found a substantial distance inland if the landscape is flat.

Why is the sea-level falling?

During the last 10.000 years most of the glaciers formed during the last ice age melted, resulting in a worldwide rise in a sea-level. So how can it be that the rocks here seem to indicate the opposite, that the relative sea-level gets lower and we find beach deposits overlain by deposits that are normally found further inland? One explanation might be found in plate tectonics. The Arabian plate, of which Kuwait is a part is moving towards the plate Iran is situated on. Both plates collide and the Iranian plate gets pushed together. This collision is responsible for the Zagros Mountains in Iran. The effect is less pronounced on the Arabian peninsular, but some slight crumbling does form locally and in Kuwait the rate of uplift caused by this crumbling was slightly higher than the rise in sea-level. The rising land meant that the sea moved further away from the shore, and thus gave the impression of a sea-level fall while the actual sea-level was in fact not falling, but rising. The two images below show the effect of a relative rising and falling sea level. The lowest image presents what happened here: with the sea further away from the shore, the beach deposits become covered in sand flat deposits such as the crossbedded and bioturbated rocks.

Depositional model

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References

Tanoli, Al-Farez, Al-Salan, 2012, The Enjefa Beach Exposure in Kuwait, Northern Gulf: Evidence of Late Holocene Regression.. Search and Discovery Article #50602.

Walker and James (editors), 1992, Models, Response to Sea Level Change.. Geotext 1, seventh edition 2004.