**Comparison of Modern and Pleistocene (MIS 5e) Coastal Boulder Deposits from Santa Maria Island (Azores Archipelago, NE Atlantic Ocean)**

**Sérgio P. Ávila 1,2,3,4,\*, Markes E. Johnson 5, Ana Cristina Rebelo 1,3,6,7, Lara Baptista 1,3,4 and Carlos S. Melo 1,3,8,9**


Received: 21 April 2020; Accepted: 26 May 2020; Published: 28 May 2020

**Abstract:** Modern and palaeo-shores from Pleistocene Marine Isotope Substage 5e (MIS 5e) featuring prominent cobble/boulder deposits from three locations, on the southern and eastern coast of Santa Maria Island in the Azores Archipelago, were compared, in order to test the idea of higher storminess during the Last Interglacial. A total of 175 basalt clasts from seven transects were measured manually in three dimensions perpendicular to one another. Boulders that exceeded the minimum definitional diameter of 25 cm contributed to 45% of the clasts, with the remainder falling into the category of large cobbles. These were sorted for variations in shape, size, and weight pertinent to the application of two mathematical formulas to estimate wave heights necessary for traction. Both equations were based on the "Nott-Approach", one of them being sensitive to the longest axis, the other to the shortest axis. The preponderance of data derived from the Pleistocene deposits, which included an intertidal invertebrate fauna for accurate dating. The island's east coast at Ponta do Cedro lacked a modern boulder beach due to steep rocky shores, whereas raised Pleistocene palaeo-shores along the same coast reflect surged from an average wave height of 5.6 m and 6.5 m. Direct comparison between modern and Pleistocene deposits at Ponta do Castelo to the southeast and Prainha on the island's south shore produced contrasting results, with higher wave heights during MIS 5e at Ponta do Castelo and higher wave heights for the modern boulder beach at Prainha. Thus, our results did not yield a clear conclusion about higher storminess during the Last Interglacial compared to the present day. Historical meteorological records pit the seasonal activity of winter storms arriving from the WNW-NW against the scant record of hurricanes arriving from the ESE-SE. The disparity in the width of the marine shelf around Santa Maria Island with broad shelves to the north and narrow shelves to the south and east suggested that periodic winter storms had a more regular role in coastal erosion, whereas the rare episodic recurrence of hurricanes had a greater impact on southern and southeastern rocky shores, where the studied coastal boulder deposits were located.

**Keywords:** coastal boulder deposits; storm surge; hydrodynamic equations; Holocene; Pleistocene; MIS 5e (Marine Isotope Substage 5e); NE Atlantic Ocean

#### **1. Introduction**

Survey models regarding the level of storm intensity during the Last Interglacial stage of the Pleistocene, specifically the Marine Isotope Substage 5e (MIS 5e), have been conducted on a global scale [1], but studies organized on a more regional scale provide the potential for higher resolution. Localized studies on the propensity for storms in the Bahamas and Bermuda (Western Atlantic) [2,3] reveal patterns in agreement with global results. However, for higher latitudes in the North Atlantic Ocean (e.g., the Azores Archipelago), such analyses have been scarce [4]. Climatological reconstructions are of utmost importance because they allow the scientific community, policymakers, and the general public to better predict and plan for future hazardous events. With a worldwide extension of 500,000 km, coastlines are highly complex and dynamic geomorphological features [5] that commonly correspond to areas of high-density human habitation [6]. With ongoing conditions of global warming, there is even more urgency for increased knowledge about the deep history of storm patterns associated with oceanic circulation.

The shorelines of volcanic oceanic islands are highly dynamic in nature as a result of volcanism, mass wasting, and exposure to the energetic action of the open sea [7–10]. Exposure to wave action is the primary factor that shapes coastlines, as wave surge crossing island shelves acts continuously, but at different levels of energy [11]. In the Azores Archipelago, all islands are subject to the direct action of waves [11]. Unprotected by the absence of barriers (e.g., reefs) and with narrow insular shelves [12], erosion rates are high [6]. The result is the production of variable amounts of detrital materials that are readily shaped and transported, commonly including coastal boulder deposits (CBDs). Sometimes, transported materials are configured in peculiar geomorphologies, such as fajãs [13]. In the Azores, processes that lead to the production of modern CBDs allow for comparisons with morphologies deposited during the Last Pleistocene Interglacial episode (MIS 5e). Santa Maria is the only island in the Azores with marine fossiliferous sequences that date back to the Pliocene and late Pleistocene (MIS 5e) [14–16]. The presence of such sequences makes this island an ideal place for studies testing the postulated higher storminess and inferred palaeo-wave heights that affected the wider archipelago approximately 125,000 years ago.

Inference on palaeo-wave-heights through measurements of eroded blocks in Pleistocene settings has been conducted by Johnson et al. [17–19] at localities in the Mexican Gulf of California, providing a useful methodology for application elsewhere. Here, we adapted the program using mathematical formulas to compare the storminess during the Last Interglacial (MIS 5e) and the modern situation, deduced from storms imprinted in the CBDs. Herein, we presented the first estimations for palaeo-wave heights from the Pleistocene (MIS 5e) in the Azores. Data on the shape, size, and calculated weight of palaeo-shore boulders from Santa Maria Island had the advantage of being sourced from well-dated MIS 5e outcrops containing thermophile fauna typical of the Last Interglacial. Crucially, estimates on wave heights from MIS 5e CBDs might be compared with adjacent modern beach boulders. Work was limited to localities on the island's south and east coast, where marine shelves were narrower. The island's broader northern shelf provided the basis for conjecture on differences in storm patterns that led to that outcome.

#### **2. Geographical and Geological Setting**

#### *2.1. Position and Geotectonic Setting*

The Azores is an oceanic volcanic archipelago with nine islands, found in the NE North Atlantic Ocean, between latitudes 36◦55 and 49◦43 N and longitudes 24◦46 and 31◦16 W, and spreading along a distance of 650 km (Figure 1a). Seated on the Azores Plateau, the archipelago straddles a triple junction between the Eurasian, North American, and Nubian (African) tectonic plates [20]. Santa Maria Island is the most southeastern and oldest among these and is seated on the Nubian plate. It exhibits a geologic record, marking the earliest emergence from the sea due to Surtseyan volcanic activity approximately 6 million years ago [8]. Much of the ensuing rock record, which includes intercalated sedimentary strata and extensive volcanic flows, is restricted to the Pliocene Epoch [21,22]. Island uplift, which commenced 2.8 million years ago, has resulted in more than 200 m of tectonic rise, as a result of which older Pliocene marine strata are well exposed in sea cliffs on all sides of the island, but most accessible along the south and eastern shores [23]. Santa Maria ranks seventh in size compared to the other islands in the Azores Archipelago, with an area of 97 km2 and a coastal circumference of 53 km. The island presents a peculiar orography, with a flatter Western part and a rougher Eastern part, as a result of its unusual geological evolution (i.e., different erosional rates and off-center volcanism during a rejuvenated stage, mostly located on the eastern section of the older edifice [8,24]; Figure 1b). In terms of bathymetry, the island is much reduced in size with an asymmetrical marine shelf that is broadest to the north and narrowest to the south and east [24]. The shelf is also characterized by a suite of submerged terraces, all presumably younger than ~1 Ma, which are more developed and preserved in wider and low-gradient sectors (Figure 1b) [25]. During the Last Interglacial epoch of the Pleistocene, extensive boulder beds were emplaced all around the island that incorporates marine fossils attributed to the MIS 5e [4]. Three localities, including Prainha and Ponta do Castelo on the south shore and Ponta do Cedro on the east coast, were chosen to highlight lateral variations in Pleistocene boulder size together with a review of the associated fauna. Comparisons were drawn with modern boulder beds at the two southern localities, whereas the plunging coastal cliffs at Ponta do Cedro lacked any such development of a coastal boulder deposit (CBD) at or close to modern sea level.

**Figure 1.** Maps covering the Azores Archipelago in the NE Atlantic Ocean: (**a**) Geographic and geotectonic setting of the Azores Archipelago (modified from [26–28]). NA—North American plate; Eu—Eurasian plate; Nu—Nubian (African) plate; MAR – Mid Atlantic Ridge; TR—Terceira Rift (grey area); EAFZ—East Azores Fracture Zone; GF—Gloria Fault; (**b**) Topography/bathymetry of Santa Maria Island. Black stars mark the studied sites. The bathymetric data was extracted from GEBCO 2019 (https://www.gebco.net); subaerial topography was generated from a 1:5000 scale digital altimetric database.

#### *2.2. Sources and Shaping of the Boulders*

The Prainha site is situated within a bay, thus affording a higher potential for the deposition of clasts and finer sediments. The analysis of local currents showed a confluence of waves to the south coast of the island (https://www.ipma.pt/pt/maritima/hs/index.jsp?area=acores-east) that explains how during the summer season the nearby coastlines collect extensive (on a local scale) sand beaches. The area is also the river-mouth of some streams. As the archipelago's climate is classified as "Warm Temperate" [29], higher precipitation events are common. Such events are responsible for the

transport of varying amounts of boulders and sand to the intertidal zone. As rough seas are commonly registered in the archipelago, the boulders are easily rounded and shaped.

At the southeasternmost tip of the island, Ponta do Castelo presents the most complex wave regime from the three sites studied. In the area, perpendicular (in direction) wave currents interact, promoting rougher seas, even under fair-weather conditions. As a result of this exposure, no fine sediment is deposited, and only a well-developed CBD is present.

In Ponta do Cedro, however, despite being found inside a bay, like Prainha, a plunging coastline allows for no accumulation of sandy deposits, and a modern CBD also is absent. Ponta do Cedro and Ponta do Castelo, however, share the same type of boulder source resulting from erosion of the area's high sea cliffs. Both these sites are situated in the rough eastern end of the island, where sea cliffs are impressive in height.

#### *2.3. Wave Energy and Direction*

The Azores occupies a region in the NE Atlantic Ocean encompassing approximately 22,500 km<sup>2</sup> (Figure 1a) and is characterized by a high level of marine storm activity. Hurricanes rarely cross through the region during the annual Atlantic hurricane season, although tropical cyclones are common. Such storms arrive from a zone off the west coast of Africa in the vicinity of the Cabo Verde Archipelago, located at a lower 17◦ N latitude. Notwithstanding, extreme storm events tend to arrive once every seven years, on average [6,30]. In contrast and as a result of the exposure to the strong NW North Atlantic winds – the Westerlies [31], the northern exposures of islands are subject to winter storms. The archipelago is also in the pathway the Gulf Stream makes from the North American coasts towards the central zone of the North Atlantic [32,33], being so a source of many instability processes, meanders, and eddies [34]. The main wave direction that affects the archipelago comes from WNW-NW [5,8,23]. Such predominance is reflected in the island's bathymetrical morphology that shows higher erosional rates on the W and N coasts. According to Rusu and Soares [11], and using the closest station to Santa Maria Island (P18), present-day mean wave heights during summer is 1.75 and during winter is 3.20 m. For the Azores archipelago, Santa Maria presents the lowest values regarding the mean values for both seasons, and Corvo presents the highest. This variation in values has been related to the shadow effect promoted mainly by São Miguel Island (North of Santa Maria), protecting it from the high wave energy [13].

#### **3. Methods**
