Space-time evolution of a large field of pockmarks in the Bay of Concarneau (NW Brittany)

About a decade ago, a large field of pockmarks, covering an overall area of 36 km2 was discovered in water depths of < 30 m in the central part of the Bay of Concarneau (Southern Brittany, France). This field, composed of features from 5 m to 35 m in diameter and < 1 m in depth, is characterized by unusual high densities of pockmarks, up to 5840 per square kilometre. Geophysical data correlated with sedimentary samples acquired in 2005 and 2009 show that pockmarks and their immediate surroundings are associated with dense tubes benches cover, built by a filter-feeding amphipod: Haploops nirae, forming original benthic communities. Two complementary surveys were carried out in April 2011 (Pock and Ploops) and April 2014 (Pock and Tide), on the Oceanographic Vedette (O/V) Haliotis (Ifremer/Genavir), to map the limit of the pockmarks and Haploops fields with the maximum accuracy. The link between the presence of the Haploops nirae communities and the occurrence of pockmarks /gas was then established and the proposed hypothesis was that tidal cycles may provide a good candidate for a short-term (monthly) triggering mechanism of fluid expulsion (Baltzer A, Ehrhold A, Rigolet C, Souron A, Cordier C, Clouet H. 2014. Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders. Geo-Marine Letters, 34, 215–230). Due to the high-level precision (50 cm) of the positioning system (Magellan Aquarius Ixsea Hydrins) coupled with the RTK attitude system, these new bathymetric and imagery maps together with the sub-bottom Chirp profiles, allow us to compare the data sets from April 2011 and April 2014. The superimposition of the two data sets shows that the distribution of the pockmarks remains similar between these 2 dates (i.e. for 3 years), for the group of large, widely scattered pockmarks, which are deeply rooted in the Holocene palaeo-valley infills and for the group of pockmarks identified as the trawl-scour pockmarks, initiated by trawling action. Most of the pockmarks present very recent shapes without any infilling but sonar imagery reveals that some of them have been covered by a thin muddy layer, thereby reflecting, at least, a temporary cessation of expulsion or a different activity. Chirp profiles indicate some acoustic flares above the pockmarks, revealing gas/fluid expulsion. Different gas clues within the sedimentary column, such as acoustic turbidity, enhanced gas reflectors (EGR), chimneys pipes, occur at exactly the same places on the chirp seismic profiles from 2011 and 2014. Therefore, contrary to most examples described in the literature, this pockmarks field is still active.

SUBMARINE VOLCANO CHARACTERISTICS IN SABANG WATERS

The aim of the study is to understand the characteristics of a volcano occurred in marine environment, as Weh Island where Sabang City located is still demonstrated its volcanic cone morphology either through satellite imagery or bathymetric map. Methods used were marine geology, marine geophysics and oceanography. Results show that surface volcanism (sea depth less than 50 m) take place as fumaroles, solfataras, hot ground, hot spring, hot mud pool and alteration in the vicinities of seafloor and coastal area vents. Seismic records also showed acoustic turbidity in the sea water column due to gas bubblings produced by seafloor fumaroles. Geochemical analyses show that seafloor samples in the vicinities of active and non-active fumarole vent are abundances with rare earth elements (REE). These were interpreted that the fumarole bring along REE through its gases and deposited on the surrounding seafloor surface. Co-existence between active fault of Sumatra and current volcanism produce hydrothermal mineralization in fault zone as observed in Serui and Pria Laot-middle of Weh Island which both are controlled by normal faults and graben.Keywords: submarine volcano, hydrothermal mineralization, Sabang-Weh-Aceh. Tujuan kajian adalah memahami karakteristik suatu gunungapi yang berada dalam lingkungan marin, sebagaimana Pulau Weh dimana Kota Sabang terletak masih menunjukkan morfologi kerucut volkaniknya baik melalui citra satelit maupun batimetri. Metoda yang digunakan adalah geologi kelautan, geofisika kelautan dan oseanografi. Hasil menunjukkan bahwa volkanisma permukaan (kedalaman laut kurang dari 50 m) terdapat dalam bentuk fumarola, solfatara, lahan panas, mata air panas, kolam lumpur panas dan alterasi sekitar lobang kepundan dasar laut dan pantai. Rekaman seismik juga menunjukkan turbiditas akustik dalam kolom air laut akibat gelembung gas yang dihasilkan oleh fumarola dasar laut. Analisis geokimia menunjukkan bahwa contoh-contoh dasar laut sekitar lobang kepundan fumarola yang aktif maupun tidak aktif kaya akan logam tanah jarang. Ini ditafsirkan bahwa proses fumarola tersebut membawa REE melalui gas-gasnya dan mengendapkannya pada permukaan dasar laut di sekitar. Ko-eksistensi antara Sesar Sumatera aktif dan volkanisma Resen menghasilkan mineralisasi hidrotermal dalam zona sesar seperti teramati di Serui dan Pria Laot - bagian tengah Pulau Weh yang keduanya dikontrol oleh sesar normal dan graben. Kata kunci: gunungapi bawah laut, mineralisasi hidrotermal, Sabang-Weh-Aceh.

SEAFLOOR FAULTING AND ITS RELATION TO SUBMARINE VOLCANIC ACTIVITIES BASED ON SUB BOTTOM PROFILING (SBP) ANALYSES IN WEH ISLAND WATERS AND ITS SURROUNDING, NANGROE ACEH DARUSSALAM PROVINCE

Sub bottom Profiling survey using strata box, a specially designed low penetration sub bottom Profiling (< 80 m) for coastal waters exploration, found out evidence of submarine volcanic activities in northern coastal waters of Weh Island, NanggroeAceh Darussalam Province. Gas bubbling could be observed at water columns of the digital sub bottom Profiling records as acoustic turbidity. There are at least 33 spots of volcanic gas bursts observed from the sub bottom Profiling. Examination of gas bursts at coastal area which show fumaroles and solfatara indicate reduce volcanic activity either at submarine or terrestrial. Identification of seafloor gas burst by diving team found out that center of such burst is occurred at a north - south opened lineation assumed as normal fault. It seems that the seafloor normal fault is the continuation of terrestrial fault of the same direction as observed from terrain earth google of Weh Island.Keywords: seafloor faulting, submarine volcanic activities, shallow sub bottom Profiling data, Weh Island Aceh Survei penampang bawah dasar laut (SBP) menggunakan strata box, suatu alat SBP penetrasi rendah yang didisain untuk eksplorasi perairan pantai, mendapatkan bukti-bukti aktivitas gunungapi bawah laut di perairan sebelah utara Pulau Weh, Provinsi Nanggroe Aceh Darussalam. Gelembung-gelembung gas dapat diamati pada kolom air rekaman digital penampang bawah dasar laut sebagai turbiditas akustik. Sedikitnya dijumpai 33 titik semburan gas volkanik yang teramati dari penampang bawah dasar laut tersebut. Pemeriksaan semburan-semburan gas pada wilayah pantai sebagai fumarola dan solfataramenunjukkan telah berkurangnya aktivitas volkanik apakah pada dasar laut maupun darat. Identifikasi semburan gas dasar laut oleh tim selam mendapatkan bahwa pusat semburan berada pada kelurusan berarah utara - selatan yang diduga sebagai sesar normal. Tampaknya adalah bahwa sesar normal dasar laut tersebut merupakan kelanjutan sesar darat yang berorientasi sama seperti teramati dari citra earthgoogle terrain Pulau Weh. Kata kunci: pensesaran dasar laut, aktivitas gunungapi bawah laut, data penampang bawah dasar laut, Pulau Weh Aceh

Seabed pockmarks in the southern Bulgarian Black Sea zone

ockmarks were discovered for the first time in the SE of the Bulgarian Black Sea zone in 1988. Echo sounder and shallow seismic data analyses of investigations conducted before and after 1988, allow the main pockmark area to be enlarged and given more details. A total of 305 pockmarks have been located within an area of more than 100 km2 (2-5 km wide and 41 km long). The bottom of the area is covered with Holocene soft silty clay. The following trends are defined when moving from the northern part of the pockmark area to the southern part: 1) increase in seabed sediments thickness from 2.5 m to more than 3.2 m and decrease in mean sediment grain size; 2) increase in general water depth from 160-300 m to 230-340 m in areas where pockmarks occur; 3) increase in average individual pockmark size from 86 to 132 m in diameter and from 2 to 4 m in depth; 4) decrease of pockmark density from 8-9 p.m./km2 to 3-4 p.m./km2 Similar trends are observed seaward where the pockmark profile shape changes from a smooth U-shaped to a sharp V-shaped form, with pockmark wall slope angle reaching more than 10°. A variety of acoustic anomalies such as acoustic turbidity, bright spots, enhanced reflections etc., are recorded in many places within the pockmark area, indicating the presence of gas in the sediments. Water column anomalies have also been recorded. The paper discusses the origin of the gas (most likely to be a mixture of thermogenic and biogenic gas) and the time of pockmark formation.