Limalok

Limalok: A Cretaceous-Paleocene Guyot in the Marshall Islands

Imagine a hidden gem, lying deep beneath the waves of the Pacific Ocean, waiting to be discovered. This is Limalok, a fascinating guyot or tablemount that has intrigued scientists for decades. Located in the southeastern Marshall Islands, this underwater mountain was formed by volcanic activity and now lies at a depth of 1,255 meters. But what makes Limalok so special? Let’s dive into its history and explore why it is such an important geological feature.

The Formation of Limalok

Imagine the Earth as a giant canvas, with hotspots painting vibrant patterns across its surface. One such hotspot created Limalok, a Cretaceous-Paleocene guyot that has stood the test of time. The volcano was active for millions of years before finally ceasing its activity and being eroded by the relentless forces of nature. This erosion led to the formation of a carbonate platform, which eventually sank below sea level during the Eocene period.

Drilling into Limalok’s History

Limalok was one of many seamounts targeted during the Ocean Drilling Program. However, due to the low proportion of material recovered from drilling, it has been challenging for scientists to fully reconstruct its geologic history. This mystery only adds to the allure of this underwater wonderland.

Geographic and Geological Context

Limalok is situated at the southernmost end of the Ratak Chain in the western Pacific Ocean, southeast of Mili Atoll and Knox Atoll. Its summit platform covers an impressive 636 square kilometers and is connected to these atolls through a volcanic ridge. The seafloor around Limalok is estimated to be between 152-158 million years old, but there’s a possibility that it rises from even older Cretaceous flood basalts.

Historical Names and Legends

Limalok has had several names throughout history. Formerly known as Harrie or Harriet Guyot, this name pays homage to a traditional chieftess of Mile Atoll. The seamount is part of a group of guyots found in the Marshall Islands, which are characterized by their steep slopes and flat tops, often covered with coral reefs.

The Geological Story

Hotspot theory suggests that Limalok formed through volcanic activity, but there are inconsistencies that hint at multiple hotspots passing through the area. Geochemical evidence points to a connection with Rarotonga and Rurutu hotspots, while reconstructions indicate involvement of the Macdonald hotspot (95-85 million years ago), Rurutu hotspot (75-65 million years ago), and Society hotspot (75-65 million years ago).

Volcanic Rocks and Minerals

Limalok erupted basaltic rocks, including alkali basalts, basanite, and nephelinite. The minerals found here include apatite, augite, biotite, clinopyroxene, olivine, nepheline, and plagioclase. Shallow crystal fractionation processes played a significant role in the genesis of these magmas.

Alteration and Sedimentation

The alteration process on Limalok resulted in the formation of calcite, chlorite, clay, iddingsite, montmorillonite, zeolite, and celadonite. Volcanogenic sandstones and hydrothermal alterations are also present here. The carbonate rocks found include various forms such as grainstone, packstone, limestone, rudstone, and wackestone. Porosity is usually low due to cementation.

The Carbonate Platform

Limalok is the youngest guyot in the Marshall Islands, with argon-argon dating yielding ages of 69.2 and 68.2 ± 0.5 million years ago. The volcano was formed as a shield volcano in French Polynesia at 15–10 degrees southern latitude. Volcanic rocks were emplaced as lava flows reaching thicknesses of 1-7 meters, with soils generated through weathering over 1-3 million years.

Life on Limalok

The early limestones dredged from Limalok date back to the Eocene age (56–33.9 million years ago). The volcano’s erosion led to the beginning of carbonate platform growth, starting around 57.5 ± 2.5 million years ago in the Paleocene. The platform reached an overall thickness of 290 meters and showed variations between individual carbonate layers, indicating that parts of the platform were submerged and emerged over time.

The Drowning of Limalok

Imagine a vibrant ecosystem thriving on Limalok’s carbonate platform. However, around 48 ± 2 million years ago during the early-middle Eocene, the platform ‘drowned’ when sedimentation could no longer keep up with sea level rise, stopping carbonate deposition. This drowning may have been due to several factors: sea level drop, emergence of the platform, nutrient-rich equatorial waters overgrowing algae, or global temperature extremes.

After Drowning

After Limalok’s drowning, subsidence lowered the tablemount below the photic zone. Hardgrounds and iron-manganese crusts formed containing Oligocene sediments and planktonic fossils. Renewed sedimentation began in the Miocene period with foraminifera and nanofossils. Sediments reached a cumulative thickness of 100–140 meters during the Miocene and Pliocene–Pleistocene periods.

Through its complex geological history, Limalok continues to fascinate scientists and offer insights into Earth’s past. Its story is a testament to the dynamic nature of our planet and the enduring mysteries that lie beneath the ocean’s surface.