Tiny Fossils, Big Impact: Sedimentology of a Foraminifera-Enriched Detachment Horizon of a Large Retrogressive Submarine Landslide in the Gulf of Mexico

Abstract:

A 2-meter thick condensed section enriched in foraminifera is the final detachment horizon of a retrogressive submarine landslide, in the Ursa Basin, northern Gulf of Mexico. The high concentration of foraminifera (up to 900% relative to background values) in this layer produces a high porosity (up to 5 porosity units), low sonic velocity, low resistivity, and high gamma ray zone relative to the background clay properties. A high acoustic impedance contrast occurs at this interface and results in a high-amplitude reflection that is widespread and easily mappable in seismic. Integrated Ocean Drilling Program Expedition 308 Site U1324 cored and logged this layer. We acquired 31 samples: 22 from the foraminifera-enriched layer and 9 to establish background values. Objectives were to (1) quantify foraminifera enrichment relative to background values, (2) describe the spatial arrangement of foraminifera within the clay, (3) identify the foraminifera assemblage, and (4) determine particle size. CT images show individual foraminifera dispersed within the clay and are predominantly in-tact. Foraminifera concentration is variable within the layer but average 0.75 grams of foraminifera per gram of sediment (maximum = 0.97 grams of foraminifera per gram of sediment). Foraminifera concentration correlates to particle size of the hosting sediment: highest foraminifera concentrations occur within the finest-grained samples and vice versa. This explains the high gamma ray response. The assemblage is expected for this time interval in the Late Pleistocene (~24 kya) with an abundance of Globorotalia ruber and Globorotalia sacculifer. This suggests the layer is in-situ and a result of a pause in terrigenous sedimentation. The landslide was a multi-detachment event that initiated in a deeper horizon but retrogressively cut upwards. The condensed section was a preferred detachment horizon but only minimal sliding occurred before further movement ceased. One potential mechanism to explain this is the presence of foraminifera causes a dilational shear strengthening behavior, which acts to arrest movement. On a broader scale, condensed sections with abundant microfossils, may play a key role in landslide mechanics by altering the shearing properties of the parent material.