The first purusa-avatara, Maha-Visnu in the Causal Ocean, who is the creator of the aggregate material energy, is an expansion of Sankarsana
“You are the ultimate shelter of these three plenary portions. Thus there is not the slightest doubt that You are the primeval Nārāyaṇa.
Brahmā has confirmed that Lord Kṛṣṇa is the Supreme, the source of the three manifestations known as Kṣīrodakaśāyī Viṣṇu, Garbhodakaśāyī Viṣṇu and Kāraṇodakaśāyī Viṣṇu (Mahā-Viṣṇu). For His pastimes, Lord Kṛṣṇa has four original manifestations—namely Vāsudeva, Saṅkarṣaṇa, Pradyumna and Aniruddha. The first puruṣa-avatāra, Mahā-Viṣṇu in the Causal Ocean, who is the creator of the aggregate material energy, is an expansion of Saṅkarṣaṇa; the second puruṣa, Garbhodakaśāyī Viṣṇu, is an expansion of Pradyumna; and the third puruṣa, Kṣīrodakaśāyī Viṣṇu, is an expansion of Aniruddha. All these are within the category of manifestations of Nārāyaṇa, who is a manifestation of Śrī Kṛṣṇa.