Whalebone Bay

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Satellite image of the Devonshire Bay Locality on the South Shore of Devonshire Parish, showing numbered Section locations which are described separately below.

Summary

Tightly cemented beach deposits thought to be of the Town Hill Formation occupy most of the northeastern shore of Whalebone Bay at Section 1 (See satellite image for section locations). A terra rossa palaeosol, which formed subsequent to the beach deposits at a lower sea level, has largely been stripped away. Evidence of its existence takes the form of  occasional remnants of truncated soil pipes exposed on the surface of the truncated beach deposits. Next in the succession, at Section 2,  are coarse weakly cemented beach sands, and locally a conglomerate, of the Rocky Bay Formation. This represents a rise in sea level approximately 120,000 years ago. Overlying this is an immature brown palaeosol, or protosol,  which slopes down to sea level towards the southwest. Last, is a Rocky bay eolianite, representing a small dune which blew in from northwest towards the end of the Last Interglacial period during a fall in sea level.

Whalebone Bay whole section
Section 2. The headland at Whalebone Bay comprises ancient cemented beach deposits overlain by a Rocky Bay shelly marine unit – the “Devonshire” – followed by a north-westward dipping protosol – the “Harrington” – and a then dome-shaped Rocky Bay dune which blew in from the north.

Locality cross section
Summary cross-section through Whalebone Bay, Section 2. Click on image to enlarge.

Section descriptions

Section 1

Planar strata, along the shore at Section 2, dip down to the northwest at approximately 10° . These deposits are interpreted as an ancient beach and are mapped as a member of the Town Hill Formation (VA6). A pre-Belmont age, is consistent with a later finding (VO4) that, based on their diagenetic history, the deposits must have been subjected to multiple submersions over a long period.  Also indicative of age are thin layers of non-carbonate black volcanic sand within these deposits.

Beach strata
Section 1. North-westward dipping planar beach-face strata occupy most of the north-eastern shore of Whalebone Bay.

Interpretation of Section 1

Gently dipping parallel planar strata (Section 1) represent the swash zone of a beach which prograded towards northwest at a time when sea level was close to its present position. Deposits, close to present sea level,  at the south-eastern end of the exposure, at the small beach, appear to be aeolian. This would suggest that the beach was formed contemperaneously with a marine transgression, as observed at other localities such as Devonshire Bay.     Based on the occurrence of several phases of marine cementation (VO4), the beach is considered to be well over 200,000 years old i.e. pre-dating the Penultimate Interglacial period. It is inferred, from the inter-layered volcanic sand, that at the time of the beach’s deposition either remnants of the volcano were still exposed to wave erosion or sediment containing volcanic debris was being re-worked.  More on the volcanic origins of Bermuda can be found in Chapter 2.

Volcanic sand with inset
Section 1.  The beach strata includes layers of volcanic sand. Close-up of such a layer is shown in the inset (top left).

Section 2

The beach deposits which occupy most of the north eastern shore of Whalebone Bay, at  Section 1, extend westward and form the basal unit below the headland at Section 2. For the most part the terra rossa palaeosol  which had overlain these beach deposits has been stripped away. Evidence of the soil’s existence is provided by truncated soil pipes. Weakly cemented marine sands with low angle near-horizontal stratification rest directly on the older beach. Large shell fragments and rounded limestone fragments are generally concentrated at the base of this younger marine limestone, which are interpreted as the marine member of the Rocky Bay Formation, traditionally known as the “Devonshire ” marine deposit.

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Section 2a. At the eastern end of Section 2. An erosional platform of ancient well-cemented beach deposits (below hammer) is overlain uncomformably by Rocky Bay, “Devonshire”, marine deposits. The intervening palaeosol has been stripped away by marine erosion. Above the Devonshire deposits is the “Harrington” protosol which merges upwards into the modern soil.  Towards the east (right) the older beach deposits rise up and the Devonshire marine unit pinches out.

The succession at Section 2 varies along its length. Towards the east, the Devonshire marine unit thins out until the overlying Harrington protosol rests directly on the old beach unit. Also, there are no dune deposits evident above the Harrington protosol at the eastern end of the section.

Towards the middle of  Section 2, aeolian slip-face strata appear above the north-westward dipping protosol (Section 2b). These belong to a relatively small dome-shaped Rocky Bay dune which blew in from the north. Up-slope (to the east), well preserved fossil Poecilozonites land snails are quite common within the protosol, down-slope (to the west) they are replaced by broken fragments of sea shells.

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Section 2b. Near the middle of Section 2. The pale brown protosol, interpreted as the intra-Rocky Bay “Harrington” soil,  is overlain by dune deposits. A thin shelly “Devonshire”marine unit,   below the protosol, rests unconformably on a platform of tightly cemented, much older, beach deposits. At the eastern end of this exposure (right of the hammer) the protosol includes some well preserved fossil  land snails. in the other direction (left of the hammer) fragments of sea shells start to appear within the soil.

 

Continuing westward to the point of the headland (preferably at low tide) the sea-shell content of the protosol increases to the point that it could be characterised as a soily marine breccia or shell hash. The Devonshire marine unit which normally underlies the protosol, disappears or is incorporated into this breccia.

The full succession, at Section 2, replicates that at Rocky Bay  (Section 7) except, the lowest beach unit in the succession at Whalebone Bay is thought not to be of the Belmont Formation but rather the older Town Hill Formation. As at Devonshire Bay the Rocky Bay marine sands are superposed by weak light coloured intra-formational soil, or protosol, named the “Harrington” followed by a Rocky Bay dune.

Interpretation of Section 2

A late Pleistocene rise in sea level, or marine transgression, removed remnants of a terra rossa palaeosol which had formed over the cemented beach-face deposits of Section 1. Coarse shelly material and carbonate sand were deposited by the sea on an erosional platform of the old beach-limestone (Figure 2a). These weakly cemented  marine deposits are interpreted as part of the marine member of the Rocky Bay Formation, traditionally known as the Devonshire marine unit.

The protosol, which formed over the Devonshire marine unit, known as the Harrington, was the outcome of a fall in sea level or  marine regression. It represents colonisation of the emergent  marine deposits by vegetation. The dune which tops the Rocky Bay succession is small at Section 2. However, it is at the end of a substantial ridge of Rocky Bay dunes which extends along much of the  north-west shore of St. George’s island. As such, it represents a significant island-wide dune-building event coincident with widening beaches towards the end of the last major interglacial period (LIG).

A potential complication in this story is presented by the increasing content of marine shells in the protosol down-slope, in a westerly direction. This  has been interpreted, by Bretz (BR1), as evidence of a second marine transgression which partially submerged the protosol and, through wave action,  reworked it at low elevations.  Alternative explanations are, first, that the intermingled soil and marine material are the result of bioturbation of the Harrington protosol and the subjacent Devonshire marine unit. A second explanation is that exceptionally high tides and storm surges may contributed marine marine to the protosol at low elevations as observed. In these circumstances, sea level change need not be invoked; albeit a plausible contending explanation.